Dynamics in European and North American Soft-Bottom Mussel Beds
47
suspension feeders have on soft-bottom community structure. The response
was an immediate flurry of research activity as soft-bottom ecologists, perhaps suffering from "rocky shore envy", sought to test her hypotheses in the
field and laboratory. Numerous experiments were carried out on deposit
feeders and tubicolous species, especially in North America, where the
tradition of rocky shore field manipulations was strong. But, surprisingly,
little work was done on the third group - the suspension feeders. One reason
might be that epibenthic predation by fish and crabs has devastating effects
on the densities of soft-bottom suspension feeders in the southeastern United
States, where much of the soft-bottom ecology research in America was being
conducted at the time. Woodin had predicted that "no infaunal forms should
consistently attain their highest densities among densely packed suspensionfeeding bivalves" and that "suspension-feeding forms ... reduce the probability of successful larval settlement by any larvae including their own" so
that "their assemblages should persist but be strongly age-class dominated".
If ecologists thought that densely packed suspension-feeding bivalves were
uncommon due to high predation rates, the hypothesis was not likely to be
tested. However, in the northeastern United States, the suspension-feeding
bivalve Mytilus edulis forms large beds that can certainly be considered
"densely packed".
Mussel beds have proven to be an excellent model system for testing
Woodin's hypotheses in the field. She argued that bivalves ingest larvae from
the water column and bury recently settled larvae in feces and pseudofeces, as
described earlier by workers such as Thorson (1966) and Mileikovsky (1974).
Mussels are known to filter enormous quantities of water, and they produce
copious amounts of feces and pseudofeces (Bayne et al. 1976; Seed 1976), so
they seem to meet the criteria for adult-larval interactions. As described
above, beds are often long-lived, so they can persist as Woodin suggested, and
their cumulative effects have time to be expressed. Commito (1987) sampled
inside and outside an intertidal mussel bed in Maine with the specific intent
of testing Woodin's hypotheses. He found a polymodal distribution of Mytilus
edulis size classes, indicating that established adults were not able to prevent
subsequent larval settlement at the sampling scale of 0.02 m 2 • Such polymodal
size- and age-class distributions are frequently reported in the literature (e. g.,
Mytilus edulis - Exe estuary, Devon, England: McGrorty and Goss-Custard
1991; Mytilus galloprovincialis - northern Adriatic, Italy: Ceccherelli and
Rossi 1984). Furthermore, total infaunal density was threefold higher inside
the bed than outside, mostly due to the enhanced abundance of the oligochaete Tubificoides benedeni. This oligochaete had higher absolute and relative abundances inside the mussel bed than outside, accounting for 97.8 % of
bed infauna. It is often found in high densities wherever the silt-clay fraction
of the sediment is high and oxygen levels are low (Hunter and Arthur 1978),
the conditions that exist in mussel beds. In addition, oligochaetes produce
47
suspension feeders have on soft-bottom community structure. The response
was an immediate flurry of research activity as soft-bottom ecologists, perhaps suffering from "rocky shore envy", sought to test her hypotheses in the
field and laboratory. Numerous experiments were carried out on deposit
feeders and tubicolous species, especially in North America, where the
tradition of rocky shore field manipulations was strong. But, surprisingly,
little work was done on the third group - the suspension feeders. One reason
might be that epibenthic predation by fish and crabs has devastating effects
on the densities of soft-bottom suspension feeders in the southeastern United
States, where much of the soft-bottom ecology research in America was being
conducted at the time. Woodin had predicted that "no infaunal forms should
consistently attain their highest densities among densely packed suspensionfeeding bivalves" and that "suspension-feeding forms ... reduce the probability of successful larval settlement by any larvae including their own" so
that "their assemblages should persist but be strongly age-class dominated".
If ecologists thought that densely packed suspension-feeding bivalves were
uncommon due to high predation rates, the hypothesis was not likely to be
tested. However, in the northeastern United States, the suspension-feeding
bivalve Mytilus edulis forms large beds that can certainly be considered
"densely packed".
Mussel beds have proven to be an excellent model system for testing
Woodin's hypotheses in the field. She argued that bivalves ingest larvae from
the water column and bury recently settled larvae in feces and pseudofeces, as
described earlier by workers such as Thorson (1966) and Mileikovsky (1974).
Mussels are known to filter enormous quantities of water, and they produce
copious amounts of feces and pseudofeces (Bayne et al. 1976; Seed 1976), so
they seem to meet the criteria for adult-larval interactions. As described
above, beds are often long-lived, so they can persist as Woodin suggested, and
their cumulative effects have time to be expressed. Commito (1987) sampled
inside and outside an intertidal mussel bed in Maine with the specific intent
of testing Woodin's hypotheses. He found a polymodal distribution of Mytilus
edulis size classes, indicating that established adults were not able to prevent
subsequent larval settlement at the sampling scale of 0.02 m 2 • Such polymodal
size- and age-class distributions are frequently reported in the literature (e. g.,
Mytilus edulis - Exe estuary, Devon, England: McGrorty and Goss-Custard
1991; Mytilus galloprovincialis - northern Adriatic, Italy: Ceccherelli and
Rossi 1984). Furthermore, total infaunal density was threefold higher inside
the bed than outside, mostly due to the enhanced abundance of the oligochaete Tubificoides benedeni. This oligochaete had higher absolute and relative abundances inside the mussel bed than outside, accounting for 97.8 % of
bed infauna. It is often found in high densities wherever the silt-clay fraction
of the sediment is high and oxygen levels are low (Hunter and Arthur 1978),
the conditions that exist in mussel beds. In addition, oligochaetes produce