Polydora ciliata, Mya truncata and solitary ascidians in variable salinity infralittoral mixed sediment.
Ecological and functional relationships
This biotope occurs in the lower estuary where the hydrodynamic regime allows a suitable environment to develop. The presence of a suitable substratum is probably the primary structuring force, rather than the interspecific relationships. Mixed sediment provides a stable substratum for the epifauna such as solitary and colonial ascidians while the soft sediment supports infaunal annelids, crustaceans and bivalves. Sediment is the most extensive sub-habitat within the biotope and hence infauna dominate.
In areas of mud, the tubes built by Polydora ciliata can agglomerate and form layers of mud up to an average of 20cm thick, occasionally to 50cm. These layers can eliminate the original fauna and flora. Daro & Polk (1973) state that the formation of layers of Polydora ciliata tend to eliminate original flora and fauna. The species readily overgrows other species with a flat morphology and feeds by scraping its palps outside its tubes, which would inhibit the development of settling larvae of other species.
Burrowing deposit feeding species potentially disturb and mobilize the sediment, but the presence of mats of Polydora ciliata and the burrowing piddock Petricola pholadiformis suggests that the sediment is relatively stable. Tube building, e.g. by Lanice conchilega and Lagis koreni, probably stabilizes the sediment and arrests the shift towards a community dominated by deposit feeders. Many of the infaunal polychaetes within the biotope are surface deposit feeders (e.g. the terebellids and cirratulids).
Amphipods, e.g. Corophium sp., and the infaunal annelid species in this biotope probably interfere strongly with each other. Adult worms probably reduce amphipod numbers by disturbing their burrows, while high densities of amphipods can prevent establishment of worms by consuming larvae and juveniles (Olafsson & Persson, 1986). For example, Arenicola marina was shown to have a strong negative effect on Corophium volutator due to reworking of sediment causing the amphipod to emigrate (Flach, 1992).
Hard substrata support suspension feeding ascidians such as Ascidiella scabra, Ascidiella aspera, Molgula spp. and Dendrodoa grossularia and tubeworms e.g. Pomatoceros triqueter, while infaunal suspension feeders include the bivalves Abra alba and Mya truncata and Mya arenaria and tubeworms e.g. Lanice conchilega.
Carcinus maenas is a significant predator in the biotope. It has been shown to reduce the density of Mya arenaria, Cerastoderma edule, Abra alba, Tubificoides benedii, Aphelochaeta marioni and Corophium volutator (Reise, 1985). A population of Carcinus maenas from a Scottish sea-loch preyed predominantly on annelids (85% frequency of occurrence in captured crabs) and less so on molluscs (18%) and crustaceans (18%) (Feder & Pearson, 1988).
Carnivorous annelids such as Nephtys hombergi, Eteone longa, Glycera spp. and Harmothoe spp. operate at the trophic level below Carcinus maenas (Reise, 1985). They predate the smaller annelids, such as Exogone naidina, and crustaceans, such as Corophium volutator and Cumacea sp.
Seasonal and longer term change
Seasonal changes occur in the abundance of the fauna due to seasonal recruitment processes. Variation in abundance is very pronounced in the polychaete Aphelochaeta marioni
. In the Wadden Sea, peak abundance occurred in January (71,200 individuals per m²) and minimum abundance occurred in July (22,500 individuals per m²) following maximum spawning activity between May and July (Farke, 1979). However, the spawning period varies according to environmental conditions and so peak abundances will not necessarily occur at the same time each year. Adult densities of the bivalve, Abra alba
, may exceed 1000 per m² in favourable conditions but typically fluctuate widely from year to year due to variation in recruitment success or adult mortality (see review by Rees & Dare, 1993). However, the sea squirt Ascidiella scabra
showed regular annual recruitment onto artificial and scraped natural substrata and was described as an 'annual ascidian' by Svane (1988).
One of the key factors affecting benthic habitats is disturbance which, in shallow subtidal habitats increases in winter due to weather conditions. 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., 1977; 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 were cast ashore and over winter survival rate was as low as 7% in the more exposed locations, whilst the survival rates of the polychaetes Eteone longa and Nephtys hombergi were 29% and 22% respectively (Rees et al., 1976). Soft bodied epifauna, such as ascidians, are likely to be very sensitive to storm damage and will probably suffer high mortality during winter storms. Rapid recolonization occurs in summer and therefore abundances are likely to vary considerably due to physical disturbance. Sediment transport and the risk of smothering also occurs. A storm event at a silt/sand substratum site in Long Island Sound resulted in the deposition of a 1cm layer of shell fragments and quartz grains (McCall, 1977).
