48
J.A. Commito and N.M.J.A. Dankers
cocoons out of which juveniles crawl. There are no free-living larvae in the
water column where they would be exposed to ingestion by mussels. Thus,
both of the hypotheses were falsified. On the other hand, non-oligochaete species as a group were less abundant inside the bed than outside, indicating that
mussel beds are not the optimal environment for infaunal species with freeswimming larvae.
Based on these results, Commito (1987) suggested a modification of the
original Woodin hypotheses. Densely packed suspension-feeding bivalves
may indeed lower the density of species with free-swimming larvae or an
inability to withstand low oxygen conditions in sediment to which finegrained feces and pseudo feces have been added. However, they may harbor or
even enhance the abundance of infaunal species whose dispersal stages can
avoid ingestion by suspension feeders or suffocation in their fecal and pseudofecal material. These species include oligochaetes that produce cocoons as
well as taxonomic groups with asexual fragmentation (some oligochaetes and
polychaetes), brooding (some polychaetes, bivalves, crustaceans), and postlarval dispersal of relatively large juveniles (many types of macrofauna, e. g.,
Commito et al. 1995).
Dittmann (1990) conducted an intensive comparison of the intertidal
macrofauna and meiofauna in samples taken from the edge and center of a
Mytilus edulis bed and the adjacent sandflat at Konigshafen on the Wadden
Sea island of Sylt. She demonstrated that the sandflat macrofauna had substantial numbers of oligochaetes (Tubificoides benedeni and two similar species), but was dominated by polychaetes with free-swimming larvae. The
center of the bed had a shift in species composition. Polychaetes were less
abundant and oligochaetes were more abundant, leading to the same absolute
and relative dominance by oligochaetes as in the Mytilus edulis bed studied by
Commito (1987). Of the polychaetes that did live in the mussel bed, there was
a dramatic shift towards Capitella capitata and Heteromastus filiformis,
deposit-feeding species very similar to the oligochaetes in size, morphology,
and feeding mode. Subsequent work on Mytilus edulis beds from two sites on
the west coast of Denmark (Svane and Setyobudiandi 1996) and the East
Frisian Islands in the German Wadden Sea has shown similar patterns for
macrofauna (Gunther 1996; Kroncke 1996). As she found for the macrofauna,
Dittmann observed that the mussel-bed meiofauna had a different relative
species composition and higher density compared to the bare sandflat, results
similar to those found by Radziejewska (1986) for a Mytilus edulis bed in
Pomeranian Bay, southwestern Baltic Sea.
It can be argued that a simple "inside-outside" sampling study is a weak test
of the adult-larval interaction hypothesis. The observed differences might be
due to correlations with some third factor, rather than a cause-and-effect
relationship caused by the activities of the mussels themselves. Field manipulations can be extremely useful in situations like these. Commito and Bon-
J.A. Commito and N.M.J.A. Dankers
cocoons out of which juveniles crawl. There are no free-living larvae in the
water column where they would be exposed to ingestion by mussels. Thus,
both of the hypotheses were falsified. On the other hand, non-oligochaete species as a group were less abundant inside the bed than outside, indicating that
mussel beds are not the optimal environment for infaunal species with freeswimming larvae.
Based on these results, Commito (1987) suggested a modification of the
original Woodin hypotheses. Densely packed suspension-feeding bivalves
may indeed lower the density of species with free-swimming larvae or an
inability to withstand low oxygen conditions in sediment to which finegrained feces and pseudo feces have been added. However, they may harbor or
even enhance the abundance of infaunal species whose dispersal stages can
avoid ingestion by suspension feeders or suffocation in their fecal and pseudofecal material. These species include oligochaetes that produce cocoons as
well as taxonomic groups with asexual fragmentation (some oligochaetes and
polychaetes), brooding (some polychaetes, bivalves, crustaceans), and postlarval dispersal of relatively large juveniles (many types of macrofauna, e. g.,
Commito et al. 1995).
Dittmann (1990) conducted an intensive comparison of the intertidal
macrofauna and meiofauna in samples taken from the edge and center of a
Mytilus edulis bed and the adjacent sandflat at Konigshafen on the Wadden
Sea island of Sylt. She demonstrated that the sandflat macrofauna had substantial numbers of oligochaetes (Tubificoides benedeni and two similar species), but was dominated by polychaetes with free-swimming larvae. The
center of the bed had a shift in species composition. Polychaetes were less
abundant and oligochaetes were more abundant, leading to the same absolute
and relative dominance by oligochaetes as in the Mytilus edulis bed studied by
Commito (1987). Of the polychaetes that did live in the mussel bed, there was
a dramatic shift towards Capitella capitata and Heteromastus filiformis,
deposit-feeding species very similar to the oligochaetes in size, morphology,
and feeding mode. Subsequent work on Mytilus edulis beds from two sites on
the west coast of Denmark (Svane and Setyobudiandi 1996) and the East
Frisian Islands in the German Wadden Sea has shown similar patterns for
macrofauna (Gunther 1996; Kroncke 1996). As she found for the macrofauna,
Dittmann observed that the mussel-bed meiofauna had a different relative
species composition and higher density compared to the bare sandflat, results
similar to those found by Radziejewska (1986) for a Mytilus edulis bed in
Pomeranian Bay, southwestern Baltic Sea.
It can be argued that a simple "inside-outside" sampling study is a weak test
of the adult-larval interaction hypothesis. The observed differences might be
due to correlations with some third factor, rather than a cause-and-effect
relationship caused by the activities of the mussels themselves. Field manipulations can be extremely useful in situations like these. Commito and Bon-