ANDREW M HOSIE
Members of the Kingdom Animalia are multicelled eukaryotic organisms characterized by cells without cell walls, membrane bound cellular organelles and DNA stored as chromosomes. During development all animal embryos pass through a spherical blastula stage, where a layer of cells surrounds a fluid filled cavity. Animals differ from plants as plants have cell walls, chloroplasts and do not pass through a blastula stage during development. Being unable to photosynthesise animals are heterotrophic, that is they need to feed through the ingestion of other organisms, whereas the plant groups are able to produce their food through solar energy. All animals are motile, capable of spontaneous and independent movement, though many become sessile during parts or most of their lifecycle.
Poriferans (Figure 1) are a conspicuous feature of marine habitats. These primitive multicellular animals lack true tissues or organs, there are no digestive, circulatory or nervous systems. Adults are sessile and use specialised cells called choanocytes to draw water through a system of water canals into the internal cavity called the spongocoel, order to feed on suspended particulate matter. Nutrients are distributed around the body by mobile cells called amoebocytes. Sponge body structure is maintained by skeletal elements, known as spicules, which can be made of silicon dioxide, calcium carbonate or collagen fibres (spongin).
Figure 1. Cutaway section of a sponge.
Cnidarians are radially symmetrical, with gelatinous bodies, and a mouth surrounded by a ring of tentacles. All cnidarians possess cnidae also called nematocysts. These are the stinging cells which are used in food capture and defence. Cnidarians have a very diverse range of life histories including being sessile, pelagic, solitary, and colonial or combinations of each over alternating generations. The pelagic form is typically called the medusa, while the sessile form is typically called a polyp.
The anthozoans (Figure 2) are a key feature of marine environments, most notably in the form of coral reefs. They are sessile and either solitary or colonial and lack a medusa stage. The oral disc is encircled by numerous tentacles (in multiples of 6 or 8) placed atop a cylindrical column. The body cavity (coelenteron) is divided by mesenteries (septa) also in multiples of 6 or 8. Anthozoans have a muscular pharynx leading from the mouth to the gut.
There are two groups within the Anthozoa: the Hexacorallia and the Octocorallia. The hexacorals have simple tentacles and internal mesenteries present in multiples of six. Hexacorals include the common sea anemones and reef forming stony corals. The Octocorals on the other hand have branching tentacles and mesenteries in multiples of eight. This group contains the so-called soft corals such as sea pens, sea fans, bamboo and bubblegum corals.
The asexual and typically colonial polyp stage (Figure 3 and Figure 4) is predominant in the lifecycle and the planktonic sexual medusa stage is often suppressed or absent. The polyps and medusae lack a pharynx and there are no mesenteries. Colonial polyps have an interconnected body cavity (coelenteron); this enables them to share nutrients and the individual polyps may be modified for different roles e.g. reproduction, feeding or defence. The polyp exoskeleton is typically chitinous but in some groups it is calcareous, as in the fire-corals and hydrocorals. Hydroids have a highly variable lifecycle, the general trend is a reduction in the medusa stage, from free living and feeding, remaining attached to the parent colony either as part of a gonophore or an unspecialised hydranth to being lost completely.
There are some exceptions to this trend, however the order Trachymedusae contains about 50 species that have lost the polyp stage altogether and exist only as medusae. While in the Portuguese man o'war, Physalia physalis, the polypoid colony has formed a float, is pelagic and lacks the medusa stage.
Scyphozoans (Figure 5 and Figure 6) are characterized by the predominant medusoid stage, which are, relative to the hydrozoan medusae, quite big at 2 cm - 2 m in diameter. The coelenteron is divided by 4 mesenteries. The medusae bell is circular in cross section and the tentacles are distributed around the circumference. Sensory organs distributed around the edge of the bell, called the rhopalia, contain the ocelli (simple eyes) and statocysts used for determining spatial orientation. The polypoid stage, known as the scyphistoma, is solitary and absent from the lifecycle of some species. Medusae are formed via budding (strobilation) from the scyphistoma. The scyphistoma can feed and may survive more than one reproductive season.
The cubozoans (Figure 7) are similar to the Scyphozoa but the bell is square in cross section, with a velum-like structure called the velarium. The velarium restricts the size of the opening through which water is expelled when the bell contracts, thus increasing thrust and making them stronger swimmers than the Scyphozoa. There are four clusters of tentacles, one at each corner of the bell. The rhopalia of the Cubozoa differ from those of the Scyphozoa in possessing very complex eyes with lenses, corneas and retinas. The lens is capable of producing very sharp images, as good as human eyes but the focal length is longer than the distance between the lens and the retina making box jellyfish strangely far-sighted. Development is also different in the cubozoans. Each scyphistoma forms a single medusa via complete metamorphosis. Cubozoa includes the highly toxic box jellyfish, found in tropical regions and often in swarms which can drift into bays, disrupting human activities.
Figure 2. A sea anemone (Anthozoa) with a cutaway section showing the internal structure.
Figure 3. A thecate hydrozoan colony.
Figure 4. The lifecycle of a thecate hydrozoan (Obelia). Note that the medusa stage is not always free-living, in the majority of hydrozoans it often remains attached to the adult colony and sometimes absent altogether.
Figure 5. Ventral view of a scyphozoan medusa (Aurelia).
Figure 6. The lifecycle of a typical scyphozoan (Aurelia).
Figure 7. A cubozoan medusa.
Ctenophore (Figure 8) bodies are gelatinous, biradial symmetrical, typically ovoid and divided by 8 ciliated bands known as comb-rows. These bands are characteristic of the group, allow the animal to swim and are often luminescent. Ctenophores are similar to cnidarians except that they lack cnidae or nematocysts. Instead the ctenophores possess colloblasts in their tentacles, which act in much the same way as cnidae but are adhesive rather than harpoon-like or toxic. There are no circulatory, respiratory or excretory systems. Individuals are hermaphroditic and fertilization is external through spawning and the eggs develop directly into juveniles.
Ctenophores make up a significant component of the plankton. Famously, the ctenophore Mnemiopsis leidyi, which was introduced into the Black sea prior to the 1980s, proved to be a major pest. The species feeds on zooplankton, including juvenile fish and their eggs, resulting in dramatic declines of fish populations in the region. It has since been recorded in the Caspian (1999), where it also caused major ecological damage, as well as the Baltic and North Seas (2006).
Figure 8. A typical ctenophore.
These unsegmented worms are dorso-ventrally flattened. The mouth is the only opening to the digestive tract (when present); there is no anus. The excretory organs which are present, are called protonephridia, and act as kidneys. Respiration is by passive diffusion through the body wall, i.e. there are no gills or lungs. Individuals are hermaphroditic and fertilization of embryos is internal. The Platyhelminthes are mostly internal parasites and have many specialised structures to adapt to this highly modified life history.
Tapeworm (Figure 9) bodies are comprised of the scolex ('head') followed by numerous segments (proglottids). The scolex possesses a variety of adhesive structures including suckers and hooks which are used to adhere to the host. The proglottids are linearly arranged with identical internal structures - mostly reproductive. There is no gut and the nutrients are taken up by the epidermal cells.
