Two species of west-coast swimming scallops Chlamys hastata and C. rubida commonly have sponges growing on their valves. While sponges will grow on both valves of a scallop, they do better on the left valve – the one normally in the upright position when the scallop is at rest. The relationship could be considered a parasitism because the growth is known to interfere with the swimming of the scallop through increased drag forces, but is generally accepted to be a mutualism because benefit is thought to accrue to both partners.

An experiment done on scallops at Friday Harbor Laboratories, Washington assesses the possible protective function of the sponge for the scallop.  The author first cleans the sponge coatings from half of a collection of scallops, then subdivides the 2 groups and binds half of each subdivision.  Each of the 4 treatment groups (sponge-coated/unbound, sponge-coated/bound, spongeless/unbound, and spongeless/bound) is now allowed to be hunted down and eaten by predatory sea stars Orthasterias koehleri over a 90-d period. Results show that unbound scallops swim to safety whether they have a coating of sponge or not.  However, bound scallops without sponge coatings are caught relatively quickly and eaten, while bound ones with their sponge coatings intact last a bit longer.  The author suggests that scallops with sponge coatings may be difficult for a sea star to grab hold of with its tube feet and therefore concludes that the sponge provides at least some protection for its host. Bloom 1975 J Exp Mar Biol Ecol 17: 311.

NOTE  the sponges are Myxilla incrustans or Mycales adhaerens, and the author remarks that it is difficult to determine the species of scallop if both valves bear a coat of sponge

NOTE  the binding involves tying elastic bands around the valves and wedging the valves open with pieces of plastic.  In the actual experiment the number of scallops in each  of the 4 treatments differs slightly from the 1:1:1:1 proportions described here

Some ideas generated by the data are presented below, some obvious; others, not so obvious.  Consider their validity, then CLICK HERE for explanations.

Sea stars Orthasterias koehleri like to eat bound and helpless scallops. 

Sponges are known to have spicules and/or distasteful chemicals and perhaps these play a role in the sea-star’s aversion to sponge-coated scallops. 

Sponge-covered or not, the scallop still swims, so the presence of the sponge is irrelevant to the health of the scallop. 

So, the scallop benefits by having a protective sponge coating, and this is so valuable that it compensates for the extra energy required to swim.  But what does the sponge get out of it? Think about the ideas listed below then CLICK HERE for explanations.

The sponge, if residing on the upper valve of the scallop, is provided a clean place to live and grow. 

The sponge gets to sample different habitats. 

The sponge gains protection from its own predators. 

A more recent study at Friday Harbor Laboratories, Washington takes a different approach to sponge/scallop mutualisms than that taken in the Research Study above. The authors start with the following premises: that scallops with sponges swim spontaneously, scallops with sponges clap their valves regularly when not swimming, sponges on scallops are usually clean, scallops with sponges are preyed upon by few sea stars in the field because most sea stars crawl too slowly and, finally, sponge coatings do not deter aggressive sea-star predators such as sunflower stars, which can crawl fast enough to catch a (tired?) scallop whether it has a sponge coating or not. These researchers set about to test the hypothesis that the major beneficiary in the relationship is the sponge, not the scallop, because it is kept clean by the swimming and valve-clapping activity of its host. 

The researchers maintain sponge-coated scallops in the laboratory to be sure that the scallops swim and clap spontaneously in contained conditions, and they do (see histogram above Right). They then record survival of 2 species of sponges on living scallops over a several weeks in the laboratory and find that both types of sponges survive well (see left set of bars in histogram above Left). Next, they ascertain how well the sponges do on dead scallop shells, but with sediments cleaned off daily by blowing with a gentle flow of clean water (middle set of bars above).  Myxilla incrustans survives well, but M. adhaerens survives less well.  Finally, they determine how well the sponges survive on a dead shell with no daily clearing of sediment, and find that survival of both types of sponges is poor (right set of bars in histogram). Both species turn grey and become necrotic. The authors conclude that the swimming and clapping behaviour of their scallop hosts increases survival of the sponges (at least in a laboratory setting), and that the sponge M. adhaerens appears to be more sensitive to sedimentation than M. incrustans.  The authors note that as clapping is likely a response to buildup of feces and pseudofeces within the scallop’s mantle cavity, then its frequency should increase with sediment load; hence, increasing the survival of the sponges.  Burns & Bingham 2002 J Mar Biol Assn UK 82: 961.

NOTE  these “false feces”, which are not feces at all, represent particles, e.g., silt, that are taken in by the scallop, filtered on its gills, then bound in mucus and rejected.  From time to time the scallop claps its valves to eject this material from the mantle cavity