Annachlamys flabellata

Lore & Background

Annachlamys flabellata has a shell that is inequivalve and moderately compressed, with the right valve more inflated and less convex than the left. The shell is circular with two broad auricles extending on either side of the umbones, and a straight hinge line exactly in line with the umbones. There are 18 to 20 broad radial ribs, more prominent on the right valve, with concentric growth rings. Some or all ribs on the left valve are pink, orange or mauve, while the right valve is entirely white. The inside of the valve often has a red tinge or a concentric ring of red dots.

This species is found on sandy seabeds in the neritic zone off the coasts of Western Australia, Northern Territory, Queensland, New South Wales, New Guinea, and Indonesia. It lies covered with a thin layer of substrate and is a filter feeder, drawing water into its gill chambers through a gap between the valves at the back of the shell. Food particles, mostly microscopic algae, are sieved out and transported to the mouth.

Individuals become mature at about 4.5 centimetres in length. They exhibit partial protandric sex reversal: smaller individuals are mostly males, but females predominate among larger individuals, though some small females and large males exist. In lagoons in New Caledonia with little thermal variation, spawning occurs at intervals throughout the year, with a maximum from November to March. Spawning shows no correlation to sea temperature, salinity changes, or moon phases. Male gonads are cream coloured, female gonads bright red; when ripe they are enlarged and visible in the live animal. Eggs are fertilised externally, and larvae develop into veliger larvae that disperse with currents before settling and metamorphosing into juveniles.

Reader's Guide

Annachlamys flabellata is notable for its distinctive shell coloration and partial protandric sex reversal, where smaller individuals are mostly male and larger ones mostly female. This species is found across a broad geographic range north of Australia, from Western Australia to New South Wales, as well as in New Guinea and Indonesia, inhabiting sandy seabeds in the sublittoral zone down to 127 metres. Its filter-feeding biology, drawing water through a gap between the valves to capture microscopic algae, is typical of scallops. The species exhibits a spawning period from November to March in New Caledonian lagoons, with no correlation to temperature, salinity, or lunar phases. The shell's inequivalve structure, with 18 to 20 radial ribs and pink, orange, or mauve coloring on the left valve, distinguishes it from the closely related Annachlamys reevei, which has narrower interstices between ribs. The presence of red tinges or concentric rings of red dots inside the valve adds to its identification features. Its life cycle includes external fertilization and a planktonic larval stage, contributing to its dispersal across the region.

Did You Know?

• Spawning in New Caledonian lagoons occurs year-round, with a maximum from November to March, and shows no correlation to temperature, salinity, or moon phases.
• The shell has 18 to 20 broad radial ribs, with the space between ribs narrower than in the closely related Annachlamys reevei.