Cestodes are endoparsitic in the guts of vertebrates, requiring 1-2 intermediate hosts (sometimes more), which are typically arthropods or vertebrates. Tapeworms are known to infect humans, and the longest tapeworm recorded from a human is 12 m long. However, the longest known tapeworm, Polygonoporus, which infects whales, can reach 40 m.
Monogeneans (Figure 10) are typically oval in outline and dorsoventrally flattened. At the anterior end the mouth is enclosed by an oral sucker (prohaptor), while the posterior end has a large attachment organ (opisthaptor) with a circle of 6 suckers and a variable number of hooks (anchors).
Monogeneans are rarely longer than 2 cm and are typically ectoparasites on the skin and gills of fish with a few species infesting amphibians, turtles and one exceptional species that infest the eyes of hippopotamuses. The have the simplest lifecycle of the parasitic platyhelminths and do not have intermediate hosts.
Trematode bodies are typically elongate and dorsoventrally flattened. There are oral and mid-ventral suckers present but unlike the monogeneans, trematodes lack the opisthaptor.
Mostly endoparsitic, the lifecycle involves 3-4 hosts, the definitive host being a vertebrate (all groups) and intermediate hosts are typically molluscs. Within the lifecycle, there are serial asexual generations, before reaching the definitive host where sexual reproduction occurs. Flukes are known to infect humans and can be fatal. Transmission usually occurs when people eat uncooked meat.
Flatworm (Figure 11) bodies are as the name suggests flattened and may be elongate to broadly oval. The anterior 'head' may be differentiated from the rest of the body by lateral projections called auricles or by eye spots. Flatworms are typically predators or scavengers and use an eversible pharynx for capturing prey. The pharynx is typically on the mid-ventral surface but can be positioned anywhere along the midline. The turbellarians lack suckers or hooks and are usually free living. While turbellarians are capable of swimming to some degree, the epidermis is covered in cilia, which are used to propel the animal along the substratum on a mucous film.
Figure 9. The scolex (left) and a mature proglottid (right) of a typical cestode tapeworm (not to scale).
Figure 10. Details of the monogenean body.
Figure 11. Image of a typical turbellarian flatworm.
These unsegmented worms are characterized by the possession of a proboscis which can be explosively everted to capture food using hydrostatic pressure. The prey can be harpooned by a stylet at the end of the proboscis or if lacking a stylet, the prehensile proboscis can coil around the victim with the aid of a sticky mucous, before being pulled back towards the mouth. The nemerteans are structurally similar to the free-living flatworms, except that they possess the proboscis and a one-way gut, with a mouth and anus. Furthermore, the sexes are separate and fertilization is typically external. These acoelomate worms are elongate, often flattened and typically less than 20 cm long but the largest species, Lineus longissimus, is known to reach 30 m.
These small unsegmented worms (Figure 12) are possibly the most abundant multicellular animals, with densities reaching several million per square metre. Nematodes can be found virtually everywhere, and are either free living or parasitic. In 1914, biologist Nathan Cobb famously wrote "If all the matter in the universe except nematodes were swept away, our world would still be recognizable, its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes." Their body is cylindrical covered in a layered cuticle that requires moulting for growth. Their mouth shows a radial symmetry, with the cephalic structures present in multiples of 3. The mouth has 'lips' with variously developed jaws and sensory structures called amphids and cephalic setae.
Free living nematodes normally very small, often less than 1 mm, but the parasitic forms can reach up to 9 m long. Marine free-living nematodes are very abundant in the meiobenthos where they play an important role in the decomposition process and aid in recycling nutrients.
Figure 12. Males and female nematodes (non-parasitic) and a close up of a nematode mouth (right).
Echiurans (Figure 13) are unsegmented worms, with a large anterior projection called the proboscis or introvert. The proboscis contains the brain, is muscular and highly mobile but is not retractable into the body (trunk). Used to collect food (and also for respiration), the proboscis forms a gutter around the mouth, which is located at its base. Its ventral side is ciliated and uses mucous to bring food to the mouth. Small bumps (papillae), a pair of strong chitinous setae and excretory pores (nephridopores) just posterior to the proboscis and occasionally small setae around the terminal anus are the only external features of the trunk.
Echiurans range in size from several millimetres to over 8 cm. Often found burrowing in soft sediments, members of the genus Urechis (the innkeeper worms) often share their burrows with other organisms such as bivalves, crabs, polychaetes and fish.
Figure 13. External features of an echiuran worm.
These unsegmented marine worms (Figure 14) are similar to the Echiura, as both have an anterior introvert. Unlike the echiurans the sipunculan introvert can be fully retracted into the trunk and the mouth opens at the end of the introvert. Terminal tentacles surround (at least in part) the mouth and are used to capture food. The gut passes posteriorly from the mouth before looping back to a dorsal anus at the base of the introvert. There is a layer of thick cuticle, which is not shed for growth. The sexes are separate and fertilization is external. Sipunculans are sedentary animals often in burrows in soft sediments or inside empty mollusc shells or polychaete tubes.
Figure 14. A generalised sipunculan worm.
Molluscs are soft bodied, bilaterally symmetrical animals and typically with a calcium carbonate-proteinaceous shell (this is absent in some groups however) secreted by the mantle membrane. The body can usually be divided into the head, foot and dorsal visceral mass enclosed by the mantle. In most groups, there is a file-like tongue called the radula inside the mouth. Individuals may be gonochoristic (separate sexes) or hermaphroditic and fertilization is either internal or external. The molluscs form an extremely diverse assemblage, from the familiar garden snails, scallops and octopus to the lesser known tusk shells, sea butterflies and shipworms.
The Aplacophora (Figure 15) are an aberrant group of molluscs which lack a shell and the foot when present is vestigial. The worm-like body has numerous calcareous scales or spines embedded in the skin. There are two groups of Aplacophora the Neomeniomorpha and the Chaetodermomorpha. In both groups the margins of the mantle membrane have rolled inward ventrally, but in the former this has created a mid-ventral groove within which resides the thin ridge of the vestigial foot. The groove and foot is absent in the Chaetodermomorpha. At the posterior end is a chamber on which the anus empties, this is thought to represent the mantle cavity and in the Chaetodermomorpha it houses a pair of gills. Little is known of the biology of this typically deep water group. Whether or not the Aplacophora represent a primitive or specialist molluscan lineage is uncertain.
Externally the monoplacophorans (Figure 16) resemble limpets with their single conical shell. The shell's apex is directed forward. They can be separated from the gastropods by the presence of five or six pairs of gills and three to eight pairs of shell retractor muscles (gastropods only have one pair of each). The serial repetition of these organs suggests a link to the chitons (Polyplacophora). The head bears a pair of post oral tentacles. Monoplacophorans are not large molluscs only reaching 3 cm in length.
Not much is known about this typically deep water group and until 1952 the group was only known from Cambrian and Devonian fossils.
Chitons (Figure 17) are similar to gastropods but the obvious difference is that the shell is composed of 8 overlapping plates. These are linearly aligned along the animal's back, and the lateral margins are embedded in the mantle, giving the animal an oval outline. The mantle edge (girdle) is thickened and often armoured with scales, or spines. The head is reduced and lacks tentacles and eyes. The radula is file-like and used for scraping surfaces as in the gastropods. The sexes are separate and fertilization is external or within the mantle cavity, brooding is common but viviparity has been recorded for only one species.
