(CN) — Despite the lack of evidence, cultural beliefs that birthdays influence destiny or relationship patterns are common — but for Japanese spear squid, there is now scientific evidence that birth date sets the reproductive strategy males will employ at maturity.
Male spear squid use two distinct reproductive tactics, known as consort and sneaker. The consorts claim female squid by fighting off other males and depositing their sperm directly into the oviduct on the female's back side, where she stores the sperm for fertilization.
The sneaker squid opportunistically deposits its sperm near the female's mouth, where it is stored for fertilization a different way.
The consorts are larger, allowing them to compete with other male consorts to inseminate the female the typical way. Their sperm is suited to that method of copulation, while the sneakers' sperm has features that improve mating success through the female’s alternate fertilization path.
Now, a study released Tuesday by The Royal Society Journal reveals the destiny of male squid to be consort or sneaker is linked to the date they hatch.
“Our results showed that the hatching date determines the whole life trajectory in this species,” associate professor Yoko Iwata from the Atmosphere and Ocean Research Institute at the University of Tokyo said in a statement accompanying the study. “The difference in hatch date means that the squid experience different environmental conditions in early life, which may influence the growth trajectory.”
The study authors gathered data from squid caught in commercial nets off the coast of northeastern Honshu, Japan. They were able to determine growth rates and birthdates of the squid by removing an equilibrium sense organ called the statolith and filing it down to reveal calcified growth rings, similar to a tree stump, although the statolith rings must be observed under a microscope.
The researchers, including lead author Shoto Hosono, who is conducting the study as part of his doctoral research, counted and measured the rings of each statolith manually in three blind counts of all samples to improve accuracy.
This meticulous work revealed that both consort and sneaker males have similar growth rates for the first 100 days and then consorts began eating more and growing larger while sneakers maintain a smaller body size. This indicates the life pathway of the squid is due to a genetic trigger that is set early in life rather than from gradual growth from hatching to maturity.
Meanwhile, the researchers found the consorts grew larger than sneaker males but had not yet reached sexual maturity. This allows the immature consort squid to grow large enough to compete with other consorts for females.
Conversely, the sneaker squid reach full maturity and begin mating while smaller because their reproductive tactics do not require direct competition with the consort males.
Whether the squid become consort or sneaker depends on environmental factors leading to alterations in DNA known as genetic polymorphisms. These genetic variations are common in many organisms and can produce a variety of genetic expressions such as the alternative reproductive tactics observed in squid as well as vertebrate fish species.
The results of the study showed that consort squid had birthdates from April to July and sneaker squid hatched between June and August.
Researchers don't yet know which environmental factors or combinations thereof lead to the polymorphism that produces the sneaker or consort variation in male squid. But the bump in seasonal temperatures in the later months of the hatching period may be one of several factors that determine the squid reproductive strategy.
Further study in this area may shed light on how climate change will impact squid and other organisms that exhibit forms of genetic polymorphism.
“If an extreme environmental event, such as an ocean heat wave, happens during the hatching season, it could affect the squid’s mature body size and subsequent mating tactic," Iwata said. "This would also impact the amount that could be commercially caught enormously.”
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