Reproductive tactics and mating contexts affect sperm traits in horseshoe crabs (Limulus polyphemus)
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In systems with male alternative reproductive tactics, discrete behavioral patterns often lead to unequal levels of sperm competition across tactics. In many of these systems, the disadvantaged tactic shows adaptations to sperm traits that increase the competitive ability of its sperm. However, many studies have found that not all sperm traits differ across tactics, and in some systems no sperm trait adaptations are found. Why some systems and some sperm traits display tactic-specific differences and others do not is not always clear. Here, we investigate three sperm traits—ejaculate size, sperm concentration, and sperm velocity—in the American horseshoe crab, Limulus polyphemus, a system where males have condition-dependent reproductive tactics and females influence the amount of sperm competition males face during spawning. Contrary to our predictions that the disadvantaged tactic would show sperm trait adaptations, we found that horseshoe crabs of the tactic that faces the least amount of sperm competition had more concentrated sperm than the males of the other tactics. We propose a number of hypotheses to explain this result and suggest that future studies of alternative reproductive tactics might benefit from considering the role of females in influencing tactic-specific sperm trait adaptations.
KeywordsSperm competition Reproductive tactics Sexual selection Horseshoe crab Limulus
This work was supported by the National Science Foundation (IOB 06-41750); the PADI Foundation; the University of Florida Division of Sponsored Research; the Department of Biology; and the UF Marine Laboratory at Seahorse Key. We thank our field assistants Melissa Clark, Jessica Diller, Mariantu Robles Zarate, and Natalie Williams; other members of the Brockmann Lab including Matthew Smith and Katie Saunders; and the many undergraduate volunteers who helped with data collection. We also thank Harvey Lillywhite, Henry Coulter, Al Dinsmore, and Bronko Gukanovich of the UF Marine Laboratory at Seahorse Key for their help with field logistics. Our research was conducted under a permit from the Cedar Keys National Wildlife Refuge. Finally, we thank two anonymous reviewers for their helpful comments in improving this manuscript.
Compliance with ethical standards
This study was funded by the National Science Foundation (IOB 06-41750) and the PADI Foundation (2008 Req. No. 391).
Conflict of interest
The authors declare that they have no competing interests.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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