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Biol. Bull. 215: 115-125. (October 2008)
© 2008 Marine Biological Laboratory

Sperm Polymorphism Within the Sea Urchin Strongylocentrotus droebachiensis: Divergence Between Pacific and Atlantic Oceans

Jessica A. Marks1,2,*, Christiane H. Biermann2,3,4, Walter F. Eanes4 and Harald Kryvi1

1 Department of Biology, University of Bergen, N-5007 Bergen, Norway
2 Friday Harbor Laboratories, University of Washington, Friday Harbor, Washington 98250
3 Department of Biology, Portland State University, Portland, Oregon 97207-0751
4 Department of Ecology and Evolution, Stony Brook University, Stony Brook, New York 11794-5245

* To whom correspondence should be addressed, at CEES, Biol. Dept., Univ. of Oslo, Norway. E-mail: Jessica.Marks{at}bio.uib.no

The rapid evolution of traits related to fertilization such as sperm morphology may be pivotal in the evolution of reproductive barriers and speciation. The sea urchin Strongylocentrotus droebachiensis has a circumarctic distribution and shows substantial genetic subdivision between northeastern Atlantic populations and northwestern Atlantic and Pacific populations. Using transmission electron microscopy, we show here that sperm shape, size, and ultrastructure differ markedly among populations of S. droebachiensis from different oceans and reflect patterns of genetic divergence. Sperm nuclei from northwestern Atlantic and Pacific populations were longer and narrower than those from the northeastern Atlantic. We additionally demonstrate population-level differences in the amount and location of filamentous actin (F-actin) prior to the occurrence of the acrosome reaction. Sperm from Pacific and northwest Atlantic populations differed from that of all other echinoids examined in that intact sperm contains a partly preformed acrosomal process, a structure more closely resembling the acrosomal rod seen in some molluscs. Immunofluorescent studies using anti-bindin antibodies and the F-actin-specific stain phalloidin confirmed these findings. Divergence of reproductive traits such as sperm morphology may be related to divergence in gamete compatibility and genetic divergence, and could represent the first stages of speciation in free-spawning marine invertebrates.






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