Foliose red seaweeds on exposed or moderately exposed lower infralittoral rock
Image Rohan Holt - Foliose red seaweeds on exposed or moderately exposed lower infralittoral rock. Image width ca XX m.
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Ecological and functional relationships
Foliose algae provide shelter for invertebrates, a substratum for attachment of some species and food for grazers. Dependant relationships develop and are noted below.
The predominant environmental factor determining occurrence of this biotope is light. In the lower infralittoral there is generally insufficient light for the growth of Laminariales and substratum is dominated by foliose and encrusting red algae.
Old stipes and midribs of Delesseria sanguinea become heavily encrusted with algae and epiphytic invertebrates such as bryozoa, sponges and ascidians (Maggs & Hommersand, 1993).
The most important grazer of subtidal algae in the British Isles is the sea urchin, Echinus esculentus. It has demonstrated a preference for red algae. Sea urchin grazing may maintain the patchy and species rich understorey epiflora/fauna by preventing dominant species from becoming established. In wave exposed situations, sea urchins may not be able to cling on or feed in shallow depths during storms and this may favour the development of algal dominated biotopes. Also sea urchin densities vary in different parts of the coast, where numbers are low the biotope may be favoured (K. Hiscock, pers. comm.). Vost (1983) examined the effect of removing grazing Echinus esculentus and found that after 6-10 months the patchiness of the understorey algae had decreased and the species richness and biomass of epilithic species increased. Algae with single attachment points became more frequent in the urchin free area and the total biomass and species richness of epilithic species increased (Birkett et al., 1998b). Echinus esculentus grazing probably controls the lower limit of kelp distribution in some locations, e.g. in the Isle of Man (Jones & Kain 1967; Kain et al. 1975; Kain 1979).
Echinus esculentus may be preyed upon by the lobster Homarus gammarus, and in the north, the wolf-fish Anarhichas lupus.
The prosobranch mollusc Lacuna parva grazes extensively upon the red algae Phyllophora crispa and Delesseria sanguinea and Phycodrys rubens. Phyllophora crispa is the main substratum for spawn deposition (Ockelmann & Nielsen, 1981).
Corallina officinalis may support epiphytes, including Mesophyllum lichenoides, Titanoderma pustulatum, and Titanoderma corallinae, the latter causing tissue damage (Irvine & Chamberlain 1994). Hay et al. (1987) suggested that grazing by amphipods and polychaetes caused damage to 1-20 % of the blade area of the foliose brown algae Dictyota dichotoma
Other grazers include topshells, e.g. Gibbula cineraria and small Crustacea (amphipods and isopods) and the painted top-shell Calliostoma zizyphinum, which feeds upon cnidarians, as well as micro-organisms and detritus.
Specialist predators of hydroids and bryozoans in particular include the nudibranch species such as Janolus cristatus, Doto spp. and Onchidoris spp. Starfish (e.g. Asterias rubens, Crossaster papposus and Henricia spp.) are generalist predators feeding on most epifauna, including ascidians.
Predation does not necessarily cause mortality. For instance, Metridium senile is attacked by Aeolidia papillosa and by Pycnogonum littorale. Alcyonium digitatum is attacked by the nudibranchTritonia hombergi and the mollusc Simnia patula, which also feeds upon the hydroid Tubularia indivisa.
Many inhabitants of the biotope are suspension feeders and are doubtless in competition for food, although moderately strong water movement and the relatively close proximity of the highly productive kelp forests of the upper infralittoral are likely to bring a plentiful supply of food. Ninety percent of kelp production is estimated to enter the detrital food webs of coastal areas, as particulate organic matter (POM) and dissolved organic matter (DOM), supporting biotopes beyond the kelp beds (Birkett et al., 1998b). Suspension feeders include barnacles, ascidians such as Clavelina lepadiformis and Aplidium punctum, and anthozoans such as Alcyonium digitatum, Urticina felina and Caryophyllia smithii and occasional sponge crusts. Larger prey items would be taken by Urticina felina and Metridium senile (Hartnoll, 1998).
Seasonal and longer term change
Many of the red seaweeds in this biotope have annual fronds, which typically die back in the autumn and regenerate in the spring. Consequently a seasonal change occurs in the seaweed cover, which is substantially reduced over the winter and becomes most dense between April to September. For example, the perennial Delesseria sanguinea exhibits a strong seasonal pattern of growth and reproduction. New blades appear in February and grow to full size by May -June becoming increasing battered or torn and the lamina are reduced to midribs by December (Maggs & Hommersand, 1993). Blade weight is maximal in midsummer, growth dropping in June and July and becoming zero in August (Kain, 1987).
