West-coast scallops Chlamys spp. often have sponges, barnacles, tubeworms, and other organisms growing on their shells.  The relationship with sponges is thought to be a mutualism, that is, one in which both partners benefit. The sponge protects the scallop from sea-star predators and the scallop provides the sponge with a clean, sediment-free place to live.  Other growth on scallops, including boring sponges, barnacles, bryozoans, and worms are likely parasites.  The rock scallop Crassadoma gigantea is often infested with the parasitic boring sponge Cliona celata.

NOTE  the defensive contribution of sponges to scallopos is considered in more detail elsewhere in this section: PREDATORS & DEFENSES/SPONGE COATINGS

Two Chamys sp., one with a coating of sponge Myxilla incrustans;
the other, with a heavy settlement of tubeworms Sprirorbis sp. 0.7X

A collection of living scallops Chlamys spp. in San Juan Islands, Washington (principally dredged from 90m depth at Rock Point, Lopez Island) reveals a variety of attached ebibionts including sponges, hydroids, tubeworms, barnacles, bivalves, bryozoans, brachiopods, and tunicates.  Both Chlamys rubida and C. hastata are represented, in the proportion 64:36 in the dredge samples. 

The two species are shown in the photograph on the Right with encrusting sponges removed.  The species can be differentiated by the fine spines on C. hastata and the more rounded shell-shape of C. rubida. Both species attach weakly to the substratum with a byssus thread that extends through a byssal notch in the right valve; hence, the left valve is uppermost and usually most heavily fouled.  The mean % cover on the left valves of 144 specimens collected is 76%, with sponges (mostly Mycale adhaerens), barnacles (Balanus rostratus), and tubeworms (Sabellaria cementarium, Serpula vermicularis, and Spirorbis sp.) accounting for 99% of the area covered. Right valves have a much lower cover of epibionts (17%). All but the sponges are likelyy parasites.  Lescinsky 1993 Palaios 8: 267. Photos courtesy Halard Lescinsky, U Cal, Davis.

NOTE  lit. “spear-shape” L.

NOTE  lit. “reddish” L.

Selected photos of fouled scallop shells:

Does a scallop benefit defensively from a barnacle being on its shell?  The parasite could possibly act as visual or tactile camouflage against predators of the scallop, or perhaps be a physical impediment to soft-mouthed predators such as sea stars.  Studies on barnacle-encrusted Chlamys at Shannon Point Marine Laboratory, Washington show, in fact, that the barnacles provide no significant protection from sunflower stars Pycnopodia helianthoides. When given a choice of barnacle-encrusted or barnacle-free scallops Chlamys hastata, the sea stars are able to catch and eat just as many scallops of each type (see histogram on Right).  Also included in the data is confirmation that a load of sponge (notably Mycale adhaerens) significantly reduces predation by sea stars on the host scallops (statistically homogenous subgroups are indicated by the lines below the bars of histograms).

What does the barnacle get out of it?  Presumably, it is eaten along with its host when captured.  Nonetheless, selection should favour adoption of this unique habitat by a barnacle even if only a small fraction were to be carried to safety by the host during predatory attacks by sea stars.  In an interesting twist, the authors show that barnacles settle significantly more on epibiont-free scallops maintained in cages in the field for 3mo than on sponge-coated scallops. Thus, the sponges potentially protect the scallops from an epibiont (barnacle) that is known to interfere with a scallop’s swimming efficacy and may be found to increase the scallop’s susceptibility to predation.  Farren & Donovan 2007 Hydrobiologia 592:225.

NOTE  the data show a non-significant trend towards greater consumption of the barnacle-encrusted scallops, possibly owing to their slower swimming speeds