Article: Trophic group and evolution in bivalve molluscs
Jeffrey S. Levinton
Deposit-feeding marine benthic invertebrates ingest sediments and feed principally upon bacteria, whereas suspension-feeders feed mainly upon phytoplankton. This distinction is important because the predictability of phytoplankton is less than that of within-sediment bacteria. As a result, suspension-feeding populations fluctuate more than deposit-feeding populations. Possible consequences of these differences include: (I) The evolutionary turnover of deposit-feeding groups should be less than that of suspension-feeders, (2) Being more subject to environmental perturbations, the longevity of suspension-feeding genera should be less than that of deposit-feeding genera, and (3) trophic structure of deposit-feeding communities should be conservative, with few changes in trophic structure since the early development of the adaptive zone. Preliminary evidence from the fossil record supports these predictions. (i) Bretsky's interpretation of Palaeozoic community evolution, as being the result of nearshore-offshore differences in environmental predictability can be shown to be strongly influenced by trophic group, (ii) If survivorship curves are constructed for genera of bivalve superfamilies, the following mortality rates obtain for genera: Nuculoida (deposit-feeder)— 0.8%/million years, Pectinacea (suspension-feeder) — 12%/my, Pteriacea (suspension-feeder)— 1.5%/my, Veneracea (suspension-feeder) — 1.5%/my. Clearly suspension-feeding bivalve genera were shorter-lived, (iii) Finally, Levinton and Bambach have shown a similarity in the ecology of Silurian and Recent deposit-feeding bivalve mollusc communities.