Several species of bryozoans and hydroids demonstrate seasonal cycles of growth in spring/summer and regression (die back) in late autumn/winter, over wintering as dormant stages or juvenile stages (see Ryland, 1976; Gili & Hughes, 1995; Hayward & Ryland, 1998). For example, the fronds of Bugula species are ephemeral, surviving about 3-4 months but producing two frond generations in summer before dying back in winter, although, the holdfasts are probably perennial (Eggleston, 1972a; Dyrynda & Ryland, 1982). The hydroid Tubularia indivisa that may occasionally occur in the biotope is an annual, dying back in winter (Fish & Fish, 1996), while the uprights of Nemertesia antennina die back after 4-5 months and exhibit three generations per year (spring, summer and winter) (see reviews; Hughes, 1977; Hayward & Ryland, 1998; Hartnoll, 1998). Many of the bryozoans and hydroid species are opportunists (e.g. Bugula flabellata) adapted to rapid growth and reproduction (r-selected), taking advantage of the spring/summer phytoplankton bloom and more favourable (less stormy) conditions (Dyrynda & Ryland, 1982; Gili & Hughes, 1995). Some species such as the ascidians Ciona intestinalis and Clavellina lepadiformis are effectively annual (Hartnoll, 1998). Therefore, the biotope is likely to demonstrate seasonal changes in the abundance or cover of both algae and fauna. Winter spawning species such as Alcyonium digitatum may take advantage of the available space for colonization.
Habitat structure and complexity
- The biotope occurs over bedrock surfaces and large boulders, the nature of which provide a variety of surface aspects. The species composition probably varies with depth from the upper limit of the lower infralittoral towards the circalittoral. For example, foliose and encrusting red algae probably out compete the faunal turf species on tops of bedrock ridges, but decline on vertical surfaces and with depth.
- The algal and faunal turf provides interstices and refuges for a variety of small organisms such as nemerteans, polychaetes, amphipods, and prosobranchs.
Larger mobile species include decapod crustaceans such as shrimps, crabs, hermit crabs, lobsters, sea urchins, starfish and fish. Such species are not highly faithful to the biotope, but probably utilize available rock ledges and crevices for shelter.
Specific information concerning the biotope was not found. Foliose and encrusting red algae are primary producers in the EIR.FoR biotope, the biomass of which will enter the food chain indirectly in the form of detritus, algal spores and abraded algal particulates, or directly as food for grazing gastropods, sea urchins or fish. The biotope is likely, however, to receive more particulate and dissolved organic matter (POM & DOM) from kelp biotopes in the upper infralittoral. Kelps are the major primary producers in UK marine coastal waters producing nearly 75 % of the net carbon fixed annually on the shoreline of the coastal euphotic zone (Birkett et al., 1998b). Kelp plants produce 2.7 times their standing biomass per year. Refer to EIR.LhypFa and EIR.LhyphR.
Recruitment into the biotope occurs as a result of spore or larval settlement and by migration. Information on some of the characterizing species is given below:
- The onset of sexual reproduction in Delesseria sanguinea is stimulated by day length, Delesseria sanguinea is a short-day plant sensitive to a night-break (Kain, 1991; Kain, 1996]. Kain (1987) suggested that the southern limit of Delesseria sanguinea may be determined by winter temperatures. Studies in Roscoff and Helgoland support that observation; new blades formed in April - June at Roscoff, males plants in October - December, cystocarps and tetrasporangia in October - December, the last cystocarps found in April. Recruitment of Delesseria sanguinea occurred between February and April/June in both Roscoff and Helgoland (Molenaar & Breeman, 1997).
- Corallina officinalis produces spores over a protracted period and can colonize artificial substratum within one week in the intertidal (Harlin & Lindbergh, 1977; Littler & Kauker, 1984).
