Biodiversity & Conservation

The biotope is characterized by tube-building polychaetes and amphipods, with errant polychaetes and nemerteans foraging in the surrounding and underlying sediment.

The dominant tube-builders are the deposit feeding polychaetes Polydora ciliata, Spiophanes bombyx and Pygospio elegans. In areas of mud, the tubes built by Polydora ciliata can agglomerate and form layers of mud an average of 20 cm thick, occasionally up to 50 cm (Daro & Polk, 1973). The tube-building, suspension feeding amphipods Ampelisca sp. are present where the biotope occurs in shallow warm waters, while they are probably replaced by the very similar Haploops tubicola in deeper, colder waters (Dauvin & Bellan-Santini, 1990).

The feeding activities of high densities of Polydora ciliata may inhibit the establishment of other benthic species by removing settling and developing larvae (Daro & Polk, 1973).

Infaunal deposit feeding polychaetes include the burrow dwelling Arenicola marina, the sedentary Chaetozone setosa, the mobile detritivore Scoloplos armiger and species tolerant of nutrient enrichment including Capitomastus minimus and Capitella sp.

The amphipods and the infaunal annelid species in the biotope probably interfere strongly with each other. Adult worms probably reduce amphipod numbers by disturbing their burrows and tubes, while high densities of amphipods can prevent establishment of worms by consuming larvae and juveniles (Olafsson & Persson, 1986).

The biotope contains a number of infaunal bivalve species, including Abra alba, Fabulina fabula and Mysella bidentata, which probably both deposit feed and suspension feed depending on local environmental conditions.

Spatial competition probably occurs between the infaunal suspension feeders and deposit feeders. Reworking of sediment by deposit feeders, e.g. Arenicola marina, makes the substratum less stable, increases the suspended sediment and makes the environment less suitable for suspension feeders (Rhoads & Young, 1970). Tube building by amphipods stabilizes the sediment and arrests the shift towards a community consisting entirely of deposit feeders.

Amphipods are predated chiefly by nemertean worms. For example, the nemertean Nipponnemertes pulcher is the dominant predator in the Haploops community in the Danish Oeresund (McDermott, 1984).

Mobile, carnivorous polychaetes, including Anaitides mucosa, Eteone longa, Nephtys hombergi and Pholoe inornata, predate the smaller annelids and crustaceans.

Temporal changes are likely to occur in the community due to seasonal recruitment processes. For example, the early reproductive period of Polydora ciliata often enables the species to be the first to colonize available substrata (Green, 1983). The settling of the first generation in April is followed by the accumulation and active fixing of mud continuously up to a peak during the month of May. The following generations do not produce a heavy settlement due to interspecific competition and heavy mortality of the larvae (Daro & Polk, 1973). Later in the year, the surface layer cannot hold the lower layers of the mud mat in place and they may be swept away by water currents. The substratum may now be colonized by the abundant larvae of other species in the water column.
There is a seasonal variation in planktonic production in surface waters which probably affects the food supply of the benthos in the biotope. Increased production by phytoplankton in spring and summer due to increased temperatures and irradiance is followed by phytoplankton sedimentation events which are correlated with seasonal variations in the organic content of benthic sediments (Thouzeau et al., 1996). These variations directly influence the food supply of the deposit feeders and suspension feeders in the biotope.
Where the biotope occurs in the shallow subtidal, it is likely to be affected by winter storms. Storms may cause dramatic changes in distribution of macro-infauna by washing out dominant species, opening the sediment to recolonization by adults and/or available spat/larvae (Eagle, 1975; Rees et al., 1976; Hall, 1994) and by reducing success of recruitment by newly settled spat or larvae (see Hall, 1994 for review). For example, during winter gales along the North Wales coast large numbers of Abra alba and Mysella bidentata were cast ashore and over winter survival rate was as low as 7% and 50% respectively in the more exposed locations (Rees et al., 1976). Sediment transport and the risk of smothering also occurs.

• Structural complexity is provided by the many tube building species in the biotope. The principal tube builders are the polychaetes Polydora ciliata and Spiophanes bombyx and the amphipods Ampelisca sp. and Haploops tubicola. The tubes built by Polydora ciliata for example are embedded in the sediment and the ends extend a few millimetres above the substratum surface. The mats of agglomerated sediment may be up to 50 cm thick.
• High densities of tube builders and the presence of tubes favours further sedimentation of fine particles (e.g. Mills (1967) for Ampelisca vadorum and Ampelisca abdita) and may be a factor in stimulating recruitment of species such as Haploops tubicola (Glemarec et al., 1986, cited in Dauvin & Bellan-Santini, 1990).
• Additional structural complexity is provided by the burrows of infauna although these are generally simple. Most species living within the sediment are limited to the area above the anoxic layer, the depth of which will vary depending on sediment particle size and organic content. However, the presence of burrows of species such as Arenicola marina allows a larger surface area of sediment to become oxygenated, and thus enhances the survival of a considerable variety of small species (Pearson & Rosenberg, 1978). Underlying sediments may also become oxygenated by the activities of amphipods within their tubes (Mills, 1967).

