Defenses in other adult bivalves, such as clams, are mostly passive, relying on thick shells and burrowing for protection against predators.  Mussels, however, live on rocks in the intertidal zone and are openly exposed to predators.  Paradoxically, most species of mussel, the bay mussels Mytilus trossulus and M. galloprovincialis being good examples, seem poorly designed for defense: too thin to withstand crushing forces of crabs and fishes, too soft to prevent boring by snails, yet still too brittle to prevent being cracked by birds, and with closing muscles too weak to prevent being pulled apart by sea stars.  Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

So, how do mussels survive in face of potential predators such as fishes & birds, considered here, and SEA STARS, LOBSTERS & CRABS, and SNAILS, considered in other sections?

Several species of birds, including black
oystercatchers Haematopus bachmani, commonly prey
on mussels. Here are a few leaving the mussel bed

Pile perches Rhacochilus vacca commonly eat bay mussels Mytilus trossulus and possibly also juvenile sea mussels M. californianus.  A perch grabs a mussel in its jaws, tears it from its attachment, and crushes it between large grinding plates situated at the back of the pharynx.  The importance of predation by perches or other fishes on the population ecology of mussels is not known.  Brett 1979 Can J Zool 57: 658. Photograph courtesy James Watanabe, Stanford Univ, California.

NOTE  the feeding behaviour of perches is considered in more detail elsewhere in the ODYSSEY: LEARN ABOUT LITTORINES: PREDATORS & DEFENSES: FISHES

Pile perch Rhacochilus vacca 0.6X

Sea mussels Mytilus californianus are generally preferred prey for oystercatchers Haematopus bachmani  and other shorebirds, but does this change seasonally as tissue content of paralytic-shellfish poisoning toxins (PSPTs,  specifically, saxitoxins) increases in the prey? Note in the graph on the Left how PSPT levels increase during summer in mussels correlative with a local bloom in PSPT-containing dinoflagellates Alexandrium catenella. This interesting question is investigated by researchers at California State University Monterey Bay.  Results show, indeed, that when tissue concentrations of PSPTs exceed 150u saxitoxin eq ^. 100g^-1 the oystercatchers tend to discard the mussel tissue and switch to non-PSPT-accumulating limpet prey Lottia spp. (see graph on Right).  Based on behavioral observations of feeding birds, it seems (not surprisingly) that they taste the prey before swallowing, and then discard any tissues with higher-than-threshold levels of PSPTs.  Kvitek & Bretz.2005 Mar Ecol Progr Ser 293: 303.

NOTE  the authors mention other shorebird species whose behaviour is modified by content of PSPTs in their prey, including godwits, sanderlings, whimbrels, and willets, but the data they present focus mainly on oystercatchers.  The authors also include another prey species in the study, namely, sand crabs Emerita analoga but, as the results for this species are similar to those for mussels, they are not included here