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First published online January 17, 2007
Journal of Experimental Biology 210, 432-437 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02673

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Dimorphic sperm and the unlikely route to fertilisation in the yellow seahorse

Katrien J. W. Van Look^1,*, Borys Dzyuba², Alex Cliffe³, Heather J. Koldewey³ and William V. Holt^1,*

¹ Institute of Zoology, Zoological Society of London, Regent's Park, London NW1 4RY, UK
² Institute for Problems of Cryobiology and Cryomedicine of the National Academy of Sciences of the Ukraine, 23 Pereyaslavskaya Street, Kharkov 310015, Ukraine
³ Zoological Society of London, Regent's Park, London NW1 4RY, UK

^* Authors for correspondence (e-mail: katrien.vanlook{at}btopenworld.com; Bill.Holt{at}ioz.ac.uk)

Accepted 22 November 2006

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.

Key words: spermatozoa, sperm transport, pouch, sperm competition, sexual selection, sperm:egg ratio

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