Fecundity and timing of oogenesis at high latitudes: Reproductive traits in the brooding bivalve Neolepton cobbi (Cyamioidea: Neoleptonidae)

Abstract

Neolepton cobbi is a small bivalve (maximum length 3.4 mm) occurring on sandy or soft muddy bottoms in the southern Magellan Region. Its life span is 12–14 months and population turnover takes place in the late austral summer (December–January). It is a protandric hermaphrodite; sex change occurs at a nonde-fined size, although females smaller than 1.3 mm length were never found. During the functional male phase, clusters of oogonia were found in the posterodorsal region of the gonad. Among 152 specimens examined histologically, only one case of functional hermaphroditism was found, in which male and female tissues occupied differentiated regions of the gonad. Neolepton cobbi is a brooding species. Secretions from a glandular region of the mantle edge contribute to the agglutination of fertilized eggs, forming elongate, paired egg capsules that are attached to the anteroventral margin of the maternal individual. Up to three pairs of egg capsules were found on each female shell, with a total of about 150 eggs/embryos. Each pair of egg capsules corresponds to one of the three different spawning events during the sole breeding season (austral spring) in which each female participates. The hatchlings are released as small (260 μm length) crawling juveniles. The shell margin at the point of attachment of egg capsules is flattened due to a disruption of growth, resulting in a secondary sexual character. The timing of egg production and the possibility of increasing the number of egg capsules per female according to the time elapsed from the start of the breeding season are viewed as a solution to the limitations on fecundity and space available for brooding, which are imposed by small adult size.