Larval development of the saucer scallop Amusium balloti (Bernardi) (Mollusca : Pectinidae)

Abstract

Larvae of A. balloti from Queensland and Western Australia were reared from ova with diameters ranging from 57 to 86µm. The straight-hinged veligers grew in shell length from 85 to 163 µm. Moderately protruding umbones developed 8 days after fertilization. The length (L) and height (H) relationship of larval shells was described by the allometric growth curve: H = 0.600L^1.064. The provinculum of 15-day-old larval shells was undifferentiated and each valve had taxodontal teeth, at each end of the hinge line. Eye spots were present and common in pediveligers of 162-242 µm in length. Metamorphosis began 20 days after fertilization at 18-19º C when larvae were 178µm or longer. Settlement began on the 22nd day and lasted 5 days. Newly settled spat developed a byssal notch on the right valve, but attachment by byssal threads was never permanent. The length (L) and height (H) relationships of spat shells was described by the allometric growth equation H = 1.782L^0.857. General appearance of A. balloti larvae was comparable to that of other pectinids. Statistical comparison of height-length relationships of the larval shells of A. balloti and Chlamys asperrimus showed larvae of A. balloti to be the more oblong at lengths greater than 128µm. The most significant characteristic of A. balloti spat shells was the formation of rounded auricles at the end of the hinge-line of each valve.