Juvenile Mating and Sperm Storage in the Tropical Corallivorous Nudibranch Phestilla sibogae

Abstract

Nudibranch molluscs typically are semelparous, undergoing a single period of repeated spawning, and these opisthobranchs generally are categorized as obligately cross-fertilizing simultaneous hermaphrodites. The tropical corallivorous aeolid nudibranch Phestilla sibogae has a sub-annual life cycle, with individuals cycling from egg to egg in -40 d and adults commonly attaining 30-40 mm in body length (-800-1700 mg live weight). Spawning (egg laying) may commence at 15-25 mm length (-90-480 mg). In a preliminary experiment, 22 individuals from batch cultures were paired at sizes expected to be pre-reproductive (5-7 mm; 3-8 mg) and the isolated couples further reared to first spawning. Larvae from each pair's first egg mass were cultured and juveniles reared to sizes at which they also could be typed electrophoretically. Comparisons between the parental and offspring genotypes for up to three allozyme loci revealed that, for 6 of the 11 pairings, certain offspring did not conform to parental genotypes: in total, 18% of all offspring typed were non-conforming, ranging up to 75% in one pair. Evidently P. sibogae can become inseminated at very small sizes, and may store sperm, despite the opportunity for continued and repeated copulation with a large adult partner. The first received allosperm may display high levels of fertilization success. Three further experiments involved the batch culture of post-larval juveniles and the subsequent isolation of individuals at varying sizes and ages. These latter experiments confirmed that P. sibogae can inseminate, and be inseminated, at extremely small sizes (-5-8 mg) and well in advance of their ability to lay fertile eggs. The relevance of sperm competition models to these data is discussed in the context of apparent first-partner precedence. Additional key words: allozyme, sperm competition, Gastropoda, Mollusca, reproduction, larva, simultaneous hermaphrodite All nudibranch molluscs are simultaneous hermaphrodites, in that the male and female reproductive systems are functional contemporaneously during adult life. Maturation of the male system, however, slightly precedes that of the female system in the annual boreoarctic dorid nudibranch Onchidoris muricata (Todd 1978). Once mature, almost all nudibranch species repeatedly deposit egg masses over a more or less defined period before death of the adult. Morphologically, the opisthobranch reproductive system is extremely complex (e.g., Kress & Schmekel 1992) and varies in structure and organization according to species; the very complexity of the system is functionally focused on the separation, not only of the male and female a Author for correspondence. E-mail: cdt@st-and.ac.uk b Present address: Department of Entomology, University of Kansas, Lawrence, KS 66045, USA systems, but also of the routes followed by autosperm and allosperm (Beeman 1970a,b,c; Thompson 1976; Hadfield & Switzer-Dunlap 1984). It is likely that no nudibranch species are self-fertile and copulation probably results in reciprocal transfer of sperm, which may or may not be simultaneous (reviewed by Hadfield & Switzer-Dunlap 1984). There are no records of unilateral copulation, or the formation of the copulatory chains typical of aplysiids (Hadfield & SwitzerDunlap 1984), and very few records of spermatophores in nudibranch reproduction (Tardy 1965). Functional allosperm may be stored, perhaps for extended periods, in a diverticulum of the reproductive system. Storage of allosperm occurs in the receptaculum seminis-a gonoduct evagination-and their heads orient to the endothelial lining of the receptacle (Hadfield & Switzer-Dunlap 1984). Spawning of unfertilized oocytes may occur in nuThis content downloaded from 207.46.13.179 on Sun, 17 Apr 2016 07:40:19 UTC All use subject to http://about.jstor.org/terms Juvenile mating and sperm storage in a nudibranch