In the gastropods (Figure 18) the head is usually well developed and bears various numbers of cephalic tentacles, each bearing an 'eye'. Gastropods have one shell that is normally external and developed into a protective retreat. There is a single pair of shell retractor muscles, gills and kidneys (nephridia). The foot is typically large and used for locomotion-creeping or swimming-and adhering to the substratum. The typically file-like radula is well developed and can be used to scrape surfaces, bore holes or harpoon food.
In certain groups such as the Opisthobranchs the gastropod shell is reduced or absent allowing some species to swim. These include the planktonic pteropods (sea butterflies and sea angels). Pteropods have very thin shells and swim by flapping their enlarged bilobed foot. They are an abundant part of plankton communities and the shells of dead sea butterflies are common components in the pelagic sediments of offshore sea beds.
Bivalve bodies (Figure 19) are laterally compressed and encased in two calcareous-proteinaceous valves. The head is vestigial lacking eyes but bearing labial palps. The molluscan foot is present and may be used in locomotion or burrowing but is often reduced in sessile species. Bivalves are normally filter feeders, drawing water into the shell through a siphon, food is then passed to the mouth by the well-developed ciliated gills (ctenidia). Bivalves can be either gonochoristic or hermaphroditic and fertilization of embryos is external and typically via free spawning in the water column.
Bivalves are mostly burrowers but there are a significant number that adhere to hard substrata. Sessile species attach to the substratum either by cementing the shell, as in oysters, or by chitinous byssal threads as in mussels. Burrowers, such as cockles, bury themselves in soft sediments (some can bore into rock and wood) using an enlarged foot. Commonly only the inhalant and exhalant siphons will be visible above the sediment surface.
Superficially scaphopods (Figure 20) resemble gastropods with their prominent shells, however, the slightly curved shell is tubular and opens at both ends. The mantle is also tubular and open at both ends. The head is reduced to a conical projection, or proboscis, bearing the mouth and lacking eyes. The radula is internal and not used directly for food capture. Scaphopods feed on small organisms like Foraminifera which are captured using numerous threadlike tentacles (captacula), each with an adhesive knob at the tip. The tentacles extend into the sand to capture food, which is brought back to the mouth via tentacular cilia. The foot is cone shaped and used in burrowing. There are no gills and gas exchange occurs across the mantle membrane.
Dentalium (a genus of scaphopod) was valued by Native American cultures on the Northeast Pacific and used as wampum-a form of currency.
Cephalopods (Figure 21) appear the least like other molluscs with few features resembling the other groups. Cephalopod heads are very well developed with complex eyes and the cerebral ganglia form a relatively large and discrete brain. The radula is tongue-like inside the beaked mouth encircled with prehensile arms or tentacles (often with suckers) used for feeding, and secondarily for locomotion in some groups. The mantle is tubular and elongate, and utilized for locomotion by pushing water through a tubular funnel (the modified foot), giving the animal jet propulsion.
An obvious trend within the cephalopods is the reduction of the molluscan shell. The shell in the Nautilida (Nautilus) is external, spiralled and internally chambered, the Spirulida (Spirula or ram's horn squid) shell is internal, spiralled and chambered; the Sepiida (cuttlefish) is also internal but straight and has a different internal structure. All of these shell forms are used as a buoyancy device. In the Teuthiida (true squid) the shell has been reduced to a chitinous 'pen', to provide structural support to the mantle while the Sepiolida (bobtail squid) have no shell at all.
For the Octopoda (octopus), only members of the genus Argonauta (argonauts or paper nautilus) have a shell, which is secreted only by the females from special glands on their tentacles. It is used as a buoyancy device but also as a brood chamber for the eggs, playing a vital role in reproduction.
Figure 15. A neomeniomorphan Aplacophora.
Figure 16. Ventral and dorsal (lower right) views of a monoplacophoran.
Figure 17. Dorsal (left) and ventral (right) views of a polyplacophoran.
Figure 18. Ventral view of a limpet showing major gastropod features. Note the foot has been cutaway to reveal position of the gill. Inset an example of an unshelled (above) and shelled gastropod (below).
Figure 19. The internal features of a bivalve clam (left valve removed).
Figure 20. Scaphopoda, with cutaway shell showing internal anatomy.
Figure 21. Posterior view of a common cephalopod (Loligo).
The important feature of the annelid worms is their segmented (metameric) bodies. Each segment (metamere) is separated by transverse septa. These septa help maintain the body structure by regulating hydrostatic pressure, creating a hydroskeleton. Annelid bodies can be divided into the prostomium, trunk and pygidium. The prostomium bears sensory organs and is the first segment, making up the anterior wall of the mouth, while the terminal pygidium bears the anus. The nerve cord is ventral, while the brain and heart are dorsal.
The prominent feature of the Clitellata, which contains the leeches and earthworms, is the presence of a clitellum. The clitellum is a reproductive structure which secretes a cocoon in which eggs are deposited.
Leeches (Figure 22) are characterized by having a sucker at both ends (oral and anal) of their elongate body. The oral sucker encloses the mouth armed with chitinous jaws. The body has a fixed number of relatively few (34) segments but may appear to have many more owing to the annulated body which give the leech the ability to stretch. The segments lack parapodia and most species lack the chitinous chaetae of the other members of the annelids. Posterior to the gonopores is the small reproductive structure called the clitellum only prominent during the reproductive season, appearing as a band of swollen segments. Unlike other annelids the leeches are unable to regenerate lost segments. Leeches are hermaphroditic, fertilization is internal and the eggs are deposited within a cocoon secreted by the clitellum.
The stereotype of the blood-sucking leech is a bit misleading, terrestrial and freshwater species are more likely to be active predators than parasites. The reverse is true for the marine species where most species are ectoparasites on fish.
The most readily recognised members of the Oligochaeta (Figure 23) are probably the earthworms. Oligochaetes have an overall simple appearance as the prostomium lacks sensory appendages and there are no appendages along the body. There are four bundles of chaetae per trunk segment but these are usually very small in terrestrial forms. Oligochaetes are hermaphroditic, fertilization is internal and reciprocal (i.e. copulation results in the fertilization of both individuals) and eggs are deposited within a cocoon which is secreted by the clitellum. The clitellum is a reproductive structure formed by a girdle of swollen segments and is posterior to the gonopores and is most notable in earthworms.
Polychaetes (Figure 24) are common members of any marine habitat. As their name suggests the chitinous chaetae are numerous and prominent on fleshy appendages (parapodia) typically used for traction in locomotion. Typically the parapodia are present on every trunk segment but are variously reduced in burrowing and tube dwelling forms. The prostomium is well developed with numerous sensory and feeding appendages; including eyes, tentacles and jaws. Polychaetes can be gonochoristic or hermaphroditic. Fertilization is typically through free spawning, though some species may brood their eggs. The cocoon secreting clitellum of the other annelid groups is absent.
