Mating behavior of nemerteans: present knowledge and future directions

Abstract

In most nemertean species, members of the two sexes aggregate before fertilization takes place. Few specific studies on the mating behaviour of nemerteans have been conducted but several observational reports indicate that important processes known from other organisms, such as sexual selection and sperm competition, may also be at work in nemerteans. Herein, we review some of these observations and discuss their possible implications. We produce a summary table and reproduce some important observations, placing them in an evolutionary context. Four types of gamete‐transfer mechanisms are distinguished: (1) free‐spawning, where members of both sexes release gametes freely into the water column; (2) mucus‐spawning, where gametes are released within a mucus matrix; (3) internal fertilization, where spermatozoans are transferred to the immediate vicinity of oviducts, which they penetrate; and (4) gamete transfer aided by specific structures. While little is known about the last transfer mechanism, anecdotal observations are mainly available for the first three mechanisms. Mating interactions frequently involve more than two individuals. In several species, individuals only invest part of their gametes in a single mating interaction, apparently saving additional gametes for potential future mating events. These observations suggest that males of some species are exposed to mating competition. Males might counteract sperm competition by producing large numbers of sperm, as suggested by their high fertilization potential. Previous authors inferred that sperm morphology may be an adaptation to spawning behavior and possibly also to the risk of sperm competition. Based on the results of our analysis and the above observations, we suggest that sexual selection and sperm competition are not uncommon in nemerteans.