- Sea urchins most likely migrate into the biotope rather than settle directly there. However, maximum spawning of Echinus esculentus occurs in spring although individuals may spawn over a protracted period. Gonad weight is maximal in February / March in the English Channel (Comely & Ansell, 1989) but decreases during spawning in spring and then increases again through summer and winter until the next spawning; there is no resting phase. Spawning occurs just before the seasonal rise in temperature in temperate zones but is probably not triggered by rising temperature (Bishop, 1985). Planktonic development is complex and takes between 45 -60 days in captivity (MacBride, 1914). Recruitment is sporadic or variable depending on locality.
- Hydroids are often the first organisms to colonize available space in settlement experiments (Gili & Hughes, 1995). For instance, Nemertesia antennina releases planulae on mucus threads, that increase potential dispersal to 5 -50m, depending on currents and turbulence (Hughes, 1977). Most species of hydroid in temperate waters grow rapidly and reproduce in spring and summer. Few species of hydroids have specific substratum requirements and many are generalists. Hydroids are also capable of asexual reproduction and many species produce dormant, resting stages, that are very resistant of environmental perturbation (Gili & Hughes, 1995).
- Sponges may proliferate both asexually and sexually. A sponge can regenerate from a broken fragment, produce buds either internally or externally or release clusters of cells known as gemmules which develop into a new sponge. However, some sponges appear to be long-lived, slow growing and recruit infrequently. For instance, monitoring studies at Lundy revealed extremely slow growth and no recruitment of Axinella dissimilis (Hiscock, 1994).
- Anthozoans, such as Alcyonium digitatum and Urticina felina are long lived with potentially highly dispersive pelagic larvae and are relatively widespread. They are not restricted to this biotope and would probably be able to recruit rapidly (refer to full MarLIN reviews). Similarly, Metridium senile has a long lived, dispersive planktonic planula larva. However, it is also capable of reproducing asexually by budding form the base, and colonizes space aggressively, forming clumps (Sebens, 1985; Hartnoll, 1998). Juveniles are susceptible to predation by sea urchins or overgrowth by ascidians (Sebens, 1985; 1986).
- Development of Lacuna parva is direct and takes about two months at 10-11 °C . After copulation females may produce fertilized eggs for two to three months. The species has an annual life cycle with mating prior to the production of spawn between March and June, death of adults occurs throughout May and June, the main hatching of new recruits occurs in June and July (Ockelman & Nielsen, 1981).
- Mobile fauna, crabs, fish and starfish, will probably recruit from the surrounding area either by migration or from planktonic larvae, as the community develops and food, niches and refuges become available, .
Time for community to reach maturity
It is likely that Rhodophyceae could recolonize an area from adjacent populations within a short period of time in ideal conditions but that recolonization from distant populations would probably take longer.
Many of the Rhodophyta e.g. Delesseria sanguinea, Plocamium cartilagineum, Dilsea carnosa and Corallina officinalis are perennial species that may persist for several years. For instance, Dickinson (1963) suggested a life span of 5-6 years for Delesseria sanguinea. However, Kain (1984) estimated that 1 in 20 specimens of Delesseria sanguinea may attain 9 - 16 years of age. Kain (1975) examined recolonization of cleared concrete blocks in a subtidal kelp forest. Red algae colonized blocks within 26 weeks in the shallow subtidal (0.8m) and 33 weeks at 4.4m. Delesseria sanguinea was noted within 41 weeks (8 months) at 4.4m in one group of blocks and within 56-59 days after block clearance in another group of blocks. This recolonization occurred during winter months following spore release and settlement, but not in subsequent samples (Kain, 1975). This suggests that colonization of Delesseria sanguinea in new areas is directly dependent on spore availability. Rhodophyceae have non flagellate, and non-motile spores that stick on contact with the substratum. Norton (1992) noted that algal spore dispersal is probably determined by currents and turbulent deposition. However, red algae produce large numbers of spores that may settle close to the adult especially where currents are reduced by an algal turf or in kelp forests.
Many of the sessile fauna present in the EIR.FoR biotope such as alcyonarians, ascidians and sponges, are present in the communities described by Sebens (1985) which were considered to be dynamic and fast growing. Smaller associated mobile species such as polychaetes and prosobranchs have planktonic larvae and would most likely colonize after a year. Large mobile species such as sea urchins, starfish and crabs would migrate into the area rapidly. The community may therefore take probably two or three years to reach maturity, but competitive interactions and the arrival of slower colonizing species could mean that dynamic stability is not achieved for several years.
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This review can be cited as follows:
Foliose red seaweeds on exposed or moderately exposed lower infralittoral rock.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=65&code=1997>