Many benthic errant polychaetes use a reproductive strategy known as epitoky where an individual swims to the surface to form large spawning aggregations. Spawning results in the death of the epitoke. The epitoke transformation involves the modification of the parapodia into swimming appendages, enhancement of sensory appendages, and degeneration of the digestive system, preventing feeding and allowing more space for the gonads to develop inside the body. The transformation is either total (epigamous) where the entire individual becomes the epitoke, or partial, whereby only the posterior segments are modified and separate from the 'parent' to swim to the surface as the epitoke (schizogamous).
Figure 22. A typical hirudinean leech.
Figure 23. The anterior segments of a typical oligochaete.
Figure 24. Dorsal view of head and first anterior segments of an errant polychaete.
As the most speciose group of organisms the arthropods are typical members of every ecosystem. In arthropods the external cuticle is strengthened and thickened into a chitinous (or calcareous) exoskeleton, giving not only support and protection to the internal organs but used for locomotion and feeding as well. The exoskeleton is articulated and metameric segmentation is evident throughout the arthropods but is reduced to varying degrees, particularly in the Acarina (mites). The heart and brain are dorsal with a ventral nerve cord with ganglia in each segment.
Chelicerates (Figure 25) are a diverse group, the most familiar of which are the Arachnida which contains the spiders, mites and scorpions. The general chelicerate body is composed of the cephalothorax and abdomen. There are no antennae, instead the cephalothorax bears the eyes, chelicerae (e.g. the fangs on spiders), palps (used for handling food) and normally 4 pairs of walking legs. Gas exchange occurs via organs in the abdomen including book-gills e.g. horseshoe crabs (Merostomata), trachea and/or book-lungs (similar to book-gills) in arachnids or through the gut e.g. Pycnogonids.
The Pycnogonida, or sea spiders, are the most common marine chelicerate group and also the most unusual group. The first body segment bears a diverse array of appendages. There are up to 4 limbs attached to the first body segments: the chelicera (chelifore - chelate limbs), palps, ovigers (palp-like appendages which the males use to carry eggs unique in the chelicerata) and the first pair of walking legs as well as an anteriorly directed proboscis that bears the mouth. The former 3 limbs are variably pronounced, reduced or even absent depending on the group. Lastly the tubercle, which is a small projection on the dorsal surface, bears four eyes. There can be three to five more segments each bearing a pair of walking legs followed by the abdomen, which is very small and only bears the anus. Sea spiders are found globally. While shallow water species are typically small and very easily overlooked, deep sea and Antarctic species can be very large with leg spans reaching 70 cm.
From barnacles, ostracods and copepods to crabs and shrimps crustaceans (Figure 26) are probably the most morphologically diverse group of invertebrates. The head, however, is relatively uniform throughout the Crustacea with five pairs of appendages - antennules, antennae, mandibles, first and second maxilla. The two pairs of antennae are unique among the Arthropoda. The trunk is highly variable within the group but often divided into thorax and abdomen, showing varying degrees of regional specialisation. Typically all limbs are biramous (sometimes polyramous or secondarily uniramous).
This diverse group of crustaceans contains the abundant barnacles and copepods as well as some very obscure parasitic groups - tantulocarids, sea lice (Branchiura) and the tongue worms (Pentastomida). They are characterized in having 6 thoracic segments and a reduced abdomen with no appendages and not more than 4 segments. In the barnacles the abdomen is present only during the cyprid larval stage.
These small crustaceans are most easily recognised by the bivalved carapace which completely encloses the body. Found in both fresh and salt water, ostracods are typically found burrowing in soft sediments during the day, emerging to feed at night.
Figure 25. Diversity of the Chelicerata. A) Pycnogonida (sea spiders); B) Arachnida (spiders, mites and scorpions); C) Merostomata (horseshoe crabs). Centre; close up of the pycnogonid body.
Malacostraca is the most speciose crustacean group and contains the most familiar crustaceans: the crabs, shrimps and lobsters. The main differences between malacostracans and the previous groups is that the abdomen is well developed, typically with limbs on each segment called pleopods. Also the free living naupliar stage is normally absent but is still present during embryo development. There are eight thoracic limbs; the first pairs may be modified into mouthparts. There are seven common orders of Malacostraca in the UK.
The mantis shrimps are highly specialised predators with well developed stalked compound eyes but the most notable feature is the preying mantis-like second thoracic limb which has rows of long sharp spines used to grab or is club-like and blunt to bash prey. There is a small carapace covering the first five thoracic segments, the abdomen is large ending in a large shield-like telson with well developed uropods.
The cumaceans, sometimes referred to as comma shrimp owing to their curved appearance, are found burrowing in sand and mud. The head and thorax is greatly enlarged relative to the slender abdomen and encased in a bulbous carapace. Cumaceans brood eggs in a marsupium formed by special plates on certain thoracic limbs called oostegites. This feature links them with the Isopoda, Amphipoda, Mysidacea and Tanaidacea inside the group known as the Peracarida.
Tanaids are generally less than 1 cm in length and can be found living in tubes, burrows or in crevices. Tanaids like cumaceans possess a marsupium for brooding eggs. Generally slender they were once classified within the Isopoda but the significant differences include a small carapace covering the head and first two thoracic segments, and the first thoracic limbs are chelate.
The opossum shrimp, so-called because of the marsupium, closely resemble the true shrimp or krill in shape and habits. They possess a well developed carapace which covers the entire thorax but is not fused with the last four segments. Unlike the other peracarids the mysids possess stalked eyes.
The typical isopod is dorsoventrally flattened, lacks a carapace and the thorax and abdomen are not necessarily well defined. They generally have seven pairs of walking legs and five pairs of pleopods which are used in respiration. This body plan is highly modified for groups with highly specialised life histories, especially the parasitic forms. Isopods are probably the most familiar of the peracarids and contain the largest group of terrestrial crustaceans-the wood lice or pill bugs.
Amphipods, unlike the isopods, are typically laterally compressed and the abdomen is divided into two 3 segmented sections known as the pleon and urosome. The anterior pleon possesses the pleopods used for swimming and the posterior urosome bears pairs of uropods which are directed backwards. The first two pairs of walking legs are usually well developed and enlarged, and referred to as the gnathopods. Amphipods are common in marine habitats and some groups are terrestrial found under rocks and logs. They can be benthic or pelagic, and some, such as the whale louse, are parasites.
The major feature of the decapods as the name suggests is that they have ten walking legs, with the first pair typically chelate. The carapace is well developed and fused to all thoracic segments and the compound eyes are on moveable stalks. Unlike the peracarids, eggs are not brooded within a marsupium on the thorax but instead held by the pleopods or swimmerets on the abdomen or less commonly shed into the water column.
Krill play a vital role in ocean ecosystems, most notably as a food source for large planktivores like whales. Krill are similar to the decapods in that the eggs are carried on the abdomen rather than the thorax but unlike the decapods the krill carapace does not enclose the gills. Thoracic limbs are biramous and of similar shape as none are specialised into mouthparts or pincers.
