Neomysis integer and Gammarus spp. in low salinity infralittoral mobile sand
Ecological and functional relationships
In the estuarine environment the highly mobile macrofauna comprises two ecologically distinct groupings: those (mainly invertebrate) species permanently resident within an estuary, and those (mainly vertebrate) species entering estuaries principally to feed at high or low tide, e.g. fish and birds respectively (Barnes, 1974). Infralittoral mobile sands provide prey for demersal fishes. Prey especially includes mobile small crustaceans which migrate from the sediment and become available to predators. The mysid shrimp, Neomysis integer is important as food for fish, especially for juvenile flounder, Platichthys flesus, in the upper parts of estuaries (Costa & Elliott, 1999; Bell, 1990).
Estuarine biotope complexes may be used by important wintering and passage birds for feeding (Elliot et al., 1998).
Neomysis integer may be parasitized by the third larval stage of the nematode Thynnascaria adunca (Astthorsson, 1980). Both Gammarus salinus and Gammarus zaddachi are important host species for the transmission of fish and bird parasites (Voigt, 1991).
Gammarus salinus has a documented role as a seaweed disperser (Breeman & Hoeksema, 1987). The red seaweed Audouinella purpurea was able to survive digestion by Gammarus salinus and grew in the field from faecal pellets.
Seasonal and longer term change
- The abundance of the important characterizing species may vary according to the season, for instance in the Severn Estuary, Neomysis integer over-wintered at relatively low densities in comparison to the summer, when brooding females swarmed inshore (Moore et al., 1979).
- Seasonal storm events are likely to change sediment distribution significantly.
Habitat structure and complexity
The habitat is not complex and consists of mobile usually coarse sand. The sand provides shelter for a very small variety of mobile species that live 'loosely' amongst the sand grains, e.g. Gammarus salinus
. Over the sand the habitat for Neomysis integer
is the water column, and whether or not sand is important to Neomysis integer
is not clear.
The physical environment of infralittoral mobile sands with strong currents is on the whole too harsh for vegetation to become established. Therefore such environments are less productive with lower levels of organic matter. Microphytobenthos may be supported in the interstices of the sand grains in the uppermost millimetres of illuminated sediments, typically appearing as a subtle brownish or greenish staining (Elliott et al., 1998), whilst macroalgae that have become detached from rocky substrata elsewhere may wash up in the estuary, eventually decomposing and contributing to the energy budget of the system. However, the principle source of production in the estuarine environment is secondary, derived from detritus and allochthonous organic matter, which is utilized by the fauna.
and Gammarus salinus
are the only two species with significant populations regularly recorded from this biotope. Both species are capable of migration over some distance and therefore colonization by adults from other biotopes is likely to occur.
- Neomysis integer reaches maturity within 2 - 3 months of release form the females brood pouch (marsuipum). It breeds between spring and autumn and typically produces three generations per year, two during the summer and one in the autumn which over-winters. In most populations of Neomysis integer breeding ceases in winter, with the exception of the population from Loch Etive, Scotland, which bred continuously throughout the year, although at low intensity (Mauchline, 1971). Neomysis integer has a lifespan of less than a year.
- Gammarus salinus typically produces two generations per year. Mature females were found from late November through to late July and the main reproduction period occurred during the winter (Leineweber, 1985). Juveniles were most numerous from April through to July, and in the warmer months between July and October a relatively stable population was attained. Gammarus salinus also has a lifespan of less than a year.
Time for community to reach maturity
Little evidence concerning community development was found. However, it is expected that the community, which consists entirely of swimming species, could establish very rapidly as migration from other populations would occur in addition to any larval recruitment. The length of time for recruitment to occur might be a few hours but 'maturity' would not be expected for several weeks in the case of extensive defaunation of the substratum.
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This review can be cited as follows:
Neomysis integer and Gammarus spp. in low salinity infralittoral mobile sand.
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=51&code=1997>