Iridescent Scallop

Overview

The Iridescent Scallop (Pedum spondyloideum) is a unique, sessile bivalve that spends its entire life embedded within the skeletons of living stony corals. As the only member of its genus, this specialized mollusk is widely distributed throughout the tropical Indo-Pacific, where it relies on its coral host for both structural support and protection.

Appearance

While the scallop's thin, fan-shaped shell remains hidden within the coral, its vibrant mantle is often visible at the opening of its burrow. This mantle typically displays striking shades of violet, mauve, or pale blue with iridescent highlights. Along the mantle's edge, the scallop possesses numerous light-sensitive eyes, which allow it to detect movement and rapidly retract into its shell when threatened.

Biology & behavior

This species is a "nestling" bivalve that secures itself to a coral host using byssal threads during its larval stage. As the coral grows, it eventually surrounds the scallop, leaving only the mantle and shell opening exposed. The scallop may actively maintain its space within the coral through chemical secretions. It is a filter feeder, drawing in microparticles from the surrounding water. When disturbed, it can defend its host and itself by expelling powerful jets of water, a behavior that may help deter corallivorous predators like the crown-of-thorns starfish. Like other members of the Pectinidae family, this species is hermaphroditic.