Despite the enormous numbers of known species, the hexapods (Figure 27) are remarkably uniform. Hexapods can be divided into three main body sections: the head, thorax and abdomen. The head bears a single pair of antennae, a pair of compound eyes, (usually) 3 medial ocelli, paired mandibles, maxillae and the labium (the fused second maxillae). The thoracic segments bear three pairs of walking legs and usually two pairs of dorsal wings (absent in the bristletails and silverfish, while secondarily lost in some groups such as the siphonaptera or fleas). The abdomen lacks appendages except the final segment which has sensory appendages called cerci and the female reproductive structure, the ovipositor. Gas exchange occurs within trachea but gills are present in freshwater larval forms. The vast majority of insects are terrestrial but some inhabit intertidal habitats.
The myriapod (Figure 28) body is composed of the head, elongate trunk and the terminal segment called the telson. The head bears a single pair of antennae, mandibles, first and second maxillae (second maxilla absent in the millipedes); and a pair of ocelli (compound eyes found only in a few centipedes). The trunk is made up of many segments each usually bearing one or two pairs of walking legs. The resulting large number of legs is the primary characteristic of the group from which the name Myriapoda is derived (myrias = ten thousand, podos = foot). While none actually have ten thousand legs, the millipede Illacme penipes, is the closest with over 750 legs recorded.
The centipedes are scavengers or predators reaching lengths of up to 30 cm. The first pair of trunk limbs called the forcipule or poison claw is robust and directed toward the mouth for capturing prey. In some species the poison is lethal to humans. There are another 15 or more segments which follow, each bearing a pair of legs, the last pair of which is often modified for defensive or sensory purposes.
The millipedes lack the forcipules of centipedes and also have two pairs of legs per segment rather than one. This is through the fusion of two segments rather than the growth of an extra leg. The first trunk segment is legless and forms a collar behind the head; the next three segments bear only one pair of legs. When threatened millipedes will characteristically roll into a ball. Each segment of a millipede typically has a pair of poison glands that secrete an irritating, or even deadly, liquid. The liquid which usually contains hydrogen cyanide is secreted onto the legs and at least one species is capable of spraying the liquid at potential threats.
Figure 26. Examples of crustacean diversity. A, Brachyura (crabs); B, Isopoda (slaters and sea lice); C, Stomatopoda (mantis shrimps); D, Branchiopoda (brine shrimp, sea monkeys); E, Cirripedia (barnacles); F, Copepoda (copepods).
Figure 27. Detail of the hexapod body plan.
Figure 28. An example of a myriapod, a centipede (Chilopoda).
The horseshoe worms live in chitinous tubes that are either attached to the substratum or buried in the sand in shallow waters. These unsegmented worms lack any external features on the trunk except for the relatively large horseshoe shaped lophophore-a band of ciliated tentacles-which surrounds the mouth used for filter feeding.
These sessile animals form erect or encrusting colonies comprised of individuals known as zooids (Figure 29). Colonies are formed by asexual budding around the ancestrula (the metamorphosed larva). Zooids may be differentiated into specialised zooids to perform specific functions such as feeding (autozooids) and reproduction (gonozooids). The zooid wall is usually chitinous and often calcareous and in some species this forms a very rigid exoskeleton. Colonies are hermaphroditic but this is not necessarily so for individual zooids. Fertilization is internal and the eggs are generally brooded while the larvae are planktonic. The animals filter feed using a lophophore-a ring of ciliated tentacles surrounding the mouth which can be retracted into the orifice. The gut is U-shaped and the anus opens outside of the lophophore. The lophophore is a feature shared with the lamp shells (Brachiopoda) and the horseshoe worms (Phoronida).
Two types of highly modified defensive zooid may be present, the avicularia and vibracula. These zooids have a much reduced internal structure and defend the colonies in two very different ways. The avicularia have a strengthened operculum which acts as a jaw that can clamp on to potential threats. Little is known what then happens to the trapped animal, but it is doubtful that they are eaten by the bryozoan. The opercula of the vibracula are modified into a long bristle, or seta, which is used to sweep away detritus and the settling larvae of other species to prevent siltation or fouling.
Figure 29 Detail of an individual zooid of a bryozoan, inset left several zooids of the bryozoan Bugula.
Brachiopods (Figure 30) are sessile animals enclosed in a bivalved calcareous shell. They are commonly attached to the substratum by a fleshy pedicle originating internally from the foramen in the larger ventral valve but a few exceptions are cemented directly to the substratum. Externally the group are similar to the bivalve molluscs, however, brachiopods lack such molluscan features as the foot and filter feed using a ciliated tentacular structure known as the lophophore similar to those of the bryozoans and phoronids. With few exceptions brachiopods are gonochorisitc (separate sexes) and fertilization is external. The extant species are a small fraction of the ~20,000 fossil species that date back to the Cambrian. Today the majority of species are found in deep water.
Figure 30. A typical articulate brachiopod in cross-section.
The Chaetognatha (Figure 31) are fish or arrow shaped, with distinctive caudal and lateral fins. On the ventral side of the head a large chamber (vestibule) leads to the mouth, either side of which are long bristles or spines forming the jaws used to capture prey. They swim by undulating their bodies and use the lateral fins for stability. Individuals are hermaphroditic with ovaries produced in the trunk section and testes in the post-anal caudal section of the body. Fertilization is internal via passing spermatophores between individuals and the eggs are released into the water column directly developing into juveniles. Reaching lengths of around 10 cm, chaetognaths are often a very conspicuous component of the zooplankton, although, 20% of species are benthic. Chaetognaths are predatory, feeding on any small planktonic animal. In turn the chaetognaths make up an important component to the diet of many fish.
Figure 31. Ventral view of a chaetognath.
These worm-like organisms possess a dorsal nerve cord and pharyngeal slits, allying them with the chordates. The body (Figure 32) can be divided into three parts: the proboscis, collar and trunk. The proboscis is typically short and conical and used in feeding by secreting a mucous to capture food and passing it back to the mouth as well as burrowing through peristaltic movements. The collar connects the proboscis to the trunk and bears the mouth. Pharyngeal (gill) slits are found on the trunk just posterior to the collar.
The hemichordates are composed of two major classes the Enteropneusta and the Pterobranchia. The enteropneusts are by far the more abundant and well known group and are commonly called acorn worms. These sedentary animals burrow in sandy habitats and can be recognised by the coiled faecal casts at the burrow entrance. The pterobranchs are a very obscure and rare group found mainly in deep waters and consisting of around 15 species. They live in secreted tubes and can reproduce by asexual budding forming dense aggregates or even interconnected colonies. The proboscis is shield shaped and is responsible for secreting the tube as well as for locomotion within the tube. The collar has a variable number of 'arms' with numerous small tentacles projecting from the dorsal side which are used to capture food.
Figure 32. An enteropneust (left) and a pterobranch (right) hemichordate and an example of a pterobranch colony (bottom right)
Commonly echinoderms are conspicuously spiny, especially the Echinoidea, hence the scientific name from the Greek echinos meaning spiny and derma meaning skin. Unique to the echinoderms is their water vascular system. This series of canals employs hydrostatic pressure and ciliated cells to direct water to the podia or tube feet which are used in locomotion or feeding. Typically the system is open to the surrounding waters via the madreporite which is connected to the ring canal. Another feature of the echinoderm body is that it is radially symmetrical in five planes (pentaradial) radiating from the mouth. The internal skeleton is comprised of calcified plates called ossicles which in most echinoderms form a protective shell around the internal organs.
