Chief predators of scallops are sea stars, most notably fast-moving species like the sunflower star Pycnopodia helianthoides, octopuses, and sea otters Enhydra lutris. Little is known about the role of predation in the population ecology of west-coast scallops. The shell is the first defense, but little or no research has been done on this topic for west-coast species.

Scallop Chlamys spp. and clam shells adorn the den of
Enteroctopus dofleini on the outer coast of
Vancouver Island, British Columbia 0.7X

Attachment of Chlamys spp. to the substratum is usually by 1-2 byssus threads.  The function of the threads is unclear. They are easily broken when the scallop starts to swim and would appear to offer no defense if the scallop is attacked by a predator.  Perhaps they function to anchor the scallop in correct orientation to the current for feeding or predator-sensing, or for correct take-off angle during swimming escape.

Rock scallops Crassadoma gigantea also use byssus threads to anchor to the substratum, but only for the first few years of life.  After that they cement themselves firmly to the substratum.  As cemented individuals are extremely difficult to remove, it is likely that the strategy has been selected for its anti-predator value.  No studies, however, have been done on this subject. Photograph of C. gigantea courtesy Dave Cowles, Walla Walla University.

A Chlamys attached to the substratum by byssus thread(s). The single adductor muscle, responsible for holding the valves closed and for rapid contraction during swimming, is visible at the back of the mantle cavity 1X

Rock scallop Crassadoma gigantea attached to a rock.
The early pre-attachment juvenile shell is visible 0.7X

After a few years of byssus-thread tethering and at a size of about 30mm in shell height, the juvenile rock scallop Crassadoma gigantea cements its right shell-valve immovably to the substratum. The firmness of the anchoring, however, is not fixed and several environmental factors may affect it.  A study by researchers at the Marine Sciences Institute, Santa Barbara shows that attachment is facilitated by flat, hard surfaces and inhibited by fibrous ones.  Second attachment, if it occurs, does not appear to be affected by previous attachment history.  Attachment is usually possible at any size, except in cases where shell-growth irregularity prevents contact of the outer mantle lobe with the substratum.  Finally, and most important for the commercial culture trade, growth is significantly greater for attached scallops (see graph).  Culver et al. 2006 Aquaculture 254: 361.