Macoma balthica and Abra alba in infralittoral muddy sand or mud
Ecological and functional relationships
Predation in the biotope can be an important structuring force. Predators in the biotope may include small fish (Pomatoschistus microps and Pomatoschistus minutus) and juvenile flatfish (Platichthys flesus) in addition to the burrowing polychaete Nephtys hombergii, and the shrimp Crangon crangon.
The brown shrimp Crangon crangon is one of the most important epibenthic predators on shallow sandy bottom communities. Mattila et al., (1990) found that Crangon crangon had a great potential to affect many infaunal species. For instance, the presence of Crangon crangon in experimental studies affected both the densities and size frequencies of Macoma balthica. At times when the shrimp is most abundant it may have some importance as a regulating predator on shallow soft substratum communities (Mattila et al., 1990).
However, surface and sub-surface deposit feeders are particularly characteristic of this biotope. Bivalve molluscs that inhabit muddy low energy environments tend to deposit feed, although several species including Macoma balthica and Abra alba may also suspension feed. For instance, switching between the modes of feeding in Macoma balthica was directly affected by food availability in the over-lying water (Lin & Hines, 1994). When deposit feeding, bivalves remove phytoplankton, microzooplankton, organic and inorganic particles, and microbes including bacteria, fungi and microalgae. They also probably absorb dissolved organic materials in much the same manner as when filter feeding (Dame, 1996). Deposit feeding bivalves adopt two approaches to feeding; bulk feeding and particle sorting. Some may ingest large amounts of sediment in a relatively nonselective manner, or may sort particles before they are ingested and reject the majority as pseudofaeces. Deposit feeding bivalves may process up to 20 times their body weight in sediments per hour with as much as 90 % of the sediment egested as pseudofaeces (Lopez & Levinton, 1987). Consequently, the resultant bioturbation is likely to alter the characteristics of the substratum and possibly the associated infaunal community. Furthermore, as a result of feeding and metabolism, bivalve molluscs excrete both particulate and dissolved materials that may be utilized by the benthos and plankton. Thus bivalves play an important role in the cycling of nutrients in such systems (Dame, 1996).
When suspension feeding, bivalves pump large volumes of water and concentrate many chemicals by several orders of magnitude greater in their body tissues than are found in surrounding seawater (Dame, 1996).Macoma balthica is not normally considered to be toxic but may transfer toxicants through the food chain to predators. Macoma balthica was implicated in the Mersey bird kill in the late 1970's, owing to bioconcentration of alklyC-lead residues (Bull et al., 1983).
Seasonal and longer term change
Seasonal changes are likely to occur in the abundance of fauna in the biotope due to seasonal recruitment processes and variations in recruitment success. For example, in the case of Macoma balthica, Bonsdorff et al. (1995) reported juvenile density in the Baltic Sea following settlement in late summer to be 300,000/m decreasing to a stable adult density of 1,000/m, and Ratcliffe et al. (1981) reported adult densities in the Humber Estuary, UK, to be between 5,000/m and 40,000/m depending on time since a successful spatfall. Furthermore, Macoma balthica may make seasonal migrations in response to environmental conditions. Beukema & De Vlas (1979) reported that 30% of the Macoma balthica population migrated into the subtidal during winter apparently in response to low temperatures. Migration was achieved by burrowing (Bonsdorff, 1984; Guenther, 1991) and/or floating (Sörlin, 1988).
One of the key factors affecting benthic habitats is disturbance which, in shallow subtidal habitats, may increase in winter due to adverse weather conditions. Storms may cause dramatic changes in distribution of 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).
Habitat structure and complexity
- The muddy sand / mud substratum of the biotope has little structural diversity provided by either physiographic features or the biota. Some 3-dimensional structure is provided by the burrows of infauna e.g. Nephtys hombergii, whilst Lagis koreni builds itself a rigid tube of sand grains which lies either diagonally or nearly upright in the sediment (Fish & Fish, 1996). 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 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).
- 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). For example, Widdows et al. (1998) reported that typical abundances (ca 100 - 1000 per m²) of Macoma balthica increased sediment re-suspension and/or erodability four fold and that there was a significant positive correlation between density of the species and sediment resuspension.
Macroalgae are absent from IMS.MacAbr and consequently productivity is mostly secondary derived from detritus and organic material, although shallower sites may develop an extensive growth of benthic diatoms in the summer. Allochthonous organic material may be derived from anthropogenic activity (e.g. sewerage) and natural sources (e.g. plankton, detritus). Autochthonous organic material is formed by benthic microalgae (microphytobenthos e.g. diatoms and euglenoids) and heterotrophic micro-organism production. Organic material is degraded by micro-organisms and the nutrients recycled. The high surface area of fine particles provides substratum for the microflora.