There are 2 main groups of crinoid (Figure 33) based on whether the calyx (equivalent to the central disc) is in contact with the surface via a flexible stalk (Isocrinida or feather stars) made up of calcareous discs (columnals) or via prehensile cirri (Commatulida or sea lilies) which enable the animal to walk or anchor. The mouth and anus are directed upward from within the membranous tegmen, which is supported by the cup-shaped calyx. The arms extend from calyx in multiples of 5, each with 2 rows of tubular pinnules along their length. The ambulacral grooves are flanked by tube feet (in triplets) near the base of the pinnules. The tube feet excrete mucous to trap food in the water column and then pass items down to the mouth.
The Isocrinida are typically found in deep water, and the Commatulida are more common in shallow water. Both groups are capable of locomotion, for instance the isocrinids use the arms to lift themselves from the substratum and crawl. As mentioned above the Commatulida are capable of walking using the cirri but also swimming short distances using powerful downward strokes of the arms.
This class contains two subclasses, the Ophiuroidea and Asteroidea, both of which bear arms or rays that extend from a central disc region. This often leads to difficulty in distinguishing the groups by the casual observer. The two classes are, however, strikingly different in a number of features in both the adults and larval stages.
Ophiuroids (Figure 34) are similar to the asteroids in appearance but with some very important differences. The central disc is very well defined and always has 5 arms, although the arms may be extensively branched, as in the basket stars (gorgonocephala). The arms are very flexible (and very fragile) and are used to push and pull the animal along as well as capturing food. The arms (Figure 36) do not have an ambulacral groove and the tube feet lack suckers and ampullae, playing no part in locomotion but are instead used to pass food to the mouth. Internally the arms do not house the gonads or gut, but are mostly taken up by muscles and vertebral ossicles, giving the arms their flexibility. The five madreporites are on the oral shields and connect to the ring canal.
The central disc of the Asteroidea (Figure 35) is not as well differentiated from the arms as in the ophiuroids. The number of arms varies from family to family and sometime within species, the minimum is 5, although there can be exceptions, and a maximum of 40. The oral surface of the arms bear the ambulacral groove, from which 2 or 4 rows of tube feet extend. The tube feet have terminal suckers and are used for locomotion. The tube feet 'walk', giving the appearance of the animal gliding across the substratum. The arms (Figure 36) also contain the gonads and pyloric caeca (intestine). In asteroids, the ossicles form a semi-rigid test, restricting the animal's flexibility. Forceps-like stalked pedicellaria, which are used in defence, prevention of fouling, and in some cases food capture, are scattered around the aboral (dorsal) surface. There is a single, typically conspicuous, madreporite found on the aboral.
Echinoid (Figure 37) bodies are typically semi-spherical (regular) or disc shaped (irregular), with a rigid test, lacking arms. However, five pairs of ambulacral plates, from which the tube feet extend, and five interambulacral plates radiate from around the mouth and meet near the anus. Tube feet in echinoids are much the same as in the asteroids and are used for locomotion and adhering to the substratum as well as feeding. There are numerous mobile spines, distributed all over the test, between which there are various types of defensive pedicellariae, most notably the toxin producing globiferous pedicellaria. The madreporite, anus and gonopores are placed on the aboral (dorsal) surface around the periproct. While the mouth central on the oral (ventral) surface, within the peristomial membrane.
Holothurians typically appear elongate and bilaterally symmetrical but the pentaradial symmetry is still present in the five longitudinal muscles, five pairs of tentacles and corresponding five paired rows of tube feet (absent in some families) that radiate longitudinally from the mouth. The first pairs of tube feet which encircle the mouth are heavily modified to form 10 branched tentacles used for capturing food. The ossicles comprising the skeleton are microscopic and do not form a rigid test giving holothurians the appearance of being soft bodied. The madreporite is internal and the water vascular system utilizes coelomic fluid instead of seawater.
Figure 33. Examples of the two typical types of Crinoidea-the sea lilies, Isocrinida (left), and feather stars, Commatulida (right).
Figure 34. The oral surface of an ophiuroid.
Figure 35. Features of the aboral (left) and oral (right) surfaces of a typical asteroid.
Figure 36. Comparison of the structure of ophiuroid (left) and asteroid (right) arms in cross section (not to scale). Note the lack of organs inside the arm of the ophiuroid.
Figure 37. Oral (left) and aboral (right) views of a regular echinoid.
The chordata contains the most familiar of all animals - humans! As well as the vertebrate groups like the reptiles, fish and birds, there are also the invertebrate members, namely the tunicates and cephalochordates. The key features linking the chordates are the presence (at some point in the life cycle) of a notochord, a stiff rod of cells similar to cartilage used to support the body during locomotion, pharyngeal slits, which form the basis of gills, endostyle, a post anal tail, a dorsal nerve cord, a ventral heart and ventral blood vessel. Along the pharynx are the pharyngeal slits that are used in feeding and respiration. In most non-chordate groups the cerebral ganglia (brain) connects to the main nerve cord that runs along the ventral side of the body, while the dorsal heart connects to a dorsal blood vessel. The endostyle is a mucous secreting groove in the pharynx that stores iodine, it is believed to be the precursor to the thyroid gland.
The tunicates resemble common vertebrates the least, except as larvae when they vaguely resemble tadpoles. During larval development they have a dorsal notochord, nerve cord and a ventral heart but these are lost during development. The adults are sessile or pelagic, solitary or colonial. Typically sack-like in appearance, tunicates get their name from the test or 'tunic' secreted by the epidermis which envelops the internal organs. Water is drawn into the oral siphon to the large basket-like pharynx that is suspended inside the atrium formed by the tunic. Food is sieved through slits or stigmata in the pharynx and trapped in the mucous secreted by the endostyle, before being taken into the stomach for digestion and the water is expelled through the atrial siphon.
The ascidians (Figure 38) known as sea-squirts are sessile and either colonial or solitary. The siphons are directed away from the substratum, often quite close together. The animal is protected by a tough often leathery tunic.
The thaliaceans or salps are pelagic, gelatinous and barrel shaped. The siphons are at opposite ends of the body and circular bands of muscle to contract the test to expel water out through the atrial siphon for locomotion. Salps can be found solitary or in large chain-like colonies.
Adult larvaceans have retained many of the larval features of the group, including the notochord, dorsal nerve cord and tail. The larvaceans secrete a test-like mucous 'house' used for protection and food capture.
Figure 38. Typical tunicate of the class Ascidiacea (sea squirts).
Figure 39. Internal anatomy of a typical cephalochordate.
The lancelets (Figure 39) loosely resemble fish. They are generally less than 10 cm in length, laterally compressed and somewhat tapered at each end. In lancelets the notochord is present in the adult and acts as a back bone does in vertebrates, to which the paired muscles are attached. They are typically found buried in soft sediments with their mouths above the surface to filter food. Food is filtered out of the water column through the gill (pharyngeal) slits and trapped in mucous produced by the endostyle before being passed to the gut. Filtered water passes into the atrium and expelled from the body via the atriopore.
