Sex is a shot in the dark for bisexual squid

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Living in permanent darkness can do strange things to a squid's sexual appetite.

This is according to research to be published in a forthcoming issue of the journal Biology Letters. Studying the mesopelagic squid Octopoteuthis deletron using remotely operated vehicles in the waters off California, Hendrik Hoving, Stephanie Bush and Bruce Robison found evidence that male squid were attempting to inseminate other male squid.

Male squid transfer spermatophores, complex structures containing millions of sperm, using a long terminal organ often referred to as a penis. The spermatophores are deposited on the female, where they discharge sperm-containing sacs called spermatangia that implant into the female's tissue.  The empty spermatangia may remain attached to a female squid's body for some time and their presence is usually taken as evidence of a recent mating.

The authors used video footage of 108 specimens of O. deletron taken by remotely operated vehicles at depths of 400-800m in the Monterey Submarine Canyon.  

Taking frame grabs of specific body regions of the squid, the authors looked for the presence of spent spermatangia, as well as a patch of rugose mantle tissue that enabled them to sex the squids (only females have this patch of tissue).

Of these 108 observations, the authors could only confidently sex 39 (19 females and 20 males), of which nine males and 10 females were seen to be carrying spent spermatangia.

The presence of spent spermatangia on females is not surprising; that on males is perhaps more so. What this indicates is that the males are indiscriminately mating with members of the same species, regardless of sex (the spermatangia on the males were found on parts of the body beyond the reach of their own penises, thus ruling out accidental self-insemination).  

There is a reason behind all this seemingly wild orgiastic madness: because the squid live in the inky blackness of the deep sea and encounters with members of the same species are rare, it makes sense for the male to literally shoot first and ask questions later rather than to hang around and wait for a conspecific of the correct gender to come along.  

Although the risk of wasting valuable sperm on other males is high in such a scattergun approach, the potential benefits of a successful mating far outweigh the costs of wastage, reason the authors. This is especially heightened by the fact that the squid typically have a single, brief reproductive period before dying.

For more information, see the paper: Hoving, HJT, SL Bush and BH Robison (2011) A shot in the dark: same-sex sexual behaviour in a deep-sea squid. Biology Letters doi: 10.1098/rsbl.2011.0680

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