J. Moll. Stud. (1989), 55, 455-459
© The Malacological Society of London 1989
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SPAWNING SEASON AND EGG PRODUCTION IN FORTH ESTUARY RETUSA OBTUSA (MONTAGU) (GASTROPODA: OPISTHOBRANCHIA)
School of Molecular and Biological Sciences, Stirling University FK9 4LA
Retusa obtusa held individually in the laboratory carried large oocytes in the gonad but did not spawn in December-late January. In late Jan-early Feb, however, the first few individuals produced batches of 8–17 eggs and, by early February, most individuals were producing means of 2–4 eggs d–1 and maxima in late Feb of <7.9 d–1. Spawning declined to low levels in few individuals in late March. By early April, spawning had virtually ceased prior to the drastic loss of adults in the population by May.
Larger R. obtusa generally produced more eggs than smaller ones and the few specimens shorter than 3.8 mm produced markedly fewer than those of 3.8–5.0 mm shell length.
Twenty of the 34 R. obtusa held in the main breeding period (4 sets started 29 Jan-3 Mar) spawned in 3 or more successive weeks whereas only 6 of them failed to spawn in 2 consecutive weeks, so indicating that several successive batches of eggs are commonly produced. Yet in virtually all individuals (except at the start of breeding) more eggs were produced in the first week of captivity than subsequently. These first-week values imply total production by individuals in the spawning season of some 112-305 eggs comprising about 2.8-7.6 mg dry weight or, by the Forth estuary population, of 26880-73200 eggs m–2 comprising 0.67-1.83 g dry weight m–2.
(Received 19 August 1988; accepted 30 September 1988)