For vertebrates many of the chordate features are only present during embryonic development. The notochord is variably replaced by cartilage and/or the vertebrae, the pharyngeal slits, while present in all vertebrate embryos, are present only in adults of fish. The endostyle is only present in larval lampreys and metamorphoses into the thyroid in adults. The vertebrates are linked by the presence of an articulated skeleton, typically consisting of a brain case and backbone or spinal column. Unlike the skeletons of other taxa, which are acellular, the vertebrate skeleton is actually living tissue that carries out complex biological functions such as producing blood cells.
Lampreys, outwardly resemble eels, and are commonly parasitic on other fish as adults and living in freshwater sediments as larvae. The major traits which separate lampreys from the other fish groups are lacking an articulated jaw and by retaining the notochord throughout life. They are the sister group to the hagfish (Myxini) and together form the group of jawless fishes known as the Agnatha. Instead of having an articulated jaw, the lampreys of a circular oral disc or sucker surrounding the mouth. Lampreys have well developed sense organs including eyes and lateral lines. Behind the head are a series of gill slits that allow water to pass through from the mouth. The skeleton of lampreys is cartilaginous, creating a skull and a rudimentary spinal column of protovertebrae.
Hagfish are the other living members of the Agnatha (other groups such as the ostracoderms are extinct). They are similar to lampreys in retaining the notochord, cartilaginous skeleton and no jaw. They differ from lampreys in lacking the cartilaginous protovertebrae and the oral sucker. Instead they possess three pairs of barbells or tentacles which surround the mouth, the pharynx is eversible and possesses teeth for capturing prey. Hagfish are typically benthic scavengers or predators.
Like the previous agnathan groups, the chimaeras are considered to be primitive and have a cartilaginous skeleton, the major difference being the presence of an articulated lower jaw formed from a modified gill arch. This character forms the basis of the Gnathostomata supergroup which comprises the majority of chordates. In adult chimaera the notochord is retained intact but strengthened by cartilaginous vertebral processes. Another significant feature over previous groups is the paired pelvic and pectoral fins, greatly increasing manoeuvrability. The lateral line canals are open in chimaeras and run the length of the body to the long whip-like tail. The gills are protected by an opercular flap. In males there are two pairs of pelvic claspers and one clasper on the head. Internally, the upper jaw is fused to the skull and the gut is a simple spiralled tube, lacking a stomach.
Superficially similar to the holocephalans except that the upper jaw articulates with rather than being fused to the skull, the gut has a stomach and the notochord is integrated into the spinal column but is still present between cartilaginous vertebrae. Also the gills are not covered by an operculum and 5-7 gill slits are present behind the head. While the skeleton is cartilaginous there is true bone present in the bases of the teeth and scales. This remnant is what remains of the dermal armour present in prehistoric forms.
The skeleton is formed of bone, although cartilage still is a major component of the skeleton. While bone was present in prehistoric non-bony fish, this was in the form of dermal bone. The bony fish have an operculum covering the gills which are rhythmically pumped for respiration, and the fins are strengthened and articulated by dermal rays. Unique to the bony fish is the swim bladder (considered to have evolved from the lung of lungfish), this organ is usually filled with air and is used to maintain buoyancy in pelagic forms (it is absent in the benthic flatfish however).
With the evolution of terrestrial life several major adaptations were needed to face the new pressures of life on land. The skin and scales thickened to prevent water loss, the skeleton was significantly hardened and expanded to replace the support given by water. Reptiles as the first fully terrestrial group lost the aquatic larval stage in favour of laying larger eggs encased in a shell and some are even viviparous. This, coupled with the absence of gills in the adult form (gills are present during embryonic development) is considered the fundamental difference between reptiles and amphibians.
The reptiles are a diverse group, and the three living groups, Chelonia (turtles), Lepidosauria (snakes and lizards) and Crocodilia (crocodiles) are not closely related to each other. The birds are believed to have evolved from reptiles.
Birds are easily distinguished from other vertebrate groups by the presence of feathers, a horny, beaked mouth without teeth and forelimbs modified for flight. Some less obvious features of birds include a four chambered heart, nucleated red blood cells, hollow bones and warm bloodedness. A great deal of avian features are modifications that enable flight. The hollow bones and the lack of teeth are efforts at weight reduction, while the warm bloodedness allows birds to maintain muscular activity and the feathers are for aerodynamics. The birds are a relatively compact group and in many respects resemble reptiles, and are considered by many to be simply reptiles with feathers having descended from the dinosaurs.
The class of animals that humans belong to is characterized by the presence of mammary glands, which secrete milk used to suckle young (a feature found nowhere else in the animal kingdom), true hair, the temporal fenestra in the skull, cusped teeth, a four chambered heart and warm bloodedness. Another typical feature is viviparity, the birth of live young. While this feature has repeatedly evolved in other vertebrate and invertebrate groups; it is most widespread in mammals. The exception is in the half dozen or so species of the monotremata (platypus and echidna) that still lay eggs that are carried by the adult in a small pouch.
The seaweeds are a loose assemblage of plant like organisms, also known as the macroalgae. Traditionally considered to have belonged to the kingdom Plantae they now span both the Plantae and the kingdom Chromista. Distinguishing between the algal groups can be difficult as they lack gross morphological features. Owing to the high proportion of algal groups containing unicellular species, the higher level taxonomy is in large part based on cellular structures and biochemical properties. The three main groups of seaweeds-green, red and brown-can be roughly separated by their colours caused by the type of pigments they use for photosynthesis, although this can be unreliable as the colour varies and for example some red seaweeds can appear green or even brown.
Members of the kingdom Plantae are eukaryotic organisms defined as having cells surrounded by a rigid cell wall, membrane bound cellular organelles and DNA stored in chromosomes. Plants are autotrophic and produce their food from solar energy via photosynthesis in the chloroplasts, which in the plants is surrounded by two membrane layers. Current evolutionary theory states that the chloroplasts, which contain their own DNA, were photosynthetic prokaryotes which were ingested or symbiotic with the eukaryote cell.
The chlorophytes mostly live in freshwater with only 10% of species living in marine habitats but they are found globally. Many species of green algae are unicellular but the common marine species are typically multicellular and membranous or tubular in structure.
Chlorophytes get their green colour from their primary pigments chlorophyll α and β used in photosynthesis. Starch is used as an energy storage product and is formed inside the chloroplast rather than in the cytoplasm like other algal groups. The chlorophyte cell wall when present is normally made from cellulose and flagella when present are typically in pairs and placed apically.
In multicellular forms the typical lifecycle (Figure 40) involves alternating between a gametophyte phase and a sporophyte phase. The male and female form from the spores of the sporophyte. Gametes are produced by the gametophyte inside the gametangia. Fusion of the gametes results in the development of the sporophyte. The sporophyte and gametophyte are may be identical morphologically (isomorphic) or obviously different (heteromorphic) depending on the species.
Figure 40. The lifecycle of the green seaweed Ulva sp.
