Pedum spondyloideum
Pedum is a monotypic genus of bivalves belonging to the family Pectinidae. The only species is Pedum spondyloideum and it is the only genus in the tribe Pedini. Sometimes referred to as the iridescent scallop, this species is found throughout the Indo-Pacific.
Description
The two shell valves of Pedum are fan-shaped and asymmetrical: the right valve has a deep byssal notch, is inflected, smoother, and is wider and larger than the left valve, which is flattened, possesses radial lines and spines.The scallop's mantle folds and tentacles are part of its visible anatomy; two tentacle types have been observed: a wholly white tentacle and a brown-striped variety. Like other scallops, Pedum possesses eyes.
Biology
Pedum occupies a unique niche among scallops; this species bores into living stony corals, burrowing into their skeleton, described as a "nestling" life habit. Phylogenetics suggests that this behavior arose from ancestors which used byssus to attach themselves to a substrate. At least one other species exhibits this life habit.The following families were observed to host Pedum in Thailand's Andaman coast:
Depending on the shape of its host, Pedum may embed itself obliquely, or deeply.
To allow for the maintenance of its crevice, Pedum secretes a chemical that erodes the coral's skeleton and tissues, which has been compared to the distantly related Lithophaga mussels. Juveniles initially settle on the surface of a coral colony, slowly embedding itself and expanding its crevice; during this stage, Shell Width increases faster than Shell Height. As it grows, the right valve firmly attaches to the crevice's walls, allowing the left valve to open and close. The scallop's growth keeps pace with its coral host's own growth rate. Additionally, the pallial tissues of the mantle along with the tentacles attack the coral's tissues immediately surrounding the crevice, which maintains the opening necessary for the scallop and prevents it from being entombed. This action leaves a "band" or rim surrounding the opening, which displays chemical etch marks when observed under a microscope. To retract, the scallop pulls their strand of byssus.
These scallops may benefit their hosts by hindering their predation by the crown-of-thorns starfish, which are repelled by jets of water from the scallops. Pedum-inhabited corals were left uneaten by the starfish, leaving a remnant region of live coral around the scallops.
This species does not seem to tolerate the adverse conditions of marine lakes, where mytilid bivalves and polychaete tube worms occupy its filter-feeding niche.
Examination of coral growth rings containing Pedum crevices suggest that the scallops may live for at least 6 years.