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.