Dimorphic sperm and the unlikely route to fertilisation in the yellow seahorse
- 1 February 2007
- journal article
- Published by The Company of Biologists in Journal Of Experimental Biology
- Vol. 210 (3), 432-437
- https://doi.org/10.1242/jeb.02673
Abstract
Uniquely among vertebrates, seahorses and pipefishes (Family Syngnathidae) incubate their eggs within a male brood pouch. This has contributed to a widespread, but poorly founded belief, that the eggs are fertilised using spermatozoa that are deposited directly into the brood pouch via an internal sperm duct. Anatomical dissections showed, however, not only that direct sperm deposition into the pouch is physically impossible, but that spermatozoa must somehow travel a significant distance (>4 mm) outside the body of the male, to reach and fertilise eggs in the pouch. Observations of courtship and mating behaviour also revealed that the pouch closes immediately after mating, and that sperm transfer must occur within a time window of no more than 6 s. In addition to this, the yellow seahorse produces extraordinarily low quantities of dimorphic spermatozoa, but is nevertheless highly fertile and can produce broods that exceed 100 embryos. The entire fertilisation process in seahorses is therefore uniquely efficient among vertebrates, yet paradoxically involves several steps that would seem to complicate, and even appear to prevent, the interaction of the gametes. Although we are still unable to describe the exact fertilisation mechanism, we speculate that spermatozoa are ejaculated into a mixture of ovarian fluid and eggs, while the male and female are in close contact. Thereafter, this mixture must enter the pouch, whereupon the spermatozoa encounter seawater. These observations also support the view, indirectly inferred in previous publications, that sperm competition in seahorses is not only non-existent but impossible.Keywords
This publication has 14 references indexed in Scilit:
- Why is there no sperm competition in a pipefish with externally brooding males? Insights from sperm activation and morphologyJournal of Fish Biology, 2006
- Pollen and sperm heteromorphism: convergence across kingdoms?Journal of Evolutionary Biology, 2005
- Testes investment and spawning mode in pipefishes and seahorses (Syngnathidae)Biological Journal of the Linnean Society, 2004
- Ultrastructural observations of euspermatozoa and paraspermatozoa in a copulatory cottoid fish Blepsias cirrhosusJournal of Fish Biology, 2004
- The Ionic and Osmotic Factors Controlling Motility of Fish SpermatozoaAquaculture International, 2004
- The long and short of sperm polymorphisms in insectsBiological Reviews, 2002
- Sperm Competition and Sexual SelectionPublished by Elsevier BV ,1998
- Sperm Competition and the Evolution of Ejaculates: Towards a Theory BasePublished by Elsevier BV ,1998
- Sperm Competition in Fishes: The Evolution of Testis Size and Ejaculate CharacteristicsThe American Naturalist, 1997
- Some problems related to the assessment of sperm motility in freshwater fishJournal of Experimental Zoology, 1992