Reproductive strategy of the semelparous clam Gaimardia bahamondei (Bivalvia, Gaimardiidae)

Abstract

Abstract. Semelparity is one of the most drastic reproductive strategies found among marine invertebrates. It is frequently found in species whose members have small adult sizes and brood embryos internally. In this study, we describe the reproductive strategy of the bivalve Gaimardia bahamondei to explore the possible causes of the association between semelparity and internal brooding. Males of this species exhibit continuous gonadal activity throughout the breeding season. Apparently continuous spawning of sperm is associated with an abundance of captured sperm in the adfrontal region of the gill filaments of both males and females. Females are capable of brooding three cohorts of embryos simultaneously while also producing three new cohorts of oocytes. This suggests that females are able to generate at least six cohorts of embryos during the breeding season. Embryos are brooded in the suprabranchial chamber and are individually surrounded by a membrane with a projecting peduncle; embryos are anchored by this peduncle to the adfrontal region of the gill filaments. Members of the youngest cohort differ in size, color, and shell ornamentation from members of the two older cohorts. There is no difference between members of the two older cohorts in size, but there is with respect to coloring and shell ornamentation. The importance of the embryonic cohorts in terms of their percentage of the total number of embryos varied among brooding females, suggesting among‐female variation in the timing of release of the oldest cohort of embryos. Members of this cohort break loose from the gills, lose their surrounding membrane, and fall into the ventral region of the suprabranchial chamber, from which they are evacuated to the exterior. Continuous sperm production in males and the production of at least six cohorts of embryos in females suggest that the costs of reproduction are high, which may partly explain the semelparity identified in this species.

Embryo brooding and its effect on feeding in the bivalve Gaimardia bahamondei Osorio & Arnaud, 1984

“…Each embryo is attached to the abfrontal region of the branchial filament by means of a peduncle [13,22]. The gill is homorhabdic and the inner demibranch is larger than the external one; embryos are attached to both [13].…”

“…During its lifetime, each individual exhibits a single, continuous and prolonged reproductive event, in the breeding season (austral spring-summer, [1], when all the gametes have been released, following the death of the adult [37]. The limited information on the species indicates great variability in the number of embryos brooded by similar-sized females, and the presence of three cohorts in different development stages within a single female at any given time [13] suggests that the release of juveniles may be more or less continuous and that G. bahamondei is a sequential brooder [6,13,37]. During the reproductive season, most females brood three cohorts of embryos on the gill simultaneously while maintaining three cohorts of oocytes under development in the gonad.…”

“…During this period, the animals were fed with the microalga Isochrysis galbana twice daily at a concentration of approximately 30,000 cells ml −1 [9,49]. Specimens were classified as female or male according to the presence or absence of embryos, following Chaparro et al [13], who determined histologically that all incubating individuals contain always embryos and developing eggs, whereas all non-incubating individuals >3 mm long contain only developing sperm and are therefore males.…”

“…Clearance rate (CR) was measured in 174 specimens of G. bahamondei of shell length 3.47-7.96 mm, corresponding to adults with developing gonads and capable of brooding [13]. In each of the 17 experimental runs, 13 or 14 glass chambers (volume 250 ml) were used, of which 10-11 contained a single specimen and 3 were kept without animals, serving as controls for particle numbers.…”

“…Each embryo is attached to the abfrontal region of the branchial filament by means of a peduncle [13,22]. The gill is homorhabdic and the inner demibranch is larger than the external one; embryos are attached to both [13]. During its lifetime, each individual exhibits a single, continuous and prolonged reproductive event, in the breeding season (austral spring-summer, [1], when all the gametes have been released, following the death of the adult [37].…”