When male squid come into contact with a chemical found on the outside of eggs laid by females, they instantly go from swimming along calmly and minding their own business to a state of extreme aggression, according to a new report published online on February 10 in Current Biology, a Cell Press publication. After just a touch of an egg, males will often fight vigorously with other males in an attempt to gain preferred access to mates, even when females aren't around.
The researchers believe that the findings represent the first detailed evidence of an aggression-inducing contact pheromone in any aquatic animal.
"The identification of this pheromone as a key component of this signaling system is highly unusual because the male squids need only to contact these protein molecules to initiate the complex cascade of behaviors that we term aggressive fighting," said Roger Hanlon of the Marine Biological Laboratory at Woods Hole.
That's particularly notable because aggression is generally thought to be a rather complex process, involving some combination of neural, hormonal, physiological, and psychological stimuli. "Here we find what appears to be a strong and seemingly direct stimulus from certain protein molecules," Hanlon says.
Squid are highly advanced marine invertebrates with a complex mating system rivaling that of vertebrates. The loliginid squid that were the focus of the current study live for about a year, with most mating and egg laying occurring in the spring, when the squids migrate from deeper offshore waters to shallower waters along the coast from North Carolina to southern Maine. Females typically mate multiple times, and males compete fiercely for the opportunity to mate. When males see eggs on the seafloor, they will typically approach and, upon touching them, immediately escalate into intense physical fighting with any nearby males.
Hanlon and his colleagues have now traced that reaction to a single protein pheromone that is produced in the female reproductive tract and embedded in the outer surface of the egg capsules. In controlled behavioral experiments, the researchers found that males would react aggressively just as they do upon touching an egg after being presented with the purified form of the protein, known as Loligo β-microseminoprotein.
The laboratory experiments show that whichever male touches the eggs first becomes aggressive faster than those that reach the eggs later. That earlier aggression leads those males to achieve greater dominance. They also win most (but not all) of the matings and most (but not all) of the fertilizations, Hanlon says.
Interestingly, the microseminoprotein under study belongs to a family of proteins found in vertebrates including humans, Hanlon says. The proteins are generally found in reproductive glands and are at high levels in human and mouse seminal fluid.
"The functions of microseminoproteins in vertebrates have not been determined, but our findings in squids may inspire other researchers to consider similar functions in higher vertebrates," he says.
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