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..., , , # Squid male alternative reproductive tactics are determined by birth date

Age, hatch date, and growth history were analyzed for consort males, sneaker males, immature males, and mature females of the Japanese spear squid, *Heterololigo bleekeri* caught throughout the reproductive season. Significant differences were seen in hatch dates between consorts and sneakers, with no difference in growth history between them.

## Description of the data and file structure:
The dataset (CSV) contains sample ID, Mantle length, Sex, Reproductive status, Catch date, Statolith radius, Hatching check radius, Statolith growth increment counts, Age, Hatch date, and Statolith increment width at day1–100 of spear squid *Heterololigo bleekeri *collected in the coast of Miyagi prefecture, Japan.

## Sharing/Access information

If you need further information, please contact the corresponding author Shota Hosono.