Habitat structure and complexity
The biotope consists of hard substrata such as cobbles and pebbles or shell debris sitting in or on consolidated sediments. The mixed substrata provides habitats for a diverse assemblage of epifaunal and infaunal species. Most of the species that occur in the biotope are not closely associated with the community and it is probably transitional between other biotopes such as Aphelochaeta marioni
(e.g. IMU.AphTub), or bivalves (e.g. IMX.VsenMtru).
- The mixed sediment in this biotope is the important structural component, providing the complexity required by the associated community. Epifauna attached to the gravel and pebbles and infauna burrow in the soft underlying sediment. Sediment deposition, and therefore the spatial extent of the biotope, is dictated by the physiography and underlying geology coupled with the hydrodynamic regime (Elliot et al., 1998).
- The presence of both sediment and hard substrata increases the range of substrata available for settlement by organism with different habitat requirements; both infaunal and epifaunal species may be abundant . Attrill et al. (1996) described a "biodiversity hot spot" in similar situations of mixed substrata in the Thames estuary.
- There is a traditional view that the distribution of infaunal invertebrates is correlated solely with sediment grain size. In reality, and in this biotope, it is likely that a number of additional factors, including organic content, microbial content, food supply and trophic interactions, interact to determine the distribution of the infauna (Snelgrove & Butman, 1994).
- Structural complexity is provided by the many tube building species in the biotope. 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 resultant mats of agglomerated sediment may be up to 50 cm thick.
- Reworking of sediments by deposit feeders increases bioturbation and potentially causes a change in the substratum characteristics and the associated community (e.g. Rhoads & Young, 1970). The presence of tube builders, such as Lanice conchilega, stabilizes the sediment and provides additional structural complexity.
- The burrows of large bivalves (e.g. Mya spp.) and piddocks provide additional complexity to the biotope and probably increase the depth to which the sediment is oxygenated.
The majority of the productivity in the biotope is secondary, derived from detritus and organic particulates. Primary production is derived from phytoplankton and converted into secondary productivity by the suspension feeders. The benthos is supported predominantly by pelagic production and by detrital materials emanating from the coastal fringe (Barnes & Hughes, 1992). Secondary productivity is probably high given the high densities attained by some species and the diversity of species within the biotope, however no specific information was found.
The recruitment processes exhibited by the major groups within the biotope are demonstrated by the examples below.
- The lifecycle of Aphelochaeta marioni varies according to environmental conditions. In Stonehouse Pool, Plymouth, Aphelochaeta marioni (studied as Tharyx marioni) spawned in October and November (Gibbs, 1971) whereas in the Wadden Sea, Netherlands, spawning occurred from May to July (Farke, 1979). The embryos developed lecithotrophically and hatched in about 10 days (Farke, 1979). Under stable conditions, adult and juvenile Aphelochaeta marioni will disperse by burrowing (Farke, 1979).
- 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 substrata covered with mud (Lagadeuc, 1991).
- Nephtys hombergi exhibits variable spawning success with failures in some years (Olive et al., 1997).
- The mating system of amphipods is polygynous and several broods of offspring are produced, each potentially fertilized by a different male. There is no larval stage and embryos are brooded in a marsupium, beneath the thorax. Embryos are released as sub-juveniles with incompletely developed eighth thoracopods and certain differences in body proportions and pigmentation. Dispersal is limited to local movements of these sub-juveniles and migration of the adults and hence recruitment is limited by the presence of local, unperturbed source populations (Poggiale & Dauvin, 2001). Dispersal of sub-juveniles may be enhanced by the brooding females leaving their tubes and swimming to un-colonized 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, aided by bed load transport of juveniles (Boström & Bonsdorff, 2000). However, dispersal of some of the infaunal deposit feeders, such as Scoloplos armiger, 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.
