Outsized sperm produced to thwart larger males’ chances of impregnating females. Strategy appears to be way for physically less impressive males to pass on their genes.
Smaller squid make up for their diminutive size and reduced sexual status by producing bigger sperm. The outsized sperm is produced in a bid to thwart the chances of larger rival males squid impregnating females. Japanese scientists made the discovery studying spear squid, also known as Bleeker's squid (Loligo bleekeri).
In a study published in the journal BMC Evolutionary Biology, Yoko Iwata of the University of Tokyo and Japan-based colleagues have revealed male spear squid employ one of two tactics when seeking to pair with a mate. Larger males, known as "consorts", court females by flashing bright displays of colour across their bodies. After competing with other large males, the winner mates with the female and deposited sperm in a package inside her oviduct and then guards her until she spawns her eggs.
Smaller males however, employ a different tactic. These so-called "sneaker" males don't advertise themselves and exhibit few of the behaviours of their larger rivals. Instead, they wait until a larger male is guarding a female and then rush in headfirst to copulate with her. Mating in a head-to-head position, the sneaker male deposits sperm in a different place than the larger males, putting his sperm package on the outside of the female's body just below her mouth. He then dashes in just as the female begins laying her eggs in the hope that they will pass over his sperm and be fertilized by him rather than the female's original suitor.
In addition, the smaller males produce larger sperm than their larger rivals. This gives them a chance to use a different mating tactic and compete with the larger males for mating opportunities. This suggests spear squid is the first known species with individuals that produce two separate types of sperm. Smaller sperm works better within the female's oviduct, while larger sperm works better when deposited on the outside of the female squid's body.
The large sperm strategy appears to be a way for physically less impressive males to pass on their genes. Overall, however, the larger males still end up producing more offspring, perhaps because their sperm reach the females' eggs earlier.