This is the most common sponge found encrusting scallop shells in the Rosario area. The sponge seems to only encrust the surface and not bore into the shell at all. Sponge predators include the nudibranchs Cadlina luteomarginata, Dialula sandiegensis
, and Archidoris odhneri;
and another nudibranch, Discodoris heathi
, has been observed clinging to them. The symbiosis is likely mutualistic. If one of the major predators of the scallop, the seastar Evasterias troschelii
, encounters the scallop (and the scallop does not swim away) it often turns away if it touches the sponge; likely in response to some secretion or to the spicules from the sponge. The sponge also appears to make it more difficult for the seastar's tube feet to adhere to the scallop. If the sponge is removed from the scallop and the scallop is prevented from swimming, it is readily captured by the seastar. The scallop will also swim from predators of the sponge, such as Archidoris
spp, so the sponge is benefited as well. The swimming scallop may also help carry the sponge into areas with clean water and good currents, and help prevent fouling of the sponge.
Research by Kirt Onthank and Thomas Ewing at the Walla Walla University Rosario Marine Laboratory indicates that sponge encrustation deters scallop predation by octopus Enteroctopus dofleini and Octopus rubescens, both of which may be important scallop predators in our area. Sponges which live on the scallop are also less vulnerable to predation by nudibranchs such as Anisodoris nobilis and Archidoris montereyensis.