Petersens soft bottom communities

During the second decade of the twentieth century, pioneer studies of marine benthos were made by Petersen, who carried out detailed investigations by grab samples of the larger animals (the macrofauna) of soft deposits in shallow water off the Danish coast. He found that different areas supported characteristic associations of animals, and he distinguished nine communities, naming each after the most conspicuous components of the population as follows:

1 Macoma communities, widespread in shallow muds (M. baltica, Mya arenaria, Cardium = Cerastoderma edule, etc.)

2 Abra communities in shallow, muddy sands, often in sheltered creeks. (A. alba, A. prismatica, Macoma calcarea, Astarte spp, etc. sometimes with Echinocar-dium cordatum.) (Note: In Petersen's original classification Abra = Syndos-mya)

3 Venus communities, widespread on shallow sandy bottoms on open coasts. (V striatula, Tellina fabula, Montacuta ferruginosa, often with E. cordatum.)

4 Echinocardium-filiformis communities, in sandy mud at intermediate depths. (E. cordatum, Amphiura filiformis, Turritella communis, Nephtys spp., etc.)

5 Brissopsis-chiajei communities on deeper, soft mud (B. lyrifera, Amphiura chiajei, Abra nitida, etc.)

6 Brissopsis-sarsi communities on soft mud below Brissopsis-chiajei depths. (B. lyrifera, Ophiura sarsi, A. nitida, etc.)

7 Amphilepis-Pecten communities on deep mud in Skagerrak. (Amphilepis norvegica, Chlamys (Pecten) vitrea, etc.)

8 Haploops communities on deep, firm mud in Kattegat. (Haploops tubicola, Chlamys septemradiata, etc.)

9 Deep Venus communities, widespread on coarse sands (V. striatula, Spatangus purpureus, Echinocardium flavescens, Spisula spp., etc.)

Thorson (1957) observed that in middle latitudes there are certain conspicuous genera of bivalves, echinoderms, polychaetes, etc., which occur in communities of generally similar appearance in widely separated places, but represented by different species in different parts of the range. He has therefore classified communities in terms of their most obvious genus rather than species, e.g. Macoma communities, Tellina communities, etc., the species varying with differences in local conditions, mainly temperature and salinity.

One drawback of classifying communities with respect to their most prominent constituents is that certain eye-catching organisms are so widely distributed as to occur in association with rather different assemblages of other species in different parts of their range. For example, Venus striatula is frequently the most obvious species in sand at shallow depths in a community that includes Tellina fabula, Ensis ensis and Echinocardium cordatum as other conspicuous components. Venus striatula also occurs in deeper water with a different group of associated organisms, notably Spisula spp., Echinocardium flavescens and Spatangus purpureus. It is therefore necessary to distinguish two different Venus communities, one in shallow water and one at deeper levels. Similarly Brissopsis lyrifera is a prominent component of different communities at different depths.

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