Smaller squid have larger sperm

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Size does matter – especially to males of the squid species Loligo bleekeri, and in this case it's a matter of smaller males attempting to gain an advantage by manufacturing larger sperm.

To ensure the smaller males of L. bleekeri are able to pass on their genes, they have evolved larger sperm than that of the larger males and combine their larger sperm with a ‘sneaker’ tactic that they have adopted to fertilise a female’s eggs.

Larger male squid use their colour changing skin to compete for females and court them.

A successful male will then mate with the female by placing his sperm inside the female’s oviduct, after the couple adopt an above and below mating position.

To ensure his will be the sperm to fertilise her eggs the male will stay with her until she spawns – the point at which the smaller ‘sneaker’ males will make their move.

At the moment the female lays her eggs, sneaker males rush in and mate with the female, head to head, placing packages of sperm by the female’s mouth. The tactic relies on the chance that the sperm will fertilise the eggs as they leave the female’s body.

The discovery was made by researchers from London and Japan, who choose to study L. bleekeri as they believed it to be an ideal candidate because of its alternative reproductive tactics. Dr Yoko Iwata from University of Tokyo said, "Sperm size is likely to be an adaptation to fertilisation 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."

Despite the sneaker tactic and the employment of larger sperm, it appears that the more traditional courtship method used by the larger males resulted in higher paternity rates. However, by adopting the sneaky approach - coupled with larger sperm – the smaller males of the species do at least create an opportunity to pass on their genes.

The article Why small males have big sperm: dimorphic squid sperm linked to alternative mating behaviours. Yoko Iwata, Paul Shaw, Eiji Fujiwara, Kogiku Shiba, Yasutaka Kakiuchi and Noritaka Hirohashi. BMC Evolutionary Biology 2011, 11:236doi:10.1186/1471-2148-11-236, is available for free from the open access online journal BMC Evolutionary Biology.

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