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Open Access Highly Accessed Research article

Why small males have big sperm: dimorphic squid sperm linked to alternative mating behaviours

Yoko Iwata12*, Paul Shaw1, Eiji Fujiwara3, Kogiku Shiba4, Yasutaka Kakiuchi56 and Noritaka Hirohashi6*

Author Affiliations

1 Institute of Biological, Environmental and Rural Sciences (IBERS), Aberystwyth University, Penglais, Aberystwyth, SY23 3DA, UK

2 Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Chiba 277-8564, Japan

3 Documentary Channel Co. Ltd., Kawaguchi, Saitama 333-0844, Japan

4 Shimoda Marine Research Center, University of Tsukuba, Shimoda, Shizuoka 415-0025, Japan

5 Science and Education Center, Ochanomizu University, Bunkyo, Tokyo 112-8610, Japan

6 School of Humanities and Sciences, Ochanomizu University, Bunkyo, Tokyo 112-8610, Japan

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BMC Evolutionary Biology 2011, 11:236  doi:10.1186/1471-2148-11-236

Published: 10 August 2011

Abstract

Background

Sperm cells are the target of strong sexual selection that may drive changes in sperm structure and function to maximize fertilisation success. Sperm evolution is regarded to be one of the major consequences of sperm competition in polyandrous species, however it can also be driven by adaptation to the environmental conditions at the site of fertilization. Strong stabilizing selection limits intra-specific variation, and therefore polymorphism, among fertile sperm (eusperm). Here we analyzed reproductive morphology differences among males employing characteristic alternative mating behaviours, and so potentially different conditions of sperm competition and fertilization environment, in the squid Loligo bleekeri.

Results

Large consort males transfer smaller (average total length = 73 μm) sperm to a female's internal sperm storage location, inside the oviduct; whereas small sneaker males transfer larger (99 μm) sperm to an external location around the seminal receptacle near the mouth. No significant difference in swimming speed was observed between consort and sneaker sperm. Furthermore, sperm precedence in the seminal receptacle was not biased toward longer sperm, suggesting no evidence for large sperm being favoured in competition for space in the sperm storage organ among sneaker males.

Conclusions

Here we report the first case, in the squid Loligo bleekeri, where distinctly dimorphic eusperm are produced by different sized males that employ alternative mating behaviours. Our results found no evidence that the distinct sperm dimorphism was driven by between- and within-tactic sperm competition. We propose that presence of alternative fertilization environments with distinct characteristics (i.e. internal or external), whether or not in combination with the effects of sperm competition, can drive the disruptive evolution of sperm size.