Sneaky squid: Why small males have big sperm

Posted by Biomechanism 

Male squid (Loligo bleekeri) employ different reproductive strategies depending on their body size. Research published in BioMed Central’s open access journal BMC Evolutionary Biology shows that the divergent mating behavior of male squid has resulted in the evolution of different sperm sizes.

Larger male squids strategy resulted in higher paternity rates - but for smaller males, who cannot win a female by fair means, being sneaky gives them a chance of passing on their genes.

Large male squid compete for females by courting them with flashy skin color-change displays. Once a female has chosen her partner they mate in an above and below position so that the male can place his sperm inside the female’s oviduct. He remains with the female until she spawns, ensuring that his sperm fertilize her eggs and that no other males have a chance to mate with her. At the moment a female lays her eggs, small ‘sneaker’ males rush in and mate with her, head to head. These small males place packages of sperm by the female’s mouth in the hope that their sperm have a chance of fertilizing the eggs as they leave the female’s body.

When researchers from London and Japan looked at the sperm produced by small sneaker males and large consort males they discovered that the sperm produced by the sneaker males was larger than that of the consorts. Dr Yoko Iwata from University of Tokyo said, “Sperm size is likely to be an adaptation to fertilization environment, either inside the female or externally, rather than competition between sperm, because the fertility and motility of sneaker and consort sperm were the same.”

Overall, the larger males’ strategy resulted in higher paternity rates – but for smaller males, who cannot win a female by fair means, being sneaky gives them a chance of passing on their genes.

The complete study is available in PDF:  Why small males have big sperm: dimorphic squid sperm linked to alternative mating behaviours.  

Authors: Yoko Iwata, Paul Shaw, Eiji Fujiwara, Kogiku Shiba, Yasutaka Kakiuchi and Noritaka Hirohashi.

Abstract From The Study

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 um) sperm to a female’s internal sperm storage location, inside the oviduct; whereas small sneaker males transfer larger (99 um) 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.