The Ulvophyceae is the main group of marine green seaweeds. The group is characterized by a cruciate microtubular-root system that is not associated with a multilayered structure, a persistent interzonal spindle that does not collapse at telophase, the motile cells lack cell walls, have a single apical flagella and near radial external symmetry. Macroscopically, the green seaweeds will typically form sheets or tubes one or two cell layers thick such the common sea lettuce (Ulva lactuca.) and gut weed (Ulva intestinalis). Others such as the spaghetti weed (Chaetomorpha linum) and Cladophora rupestris form filaments of linearly arranged cells.
The Bryopsidophyceae differ from the Ulvophyceae by the multinucleate cells without cross-walls, a condition termed coenocytic or siphonaceous. The usually large thallus resembles a garden hose without cross-walls except during reproduction. Two accessory pigments siphoxanthin and siphonein are usually present and cellulose is not usually a component of cell walls having been substituted for xylan or mannan. Species of the genus Codium are probably the most common members of this group in the British Isles and can be recognised by the large fleshy thallus and velvet-like surface. Caulerpa is another distinctive genus, the thallus has a creeping green rhizome and erect shoots with a frond like appearance.
Charophytes are predominantly a freshwater group, with a few species that are found in brackish waters. The motile cells have two apical or subapical flagella and almost always lack eyespots, the microtubular root system is associated with a multilayered structure, there are no rhizoplasts and cell division occurs via a phragmoplast. A few of the orders contain unicellular species closely related to the other green algae but the multicellular stoneworts are relatively complex and share features with the terrestrial plants such as cell differentiation and apical growth. As a result the charophytes have been classified within the Chlorophyta, Bryophyta or on their own as the Charophyta. The true plants are believed to have evolved from the charophytes.
The rhodophytes are most abundant in temperate and tropical seas but are rare in polar and sub-polar regions where the green and brown algae dominate. Red algae are able to live at greater depths than other seaweeds, up to 270 m, due to the type of accessory pigments they possess. Most rhodophytes are multicellular and come in a variety of forms, from filamentous, fleshy to calcareous species such as those that form rhodoliths or maerl.
Rhodophytes are characterized by the utilization of chlorophyll α for photosynthesis, however, the presence of phycobilins, an accessory pigment, gives them their typical red colour. The colour is not consistent ranging from green to brown to purple and can vary considerably within a species. For example, Irish moss (Chondrus crispus) will turn from red to green in strong sunlight. Rhodophytes use floridean starch as an energy storage product which is formed in the cytoplasm of the cell. Rhodophyte cells are not flagellated and surrounded by a cell wall made of cellulose.
The life cycle of red seaweeds (Figure 41) is complicated and involves the alternation of three phases: the gametophyte, carposporophyte and the tetrasporophyte. The gametophyte and tetrasporophyte are the dominant phases, and usually identical (isomorphic), while the carposporophyte is microscopic and is attached to the female gametophyte. The tetrasporophyte produces tetraspores which germinate into the male and female gametophytes. The male gametophytes produce non-flagellated sperm which fertilize the egg produced inside the carpogonium on female gametophytes. This then becomes the carposporophyte, which release carpospores that germinate into the tetrasporophyte.
Figure 41. The lifecylce of a red seaweed.
The Chromista range from giant, multicellular kelps to unicellular diatoms. The group has chlorophyll α and c but get the typical brown or yellow colour from their accessory pigment, fucoxanthin. Forms of laminarin (a glucose polymer) are used for food storage and stored in vesicles in the cytoplasm. They share many features with members of the kingdom Plantae but the cell wall is composed of two layers of cellulose. The chloroplasts are surrounded by four membranes and are believed to be the result from ingesting or being symbiotic with a green or red alga.
There are no unicellular or colonial brown seaweeds and virtually all are found in marine habitats particularly the intertidal. Their cells contain large amounts of fucoxanthin but may also contain tannins which contribute to their brown colouration. The gametes are motile and have an anterior and posterior flagellum with an eyespot located at the base of the posterior flagellum. The brown algae are most diverse in temperate and polar regions and are quite often the dominant organisms, forming extensive habitats and providing food for other organisms. A curious feature of some brown seaweeds are the presence of air filled cavities, called air bladders, in the fronds, used in keeping the thallus erect. These are common in the fucoids (wrack) and laminarians (kelp), which are prominent members of temperate intertidal and shallow water communities.
As with the red and green seaweeds the brown seaweeds have a complex lifecycle (Figure 42) normally involving the alternation of phases between a sporophyte and a gametophyte. Typically the phases are heteromorphic, for example in the kelps (Laminariales) the gametophytes are microscopic. In the Fucales (e.g. Fucus, Sargassum), however, there is no sporophyte stage. The mature diploid individual forms specialised structures called the antheridium and oogonium to form sperm and eggs respectively, which are then released into the water column.
Figure 42. The lifecycle of the brown seaweed Fucus sp.
The Bacillariophyta contains the abundant diatoms found in both the plankton and on the benthos of both freshwater and marine habitats. These relatively large unicells (up to 2 mm) are characterized by the two part box-like cell wall called the frustule. The valves are made of silica and fit together much like a petri dish does. Diatoms reproduce asexually through cell division. The two valves of the frustule part, each with a naked cell inside, and a new valve is formed before final separation. This results in a decrease in size of the average cell. However, during sexual reproduction the fusion of gametes forms an auxospore, resulting in a new vegetative cell that re-establishes the original cell size.
The angiosperms or flowering plants make up the majority of terrestrial plants, and some, such as sea grasses (Zostera sp.) and saltmarsh (Spartina maritima), have invaded marine habitats. Angiosperms belong to the kingdom Plantae like the green and red algae but unlike the algae they are much more structurally complex. The transition to land created new challenges such as structural support, and water and nutrient supply. This led to cell differentiation into tissues for specific roles such as leaves for photosynthesis, non-photosynthetic branches and bark for support and protection, complex reproductive structures (flowers) and roots for nutrient and water uptake. The vascular system of plants used to transport water, minerals and food around the thallus is another adaptation to terrestrial environments. In flowering plants the sporophyte is the dominant phase and the gametophytes are reduced to the pollen and the ova. The flowering plants reproduce via seeds which differ from spores in that seeds result from the fusion of gametes, while spores are an asexual form of reproduction.
Lichens are not a single taxonomic entity in themselves. Instead these plant-like organisms are actually the result of a symbiosis between fungi and algae. Typically the fungus component belongs to the group known commonly as the sac fungi or Ascomycota, which includes the truffles, morels and yeast, while the algae are cyanobactaria or green algae. However, members of other fungal phyla, yellow-green algae and in a single case a brown alga may form lichens. The fungus provides minerals to the algae, which it absorbs from the substratum. In return the algae provide food for the fungus through photosynthesis. The relationship is complex and the fungus cannot survive without its particular algal species, the algal species on the other hand can be found free-living on their own. Owing to this and because the fungus is the larger and visible component, the naming of lichens is based on the fungus.
Lichens are known for their hardiness and are capable of withstanding long periods of dehydration. As such lichens are common in habitats with harsh conditions where plants are unable to live. Typically found on rocks, masonry, trees and wooden structures, lichens grow in three distinctive forms: crustose, foliose and fruticose. Most lichens are terrestrial but about 60 species are found in the high intertidal and splash zones of the Britain and Ireland.
MarLIN would like to thank the BIODIDAC for the illustrations used in this resource.