Pedum spondyloideum

Lore & Background

Pedum spondyloideum has two fan-shaped, asymmetrical shell valves: the right valve is wider, larger, smoother, and has a deep byssal notch, while the left valve is flattened with radial lines and spines. Its mantle folds and tentacles are visible; two tentacle types have been observed: wholly white and brown-striped. Like other scallops, it possesses eyes. The species bores into living stony corals, a behavior described as a 'nestling' life habit. Phylogenetics suggests this behavior arose from ancestors that used byssus to attach to a substrate. At least one other species exhibits this life habit. Depending on the shape of its host coral, Pedum may embed itself obliquely (in plate-shaped corals) or deeply (in massive corals). To maintain its crevice, it secretes a chemical that erodes the coral's skeleton and tissues, compared to the distantly related Lithophaga mussels. Juveniles settle on the coral surface, slowly embedding themselves; during this stage, shell width increases faster than shell height. The right valve attaches to the crevice walls, allowing the left valve to open and close. The scallop's growth keeps pace with the coral host's growth. Pallial tissues and tentacles attack coral tissues around the opening, leaving a band with chemical etch marks visible under a microscope. To retract, the scallop pulls its byssus strand.

Reader's Guide

Pedum spondyloideum occupies a unique niche among scallops by boring into live coral, a behavior that distinguishes it from most other members of its family. Its significance lies in its ecological interactions: it may benefit its coral hosts by hindering predation by the crown-of-thorns starfish (Acanthaster spp.), which are repelled by jets of water from the scallops. Corals inhabited by Pedum were left uneaten by the starfish, leaving a remnant region of live coral around the scallops. This species does not 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 suggests the scallops may live for at least 6 years. Its unique boring mechanism, involving chemical erosion and tissue attack, parallels that of distantly related Lithophaga mussels. The species is the only member of its genus and tribe, underscoring its evolutionary distinctiveness within the scallops.

Did You Know?

• Pedum spondyloideum is the only species in the genus Pedum and the only genus in the tribe Pedini.
• It secretes a chemical that erodes coral skeleton and tissues, similar to distantly related Lithophaga mussels.
• Corals inhabited by Pedum were left uneaten by crown-of-thorns starfish, which are repelled by jets of water from the scallops.
• Examination of coral growth rings suggests Pedum may live for at least 6 years.