It's in the eggs - what makes male squid mad with rage

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A research team from Australia, Thailand and the USA has identified a chemical in squid eggs that makes male squid fly into a rage after touching them.

In a study to be published in a forthcoming issue of the journal Current Biology, Scott Cummins and co-authors isolate and characterise the protein that immediately and dramatically changes the behaviour of male squid from calm swimming and schooling to extreme fighting, even in the absence of females.  

Focusing their research on the Longfin inshore squid (Loligo pealeii), the authors base their study on the observation of the way male loliginid squids behave during spawning.  

In these squids, the females lay multiple egg capsules over communal spawning grounds. The male squids are visually attracted to the egg masses and approach them, blow jets of water over them, and touch them with their arms and heads.

Upon physical contact with the egg masses, the males abruptly change their behaviour from schooling to highly aggressive fighting. This observation made the authors suspect that a chemical factor in the egg capsules was responsible for this behavioural change.

The authors identified a small protein in the egg capsule, which may serve as a contact pheromone. This protein, named Loligo β-microseminoprotein (Loligo β-MSP), remains active on the surface of the eggs for days to weeks (suggesting that it is highly resistant to degradation). The authors purified Loligo β-MSP, and then developed a series of behavioural bioassays to experimentally test the reactions of live male squids to the presence or absence of this contact pheromone.

The authors found that the purified Loligo β-MSP induced aggression in groups of six squids that were tested, even when the protein was embedded in agarose gels and “painted” on glass flasks and presented to the squids in lieu of egg capsules in one of the bioassays.  

The authors then analysed the amino acid sequence of this protein, and were even able to identify the gene sequence coding for it in the ovary of the female squid.

Based on this information, the authors were then able to synthesise recombinant Loligo β-MSP using the relevant genes spliced into baculoviruses.  They found the recombinant protein to be as potent in inducing aggression in male squids as its natural counterpart.

Why is all this aggression necessary?  Male squids that touch the egg capsules first become aggressive more quickly; this quickly leads to dominance over other males. This dominance translates to increased reproductive success, as dominant males mate more often with females.

The authors also identified the source of the Loligo β-MSP as the accessory exocrine glands that embed the protein into the outer coat of the egg capsules.  

Lastly, the authors note that Loligo β-MSP is a distant member of the β-MSP family found in mammalian reproductive secretions, suggesting that this gene family may have a widespread role in sexual competition in other taxonomic groups.

For more information, see the paper: Cummins, SF, JG Boal, KC Buresch, C Kuanpradit, P Sobhon, JB Holm, BM Degnan, GT Nagle and RT Hanlon (2011) Extreme aggression in male squid induced by a β-MSP-like pheromone. Current Biology doi:10.1016/j.cub.2011.01.038.