Production in IMU.TubeAP is mostly secondary, derived from detritus and organic material. Where, the biotope occurs in shallow subtidal waters, some primary production comes from benthic microalgae (microphytobenthos e.g. diatoms, flagellates and euglenoides) and water column phytoplankton. Beyond 30m depth, there is unlikely to be any in situ primary production. In all cases, the benthos is supported predominantly by pelagic production and by detrital materials emanating from the coastal fringe (Barnes & Hughes, 1992). The amount of planktonic food reaching the benthos is related to:

• depth of water through which the material must travel;
• magnitude of pelagic production;
• proximity of additional sources of detritus;
• extent of water movement near the sea bed, bringing about the renewal of suspended supplies (Barnes & Hughes, 1992).

Food becomes available to deposit feeders by sedimentation on the substratum surface and by translocation from the water column to the substratum through production of pseudofaeces by suspension feeders.
Productivity in the biotope is expected to be high. The amphipods in particular have a short life span, grow to maturity quickly and have multiple generations per year.
The sediment in the biotope may be nutrient enriched due to proximity to anthropogenic nutrient sources such as sewage outfalls or eutrophicated rivers.

• The spawning period for Polydora ciliata in northern England is from February until June and three or four generations succeed one another during the spawning period (Gudmundsson, 1985). After a week, the larvae emerge and are believed to have a pelagic life from two to six weeks before settling (Fish & Fish, 1996). The larvae settle preferentially on substrates covered with mud (Lagadeuc, 1991).
• The mating system of amphipods is polygynous and several broods of offspring are produced, each potentially fertilised by a different male. There is no larval stage and embryos are brooded in a marsupium, beneath the thorax. Embryos are released as subjuveniles with incompletely developed eighth thoracopods and certain differences in body proportions and pigmentation. Dispersal is limited to local movements of these subjuveniles and migration of the adults and hence recruitment is limited by the presence of local, unperturbed source populations (Poggiale & Dauvin, 2001). Dispersal of subjuveniles may be enhanced by the brooding females leaving their tubes and swimming to uncolonized areas of substratum before the eggs hatch (Mills, 1967).
• The tube building polychaetes, e.g. Pygospio elegans, generally disperse via a pelagic larval stage (Fish & Fish, 1996) and therefore recruitment may occur from distant populations. However, dispersal of the infaunal deposit feeders, such as Scoloplos armiger and Arenicola marina, occurs through burrowing of the benthic larvae and adults (Beukema & de Vlas, 1979; Fish & Fish, 1996). Recruitment must therefore occur from local populations or by longer distance dispersal during periods of bedload transport. Recruitment is therefore likely to be predictable if local populations exist but patchy and sporadic otherwise.

A community containing Polydora ciliata is likely to reach maturity very rapidly because Polydora ciliata is a short lived species that reaches maturity within a few months and has three or four spawnings during a breeding season. For example, in colonization experiments in Helgoland (Harms & Anger, 1983) Polydora ciliata settled on panels within one month in the spring. The tubes built by Polydora ciliata agglomerate sometimes to form layers of mud up to 20cm thick. However, it may take several years for a Polydora ciliata 'mat' to reach a significant size.
The life cycles of amphipods varies between the different families. Based on the intertidal species, Corophium volutator, the Corophium sp. may produce several broods over the summer breeding season (Fish & Fish, 1996). Haploops tubicola produces 1 or 2 broods per year with a longevity of 2 or 3 years (Dauvin & Bellan-Santini, 1990) and Mills (1967) reported that Ampelisca vadorum and Ampelisca abdita produced only 1 brood per generation but there were 2 or more generations per year. In the English Channel, two reproductive patterns were identified. Species such as Ampelisca tenuicornis and Ampelisca typica produced two generations per year. The juveniles born in May-June were able to brood in September-October (Dauvin, 1988b; Dauvin,1988c). Species such as Ampelisca armoricana and Ampelisca sarsi produced only one brood per generation and per year (Dauvin, 1989; Dauvin, 1988d). Ampelisca brevicornis showed an intermediate cycle with one generation per year during cold years (cold spring) and two generations per year during warm years (warm spring) and its cycle is intermediate between univoltine cycle and bivoltine cycle (Dauvin, 1988b,c,d,e; Dauvin, 1989, Dauvin & Bellan-Santini, 1990).