Reproduction, Fertility and Development Reproduction, Fertility and Development Society
Vertebrate reproductive science and technology
RESEARCH ARTICLE

Variable sperm size and motility activation in the pipefish, Syngnathus abaster; adaptations to paternal care or environmental plasticity?

B. B. Dzyuba A , K. J. W. Van Look B , V. S. Kholodnyy A , N. Satake B , S. Cheung B and W. V. Holt B C
+ Author Affiliations
- Author Affiliations

A Institute for Problems of Cryobiology and Cryomedicine of the National Academy of Sciences of the Ukraine, 23 Pereyaslavskaya Street, Kharkov 61015, Ukraine.

B Institute of Zoology, Zoological Society of London, Regent’s Park, London NW1 4RY, UK.

C Corresponding author. Email: bill.holt@ioz.ac.uk

Reproduction, Fertility and Development 20(4) 474-482 https://doi.org/10.1071/RD07221
Submitted: 10 December 2007  Accepted: 24 January 2008   Published: 11 April 2008

Abstract

Like seahorses, some of the closely-related pipefish species (Family Syngnathidae) incubate their eggs within a male brood pouch. This has contributed to considerable confusion about sperm transfer mechanisms to the eggs; some authors have reported that ejaculates are released directly into water before they reach the eggs, while others have suggested that eggs are fertilised using spermatozoa deposited directly into the brood pouch via an internal sperm duct. Here we present anatomical evidence from the freshwater pipefish, Syngnathus abaster, showing not only that direct sperm deposition into the pouch is 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. We have also used several putative sperm-activating solutions to identify the type of environment most conducive to sperm activation. Spermatozoa released from the testis were active for a brief period (<5 min) in water or 150 mm saline, but showed prolonged (>25 min) motility in ovarian fluid. This suggests that spermatozoa are released into a mixture of ovarian fluid and eggs while the male and female are in close contact. Our data also suggest that the fertilisation mechanism is highly efficient (sperm : egg ratio <200 : 1) even though this pipefish species produces dimorphic spermatozoa (with long and short flagellae). The shorter (<40 μm) morphotypes were not capable of motility activation, and are therefore probably incapable of fertilisation. If so, the sperm : egg ratio reported here would represent an overestimate.

Additional keywords: dimorphism, pouch, spermatozoa, sperm : egg ratio, sperm morphology, sperm transport.


Acknowledgements

This study was supported by grants from The Royal Society and the North of England Zoological Society.


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