The myth of the sperm cloud – or why is testes size not affected by spawning mode in seahorses and pipefishes

Abstract

One can predict fish species that spawn externally to have larger testes for their body size than internal fertilizers, to produce greater numbers of sperm as a response either to increased risk of sperm competition from sneaker males, or to counter the dilution of sperm in open water. Using museum specimen, we measured testis and body mass of 95 mature males, belonging to 21 genera of the family Syngnathidae (pipefishes and seahorses). In this family all species show paternal care, ranging from eggs being simply attached to the skin of the male, to completely enclosed in a brood pouch. The former unprotected group is thought to fertilize the eggs externally, whereas the latter protected group has internal fertilization, as the male fertilizes the eggs inside his own pouch. Hence, expecting smaller relative testis investment in the genera that have protected compared to unprotected brood care, to our surprise we found this not to be the case. Instead, all genera showed the same relationship between testis and body mass, regardless of brooding type. New results of ours, however, from Nerophis ophidion, a pipefish species that lacks brood pouch, show that the mode of fertilization in older literature (external fertilization after egg transfer when the male sinks through a sperm cloud) has been misjudged, and that this pipefish in fact, has internal fertilization inside the female before egg transfer. If this exciting finding proves to be true for other pouchless Syngnathids as well, the mystery of the museum data will be resolved.