Scallops are one of the few bivalve molluscs that can swim. They are not graceful and they are not fast, but they can move when threatened. While temperate and tropical scallops are only just able to swim in warm waters, it is remarkable that Antarctic scallops manage at all in Antarctica's exceedingly cold and viscous environment; yet they have somehow overcome this challenge. However, Mark Denny and Luke Miller were quite surprised that the only evolutionary adaptation to the cold they could find was in the properties of the rubber hinge that holds the shell together(p. 4503).
Scallops swim using jet propulsion. The two halves of the shell - the valves - quickly clamp together when the muscle connecting them contracts and the water trapped inside is squirted out. When the muscle relaxes, the properties of the rubber hinge cause the valves to spring open. But having to move a large flat shell through water and having to expel the trapped water is a huge strain on the scallops. `It's my impression that they are hanging on to their swimming ability by the tips of their toes', remarks Denny, a biomechanist who studied these scallops while on a trip to the Antarctic.
Denny and Miller started checking for adaptations that would help these cold-water scallops to swim. The most obvious change is that the shell is much lighter and easier to lift off the seabed. This may be an adaptation for swimming, or it may be down to the fact that there are far fewer predators in the Antarctic. While the thin shell will help, Denny expected to find that the muscle that closes the shell would be larger, so that the shell can clap open and closed at the same rate as the temperate species. However, the Antarctic muscle was only half the size of temperate scallops', which makes sense as the lighter shell is fragile and a large muscle might pull away from the shell or even break it when snapping shut. Unfortunately, these two adaptations cancel each other out and fail to explain how the Antarctic scallops swim in freezing conditions.
The only other explanation for the Antarctic scallop's mobility was the mechanism that springs the shell open again - the hinge. Made of a protein called abductin, this biorubber has several properties that could make a difference. Denny explains that abductin is an entropic elastomer; as the hinge is stretched the tangled protein chains rearrange and become more ordered, but quickly spring back into their original disordered state when the hinge is released. Unfortunately, entropic elastomers become less stiff and more viscous when cooled, making abductin a strange choice for a scallop that lives in the cold.
However, when Denny and Miller measured the hinge's resilience - the property that returns the rubber back to the original state - they found that it was slightly higher than temperate abductin's resilience, giving the cold-adapted molluscs the extra edge they need to keep moving in icy waters. Denny comments, `It is one of the intriguing things about evolution - it can be a minor something that it comes down to'. In this case, that minor something appears to be enough to keep these scallops swimming.