Squid male alternative reproductive tactics are determined by birth date

Alternative reproductive tactics (ARTs) are discontinuous phenotypes
associated with reproduction, observed in males of many species.
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 “birthdate 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. Here, the birthdate
hypothesis was tested in ARTs of Japanese spear squid Heterololigo
bleekeri (consort/sneaker) by analyzing statolith growth increments. The
birth date significantly differed between consorts (early-hatch) and
sneakers (late-hatch). However, no differences were detected in growth
history up to 100 days from hatching. Most immature males caught during
the reproductive season were larger than sneakers, and their hatch date
was similar to that of consorts, suggesting that these immature males had
already been following a life history pathway as a consort. These results
indicate that ARTs of H. bleekeri are determined based on their hatch date
in early life. This study firstly demonstrated the birthdate hypothesis in
aquatic invertebrates, suggesting that the mechanism by which birth date
determines the individual phenotype is a phenomenon more common than
previously believed.