- The bivalves which characterize the biotope are capable of high recruitment and rapid recovery. For example, adult Macoma balthica spawn at least once a year and are highly fecund (Caddy, 1967). There is a planktotrophic larval phase which lasts up to 2 months (Fish & Fish, 1996) and so dispersal over long distances is potentially possible given a suitable hydrographic regime. Following settlement, development is rapid and sexual maturity is attained within 2 years (Gilbert, 1978; Harvey & Vincent, 1989). The exact time at which maturity was attained depended upon the size of the individual, but it seemed that a minimum shell length of between 7-9 mm was typical (Nott, 1980). Normally, for Abra alba there two distinct spawning periods, in July and September and according to the season of settlement, individuals differ in terms of growth and potential life span (Dauvin & Gentil, 1989). Autumn settled individuals from the Bay of Morlaix, France, initially showed no significant growth; they were not collected on a 1 mm mesh sieve until April, 5 to 7 months after settlement. Such individuals were expected to have a maximum life span of 21 months and could produce two spawnings. In contrast, veliger larvae that settled during the summer grew very rapidly and were collected on a 1 mm mesh sieve just one month after settlement. They lived about one year and spawned only once (Dauvin & Gentil, 1989).
- Recruitment in bivalve molluscs is influenced by larval and post-settlement mortality. Typically bivalves are fecund and egg production increases with female size, however, the high potential population increase is offset by high larval and juvenile mortality, but, juvenile mortality rapidly decreases with age (Brousseau, 1978b; Strasser, 1999). Larval mortality results from predation during pelagic stages, predation from suspension feeding macrofauna (including conspecific adults) during settlement and from deposition in unsuitable habitats. Mortality of the juveniles of marine benthic invertebrates can exceed 30% in the first day, and several studies report 90% mortality (Gosselin & Qian, 1997). In addition to larval dispersal, dispersal of juveniles and adults occurs via burrowing (Bonsdorff, 1984; Guenther, 1991), floating (Sörlin, 1988) and probably via bedload transport (Emerson & Grant, 1991). It is expected therefore that recruitment can occur from both local and distant populations.
For specific information concerning the reproduction and longevity of Macoma balthica, Abra alba, Fabulina fabula and Mya arenaria, refer to MarLIN reviews for these species.
- Lagis koreni has separate sexes and breeding occurs during spring and summer. The larvae have a planktonic life of about one month and total length of life is thought to be about one year. The worms breed once then die (Fish & Fish, 1996).
- Nephtys hombergii matures between two and three years of age and breeds during April and May. The worms remain in situ within the sediment during spawning and eggs and sperm are released on to the surface of the sediment, fertilization occurs when gametes are mixed by the incoming tide or by water currents. Larval development occurs within the plankton. Nephtys hombergii may live for up to six years (Fish & Fish, 1996).
- The brown shrimp, Crangon crangon, reaches maturity after 1-2 years and the sexes are believed to be separate, although there are suggestions that the species is a protandrous hermaphrodite. Once hatched the larval life lasts for five weeks. A typical life span is three years and during that time a female may produce over 30,000 eggs (Fish & Fish, 1996).
- Subtidal populations of Echinocardium cordatum are reported to reproduce sporadically. One population recruited in only three years over a ten year period (Buchanan, 1966). The species is fecund (> 1, 000, 000 eggs), breeds between spring and summer, with a life span of between 10 and 20 years.
Time for community to reach maturity
The life history characteristics of the species which characterize the biotope suggest that the biotope would recover from major perturbations and be recognisable as the biotope within 5 years. For instance, Abra alba and Macoma balthica demonstrate an 'r' type life-cycle strategy and are 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. Bonsdorff (1984) studied the recovery of a Macoma balthica population in a shallow, brackish bay in SW Finland following removal of the substratum by dredging in the summer of 1976. Recolonization of the dredged area by Macoma balthica began immediately after the disturbance to the sediment and by November 1976 the Macoma balthica population had recovered to 51 individuals/m. One year later there was no detectable difference in the Macoma balthica population between the recently dredged area and a reference area elsewhere in the bay. In 1976, 2 generations could be detected in the newly established population indicating that active immigration of adults was occurring in parallel to larval settlement. In 1977, up to 6 generations were identified, giving further evidence of active immigration to the dredged area. 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). Such evidence suggests that recoverability of the key functional and important characterizing species of the IMS.MacAbr biotope would be typically be high. However, the recovery of Echinocardium cordatum may take longer owing to recruitment that is frequently unsuccessful (Rees & Dare, 1993).
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This review can be cited as follows:
Macoma balthica and Abra alba in infralittoral muddy sand or mud.
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=173&code=1997>