50
J.A. Commito and N.M.J.A. Dankers
producing oligochaetes that thrive in organically enriched sediment, amphipods probably do not do well in fine sediments with low oxygen levels.
Interestingly, the New Zealand site where amphipods were less abundant
inside pinnid patches was the site that also showed dramatically higher siltclay fractions and organic content and smaller median grain size inside
pinnid patches. Sediment characteristics were virtually identical inside and
outside pinnid patches where the infaunal densities were not significantly
different. Thus, the results from Chile and New Zealand seem to be consistent
with expectations from the intertidal investigations carried out on dense beds
of Mytilus edulis. Clearly, however, we need more studies, including experimental manipulations, to reveal the differences and similarities in mussel-bed
impacts from different parts of the world.
2.5 Mechanisms of Mussel-Bed Impacts on Soft-Bottom
Community Structure
The mechanisms that cause mussel beds to alter the relative densities and
species composition in soft-bottom systems have not been extensively
studied. The explanation offered above is that some species of infauna can
avoid ingestion and suffocation by mussels because they do not have freeswimming larvae, can withstand the low oxygen and high sulfide levels of
mussel bed sediment enriched with mussel feces and pseudofeces, and are
able to exploit that material as a food source. These species have enhanced
densities in mussel beds, while other species experience density reductions.
However, this explanation has not been explicitly tested and fails to include
other possibilities described by Commito (1987) and Commito and Boncavage (1989).
Recent studies of invasive mussels provide some insights into the mechanisms causing mussel-bed effects on infauna. They have been directed
specifically at the impacts of invaders on existing community structure. They
emphasized experimental manipulations, including experiments designed to
separate active effects caused by the biological activities of the mussels from
the passive effects due to the presence of physical structure. For example,
rubber mussel mimics and empty mussel valves had many of the same effects
on community structure as live zebra mussels, Dreissena polymorpha, on hard
substrates (Slepnev et al. 1994; Ricciardi et al. 1997).
Mytilus edulis impacts might be similar to those observed by Crooks (1998)
and Crooks and Khim (1999) for an introduced mussel, Musculista senhousia,
in California. This marine species forms dense byssal mats in relatively shortlived patches in the soft-bottom intertidal zone. The presence oflive or mimic
J.A. Commito and N.M.J.A. Dankers
producing oligochaetes that thrive in organically enriched sediment, amphipods probably do not do well in fine sediments with low oxygen levels.
Interestingly, the New Zealand site where amphipods were less abundant
inside pinnid patches was the site that also showed dramatically higher siltclay fractions and organic content and smaller median grain size inside
pinnid patches. Sediment characteristics were virtually identical inside and
outside pinnid patches where the infaunal densities were not significantly
different. Thus, the results from Chile and New Zealand seem to be consistent
with expectations from the intertidal investigations carried out on dense beds
of Mytilus edulis. Clearly, however, we need more studies, including experimental manipulations, to reveal the differences and similarities in mussel-bed
impacts from different parts of the world.
2.5 Mechanisms of Mussel-Bed Impacts on Soft-Bottom
Community Structure
The mechanisms that cause mussel beds to alter the relative densities and
species composition in soft-bottom systems have not been extensively
studied. The explanation offered above is that some species of infauna can
avoid ingestion and suffocation by mussels because they do not have freeswimming larvae, can withstand the low oxygen and high sulfide levels of
mussel bed sediment enriched with mussel feces and pseudofeces, and are
able to exploit that material as a food source. These species have enhanced
densities in mussel beds, while other species experience density reductions.
However, this explanation has not been explicitly tested and fails to include
other possibilities described by Commito (1987) and Commito and Boncavage (1989).
Recent studies of invasive mussels provide some insights into the mechanisms causing mussel-bed effects on infauna. They have been directed
specifically at the impacts of invaders on existing community structure. They
emphasized experimental manipulations, including experiments designed to
separate active effects caused by the biological activities of the mussels from
the passive effects due to the presence of physical structure. For example,
rubber mussel mimics and empty mussel valves had many of the same effects
on community structure as live zebra mussels, Dreissena polymorpha, on hard
substrates (Slepnev et al. 1994; Ricciardi et al. 1997).
Mytilus edulis impacts might be similar to those observed by Crooks (1998)
and Crooks and Khim (1999) for an introduced mussel, Musculista senhousia,
in California. This marine species forms dense byssal mats in relatively shortlived patches in the soft-bottom intertidal zone. The presence oflive or mimic