Talitrid amphipods in decomposing seaweed on the strand-line
Image Paul Brazier - View along sand and gravel shore backed by seawall (strandline debris). Image width ca XX cm.
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Ecological and functional relationships
Of marine invertebrates, the order Amphipoda (sand hoppers) is dominant in the biotope. Three genera are common amongst the strand-line, Talitrus, Talorchestia and Orchestia, feeding on the stranded seaweed. Such amphipods are responsible for most of the primary consumption of surface material. The feeding activity of the amphipods serves to fragment algal matter (Harrison, 1977). Fragmentation has been identified as being central to the control of decomposition rates and subsequently the productivity of food chains based on algal material (Robertson & Mann, 1980). Fragmentation of macroalgae increases the decomposition rate by reducing particle size, allowing a greater surface area for microbial action and the excretion of nitrogen rich materials enhances microbial growth (Robertson & Mann, 1980).
Large numbers of Coleoptera such as the black, smooth ground beetle, Broscus cephalotes, and the scarce Nebria complanata (restricted to the south west: Davidson et al., 1991; Fowles, 1994), frequent the biotope to feed on talitrids and insect larvae (Llewellyn & Shackley, 1996). Several species of darkling beetles (Tenebrionidae) and rove beetles (Staphylinidae), live in the sand several centimetres below the strand-line deposits.
Koop & Griffiths (1982) reported a distinct relationship between the distribution of strand-line macro and meio- fauna and their food source: >82% of the biomass of both groups was concentrated beneath the most recent strand-line on a beach on the Cape Peninsula, South Africa.
Mites of the genera Halolaelaps and Phaulodinychus (Gamasida), in addition to the Histiostoma (Acaridida) (Acarina: Chelicerata), occur in strand-line debris. All three mites disperse between strand-lines via talitrid amphipods. Talitrid amphipods offer two principle advantages over insect hosts. Firstly, talitrid amphipods, such as Talitrus saltator, migrate between strand-lines throughout the year allowing continual mite dispersal and secondly juvenile talidrids are sufficiently large to support several mites (Pugh et al., 1997).
Large numbers of birds feed along the strand-line at certain times of year, including waders, corvids and passerines, in addition to many seabirds (Pienkowski, 1982; Cramp & Simmons, 1983; Lack, 1986, Cramp, 1988; Davidson et al., 1991).
Terrestrial mammals such as foxes, voles, mice and rats also frequent the strand-line to feed (Shackley & Llewellyn, 1997).
Seasonal and longer term change
- The biotope is ephemeral. The amount of macroalgae stranded on the shore is likely to vary seasonally, with deposits being particularly plentiful after winter storms that dislodge algae from the rocks.
- Griffiths & Stenton-Dozey (1981) attempted to follow successional changes in the fauna of strand-line algal detritus and changes in its condition.
- Bacteria colonized the stranded macroalgae within 24 hours (Koop & Lucas, 1983).
- Amphipods and dipteran flies dominated the biomass during the early stages, followed by beetles later on as the algae dried.
- Changes were apparent between algal debris that had been deposited singly and that deposited in banks. Both types of deposit lost half of their dry mass within the first seven days following stranding indicating a rapid rate of utilization by consumer organisms.
- Single strands of algae lost moisture more rapidly than banked algae. For instance, the moisture content of single strands fell from 80% at 3 days to 22% after 6 days, whilst banks of algae retained a moisture content of 53% after 6 days.
- Adults of the most common genus of dipterous flies, Fucellia and Coelopa, are always present, as they are opportunistic colonizers, able to move rapidly between strand-line deposits. However, they are exceptionally abundant in summer and autumn, coinciding with the presence of many larvae. This probably reflects a summer/autumn breeding peak (Stenton-Dozey & Griffiths, 1980).
- In Britain during the day, Talitrus saltator occurs above the high-tide line, either buried within the sand at depths of between 10-30 cm or within high shore deposits of stranded algae (Keith Hiscock, pers. comm..), prior to emerging at night to forage intertidally on the strand-line (Williams, 1983b). During the winter, quiescent populations are found burrowed above the extreme high water spring level (EHWS), as deep as 50 -100 cm (Bregazzi & Naylor, 1972; Williams, J.A., 1976)
Habitat structure and complexity
On sandy shores, the strand-line is an ideal place for many species to live. The high organic content and water retaining capacity contrast with the relatively sterile and fast draining sand elsewhere. Seaweed in the strand-line is likely to be in various states of decay, older dryer material towards high water spring mark and fresh material towards low water, so that a plethora of microhabitats is available for colonization and utilization by the different life stages of different species, e.g. adult and juvenile wrack flies prefer different locations (see recruitment processes). In addition, differences in microclimate and decomposition rate exist between algae that is deposited in banks and algae deposited singly. The interstitial environment of the sand beneath the strand-line also differs to that of the surrounding area owing to the high concentrations of dissolved organic matter (DOM) (Koop & Griffiths, 1982).
