The growth, maturity, reproductive biology and life cycle of the blackbelly lanternshark Etmopterus lucifer, a small deep-sea species with an extremely high population density off the Pacific coast of northeastern Honshu, Japan, were examined. Age was estimated by counting growth bands in cross-sections of the second dorsal fin spine and validated through marginal increment ratio (MIR) analysis, which revealed an apparent decrease in June-July values compared to other months, indicating that growth bands are formed annually. The females examined were 0-13 years old and the males were 0-10 years old. The size and age at 50% maturity were 344 mm TL and 6.53 years for females, and 288 mm TL and 3.82 years for males, respectively. Although the blackbelly lanternshark exhibited a high von Bertalanffy growth coefficient and early age at maturity compared to other squalids, its fecundity was relatively low, with an average of 8.6 ripe oocytes per female. These results indicate that the blackbelly lanternshark is an r-selected life-history species with a high growth coefficient and early maturity that is achieved by limiting its asymptotic length. The mating season was inferred to be in early summer, corresponding to a decline in the male gonadosomatic index (GSI). In contrast, based on increases in female GSI values and oocyte diameters, ovulation likely occurs from autumn to spring. The similarity in oocyte diameters in the resting and maturing stages, combined with an intermediate uterine state between mature and gravid, indicates that this species does not achieve immediate postpartum gestation, instead requiring a prolonged interval for ovarian maturation and uterine contraction before the next reproductive cycle. Therefore, although the blackbelly lanternshark has relatively low fecundity within one reproductive season, this species has fast growth and maturity, which may contribute to its prosperity in the study region.
