Squid mating tactics are determined by birth date

The mating strategies of squid are determined by their date of birth, according to a recent study from the University of Tokyo. The research sheds new light on the reproductive tactics of male spear squids.

While humans might consider how their birth date influences personality or destiny, for these marine creatures, the timing of birth crucially influences their approach to mating.

“Alternative reproductive tactics (ARTs) are discontinuous phenotypes associated with reproduction, observed in males of many species,” wrote the study authors. “Typically, large males adopt a tactic of competing with rivals for mating, while small males adopt a tactic of stealing fertilization opportunities from the large males.”

“The ‘birth date hypothesis,’ proposing that the date of birth influences the determination of each male’s reproductive tactic, has been tested only in teleost fish to date.”

A date with destiny

Scientifically known as Heterololigo bleekeri, spear squids adopt one of two mating strategies. Males born earlier in the breeding season grow larger and become “consorts,” aggressively competing with rivals to mate and protect females during egg-laying.

On the other hand, males that hatch later in the season are typically smaller when they start reproducing and choose a “sneaker” tactic. These squids deposit their sperm on the outside of a female near where she lays her eggs – hoping to fertilize them.

The unchanging nature of squid mating tactics

The experts discovered that once a male squid’s mating tactic is set by his birth date, it remains unchanged throughout his life.

This pattern is so robust that even if an early-born squid could theoretically adopt the sneaker strategy during the early breeding season due to size, he would instead delay his maturation to grow sufficiently large to become a consort.

“The presence of large immature males appears to support this (i.e. these immature males may have been delaying their maturation in a non-reproductive state as a tactic to become a successful consort later in the season),” noted the researchers.

This all-or-nothing approach to mating tactics is remarkably consistent, with squids born around early July displaying a fifty-fifty chance of adopting either strategy.

Ecological and commercial ramifications

Professor Yoko Iwata of the Atmosphere and Ocean Research Institute at the University of Tokyo explained the significance of these findings. “The hatching date determines the whole life trajectory in this species. The difference in hatch date means that the squid experience different environmental conditions in early life, which may influence their growth trajectory.”

She further elaborated on the potential repercussions of environmental changes, such as ocean heat waves, on squid sizes and their mating tactics, which could significantly impact commercial fishing yields.

The research is notable as it extends the “birth date hypothesis” to aquatic invertebrates, suggesting its broader applicability across more animal species.

New insights into squid growth rates

The team analyzed more than 350 specimens collected between 2020 and 2023, sourced directly or from commercial catches. The results challenge previous beliefs about the growth rates among different squid mating types.

Although environmental factors like water temperature are well-known to influence squid development, the researchers discovered surprisingly little variation in the early life growth rates of consorts and sneakers.

This discovery raises new questions about the factors influencing the growth and reproduction of male spear squids.

The significance of statoliths in squid research

Looking ahead, the researchers will turn their attention to the statolith, a key structure within the squid that documents daily growth in a manner akin to tree rings.

By analyzing microelements within the statolith, experts can determine the age of squids as well as the environmental conditions they encountered during various life stages.

Professor Iwata expressed excitement about the potential implications of their findings. “I am interested in the evolution of animal survival and reproduction strategies, or life history; and also phenotypic plasticity, that is, how individuals respond to environmental changes. Spear squid provide an ideal model for studying the impact of environmental and ecological conditions on life history.”

The research will further illuminate how early life conditions affect squid mating tactics and provide deeper insights into how other marine species adapt to environmental changes.

The study is published in the journal Proceedings of the Royal Society B Biological Sciences.

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