Several studies on South African beaches (Robertson & Mann, 1980; Koop & Griffiths, 1982; Koop et al., 1982; Griffiths & Stenton-Dozey, 1981) have examined aspects of the roles of macrofauna, meiofauna and bacteria in the productivity of the strand-line environment.
Production is predominantly secondary. Carbon fixed during primary production by macroalgae in other habitats enters the detrital pathway, the decomposition of which is a key process in the channelling of energy and cycling of nutrients. The feeding activity of amphipods in particular serves to fragment algal detritus (Harrison, 1977). Fragmentation has been identified as being central to the control of decomposition rates and subsequently the productivity of food chains based on algal material (Robertson & Mann, 1980), its role in stream and lacustrine ecosystems has been well documented (e.g. Cummins, 1974). Fragmentation of macroalgae increases the decomposition rate by reducing particle size, allowing a greater surface area for microbial action and the excretion of nitrogen rich materials enhances microbial growth (Robertson & Mann, 1980). Whilst macro- and meiofauna play a vital role in fragmentation of organic particles, bacteria are overwhelmingly important in the productivity of strand-line ecosystems (Koop et al., 1982). Annual turnover estimates (P/B) suggest that bacteria may account for about 87% of annual strand-line production, with meiofauna and macrofauna accounting for 10% and 3% respectively (Koop et al., 1982).
Many marine and otherwise terrestrial species utilize the strand-line deposits of wrack as a refuge in which to breed. Within the strand-line algal debris, it is possible for the vulnerable juvenile life stages to survive in a favourable microclimate and ready supply of nourishment. For instance:
- The sand hopper, Talitrus saltator broods its eggs and has an annual univoltine reproductive cycle (one generation reaching maturity each year). As in all crustaceans, mating and the release of juveniles is synchronised with the moult cycle. Juveniles may be found from May through until September, but peak reproductive activity occurs in August. The breeding cycle in Talitrus saltator is shorter than in other intertidal amphipods and is controlled by daylength irrespective of air and sea temperature (Williams, 1978). Juveniles reach maturity before autumn, over-winter and breed the following summer.
- Terrestrial dipterous flies, especially Fucellia maritima and Fucellia fucorum, utilize the strand-line macroalgae for the deposition of their eggs. The flies favour the drier wrack beds, and three larval instars (stages between moults) are passed there prior to pupation. Larvae are not adapted to living in wet wrack owing to a lack of hairs on their posterior spiracles and large spines on the ventral surface which are present in other wrack flies. Emergence of adults is sudden towards the end of March and adults remain in abundance until the end of September (Egglishaw, 1960). In contrast to the larvae, adults are most attracted to wet wrack. Other flies, such as Coelopa frigida, Coelopa pilipes and Thoracochaeta zosterae breed in the wrack beds and would not be found on the shore but for the accumulations of wrack in which to live and breed (Eltringham, 1971).
- Some beetles and centipedes also complete their reproductive cycle in the wrack beds. The staphylinid beetle, Bleduis spectabilis burrows in the sand, its tunnels have a side chamber in which the larvae develop. Parents supply the larvae with food and ventilate the burrow. The centipede Hydoschendyla submarina has become wholly adapted to life in the littoral zone. The female lays eggs that are impermeable, and so are not affected osmotically if inundated by seawater (Eltringham, 1971).
Time for community to reach maturity
The biotope is ephemeral in nature, consequently in order to utilize the resources that the stranded debris provides, the community reaches maturity within a few weeks. Such rapid colonization is achievable owing to the fact that species of the community originate from both terrestrial (e.g. flies, centipedes and beetles) and marine (e.g. sand hoppers) environments so can migrate quickly from adjacent habitats.
Amphipods, such as Talitrus saltator
, are useful in strand-line population assessments and monitoring surveys (Shackley & Llewellyn, 1997). They are always present at and around the most recent high water strand-line deposit and possess a well defined endogenous and circadian locomotor activity pattern (Bregazzi, 1972; Bregazzi & Naylor, 1972; Williams, 1980) that controls their daily migration to recently deposited strand-line algae.
This review can be cited as follows:
Talitrid amphipods in decomposing seaweed on the strand-line.
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=176&code=1997>