Copulation duration does not predict sperm transfer in a marine hermaphrodite

Because ejaculate production can impose substantial costs, males are expected to allocate sperm prudently. Such strategic ejaculate allocation can represent male mate choice when it varies with female traits. Since sperm transfer is often difficult to measure directly, many studies rely on proxies such as copulation duration. Previous work in the simultaneous hermaphrodite sea slug Chelidonura sandrana showed that longer copulations occurred with larger or sexually isolated partners, indicating strategic ejaculation. However, the underlying assumption of a tight correlation between copulation duration and sperm transfer remained untested. Using a novel, nonlethal sperm-counting method, we show that sperm transfer in C. sandrana occurred at a predictable rate only early during copulation but subsided thereafter. As a result, total sperm counts varied independently of copulation duration, making the latter an unreliable proxy for sperm transfer. Our work offers perspectives to revisit strategic ejaculation in C. sandrana and to study in detail the fitness effects of consecutive copulatory phases.