Small squid produce bigger sperm

Smaller squid make up for their diminutive size, and reduced sexual status, by producing bigger sperm.

They produce the outsized sperm in a bid to thwart the chances of larger male squid rivals impregnating females.

While the smaller males are unable to compete with the larger males, their sperm can.

Scientists based in Japan made the discovery studying spear squid, otherwise known as Bleeker’s squid (Loligo bleekeri).

Details are published in the journal BMC Evolutionary Biology.

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.

They compete with other large males and the winner mates with the female, depositing sperm in a package inside her oviduct.

He 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 show few of the behaviours of their larger rivals.

Instead, they wait until a larger male is guarding a female, then rush in head first to copulate with her.

A sneaker male mates in a head-to-head position and deposits his sperm in a different place to the larger males, putting his sperm package on the outside of the female’s body just below her mouth.

The timing of this intervention is crucial.

The smaller male dashes in just as the female begins laying her eggs, in the hope that they will pass over his sperm and be fertilised by him rather than the female’s original suitor.

The species is unusual in another way however.

These smaller males produce different sperm to the larger males.

A study by Yoko Iwata of the University of Tokyo and colleagues based in Japan reveals that the smaller males’ sperm is actually bigger than those of the larger male squid.

That suggests that the spear squid is the first species known to have individuals that produce two separate types of sperm.

The larger sperm aren’t able to out-compete the smaller sperm, as both types are equally mobile and fertile.

But each sperm type is adapted to the environment in which it is deposited, say the researchers.

Smaller sperm work better within the female’s oviduct, while larger sperm work better when deposited on the outside of the female squid’s body.

That gives the smaller males a chance to use a different mating tactic and compete with the larger males for mating opportunities.

Overall, the larger males still end up producing more offspring, perhaps because their sperm reach the females’ eggs earlier.

But the large sperm strategy appears to be a way for physically less impressive males to still pass on their genes.


Image: Y. Iwata et al