- Mya arenaria demonstrates high fecundity, increasing with female size, with long life and hence high reproductive potential. The high potential population increase is offset by high larval and juvenile mortality. Juvenile mortality reduces rapidly with age (Brousseau, 1978b; Strasser, 1999). Strasser et al. (1999) noted that population densities in the Wadden Sea were patchy and dominated by particular year classes. Therefore, although large numbers of spat may settle annually, successful recruitment and hence recovery may take longer than a year. Recruitment of shallow burrowing infaunal species can depend on adult movement by bedload sediment transport and not just spat settlement. Emerson & Grant (1991) investigated recruitment in Mya arenaria and found that bedload transport was positively correlated with clam transport. They concluded that clam transport at a high energy site accounted for large changes in clam density. Furthermore, clam transport was not restricted to storm events and the significance is not restricted to Mya arenaria recruitment. Many infauna, e.g. polychaetes, gastropods, nematodes and other bivalves, will be susceptible to movement of their substratum.
- Ascidians such as Ascidiella scabra and Molgula manhattensis have external fertilization but short lived larvae (swimming for only a few hours), so that dispersal is probably limited (see MarLIN reviews). Ascidiella scabra has a high fecundity and settles readily, probably for an extended period from spring to autumn. Svane (1988) describes it as "an annual ascidian" and demonstrated recruitment onto artificial and scraped natural substrata. Eggs and larvae are free-living for only a few hours and so recolonization would have to be from existing individuals no more than a few km away. It is also likely that Ascidiella scabra larvae are attracted by existing populations and settle near to adults (Svane et al., 1987) . Fast growth means that a dense cover could be established within about 2 months. Where neighbouring populations are present recruitment may be rapid but recruitment from distant populations may take a long time.
Most other macrofauna in the biotope breed several times in their life history (iteroparous) and are planktonic spawners producing large numbers of gametes. Dispersal potential is high. Overall recruitment is likely to be patchy and sporadic, with high spat fall occurring in areas devoid of adults, perhaps lost due to predation or storms. The presence of fast growing space occupying species, e.g. Polydora ciliata
and Ascidiella scabra
suggests that competition for space for settling larvae is probably intense, with recruitment dependant on the coincidence of factors that free space (e.g. death of short-lived species or storm related physical disturbance) with larval supply. The presence of numerous suspension feeders an surface deposit feeders suggests that post-settlement mortality of larvae would be high.
Time for community to reach maturity
The community is dominated by fast growing opportunistic polychaete and ascidian species and the community most likely reaches maturity within one year of space becoming available. In an experimental study investigating recovery of a range of species characteristically found in this biotope after copper contamination, Hall & Frid (1995) found that recovery took up to a year. Hall & Frid (1998) found that colonization by many of the polychaetes associated with this biotope did not vary significantly with season although recruitment of Tubificoides benedii
and Ophyrotrocha hartmanni
did vary significantly with season. Polydora ciliata
is another short-lived species that reaches maturity within a few months and has three or four spawnings during a breeding season of several months. For example, in colonization experiments in Helgoland (Harms & Anger, 1983), Polydora ciliata
settled on panels within one month in the spring. The bivalve Abra alba
demonstrates an 'r' type life-cycle strategy and is able to rapidly exploit any new or disturbed substratum available for colonization through larval recruitment, secondary settlement of post-metamorphosis juveniles or re-distribution of adults. For example, Abra alba
recovered to former densities following loss of a population from Keil Bay owing to deoxygenation within 1.5 years, as did Lagis koreni
, taking only one year (Arntz & Rumohr, 1986). Mya arenaria
has a high fecundity and reproductive potential but larval supply is sporadic and juvenile mortality is high, so that although, large numbers of spat may settle annually, successful recruitment and hence recovery may take longer than a year. For example, Beukema (1995) reported that a population of Mya arenaria
in the Wadden Sea, drastically reduced by lugworm dredging took about 5 years to recover. Therefore, the polychaete infauna, ascidian and tube worm epifauna would probably colonize the habitat rapidly, producing a recognizeable biotope within 1-2 years, while the abundance of some species, e.g. Mya
sp. would take up to 5 years to develop.
This review can be cited as follows:
Polydora ciliata, Mya truncata and solitary ascidians in variable salinity infralittoral mixed sediment..
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=114&code=1997>