Japanese Spear Squid Hatch Dates Determine Mating Tactics

If you have a scientific outlook, horoscopes and their forecasts of a predestined future may hold little appeal, even when they draw on the positions of stars and planets in the sky on the night of your birth.

Yet new research from Japan indicates that some squid may be fated to become either stealthy or aggressive mating partners according to the time at which they hatch.

University of Tokyo marine biologist Shota Hosono and his colleagues examined Japanese spear squid (Heterololigo bleekeri). They discovered that the reproductive strategy used by males is linked to their hatching month.

Japanese spear squid mating strategies

Squid hatching early in the breeding season, from early April to mid-July, had generally reached a larger size by the time they mated. They therefore became consorts: squid that fend off competing males in order to fertilise a female, then remain to guard her as she lays eggs.

Those emerging later, between early June and mid-August, were usually smaller and adopted the role of ‘sneakers’. These males discreetly place sperm on the outside of a female, close to where she lays her eggs, hoping to fertilise them.

Squid that hatched in early July were equally likely to develop into sneakers or fighters.

This pattern is known as the relative age effect. It can be clearly observed among children: those born earlier in the year have a greater likelihood of becoming professional athletes than others in their age group, because they have had more time to develop before selection takes place.

Before this squid research, however, the ‘birthdate hypothesis’, as biologists also call it, had been examined only in several teleost fish species and in no other taxonomic group across the wider animal kingdom apart from humans.

“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,” Hosono and colleagues explain in their published paper, which was based on observations of 201 male squid and 68 mature females.

The findings represent the first evidence for the ‘birthdate hypothesis’ in aquatic invertebrates, indicating that male H. bleekeri may be committed to particular mating behaviours from birth.

Hatch dates and lifelong behaviour

Even when male squid born early in the season did not grow as large as expected, they did not abandon the birthday pattern by becoming sneakers. Instead, they delayed mating and carried on growing until they were large enough to act as consorts.

“Our results showed that the hatching date determines the whole life trajectory in this species,” says Yoko Iwata, a marine ecologist at the University of Tokyo.

Hosono, Iwata and their colleagues also argue that these mating strategies are not merely passed down through the paternal line.

“Female squid store sperm from both consort and sneaker males throughout the breeding season and use a mixture of sperm from both sources to fertilize their eggs during a spawning event,” the researchers explain.

Genetic variation arising from alternative reproductive strategies can shape mating behaviour in animals, including birds and damselflies, although recorded examples are thought to be uncommon.

However, the team’s results indicate that the male mating strategies of H. bleekeri are probably shaped by a potent combination of physical, biological and environmental influences present at birth. In beetles, for instance, food availability is an important factor affecting physical characteristics associated with later sexual success.

Humans also show this seasonal influence: the season of a person’s birth leaves a mark on their DNA and can alter the likelihood of conditions such as allergies.

Environmental effects on squid growth

“The difference in hatch date means that the squid experience different environmental conditions in early life, which may influence the growth trajectory,” Iwata says.

Squid are highly sensitive to environmental conditions, especially water temperature. Iwata notes that an extreme event during the hatching season, such as a marine heatwave, could alter the squid’s adult body size and, consequently, its mating strategy.

“This would also impact the amount that could be commercially caught enormously,” Iwata adds.

Nevertheless, the study found little difference in the early-life growth rates of consorts and sneakers, despite their growing up in different seasons. Further research is therefore required to clarify how environmental factors might influence squid mating behaviour.

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