Comparative reproductive biology of three dominant myctophids of the genus Diaphus on the slope region of the East China Sea

• Three Diaphus species spawned during summer with interspawn periods of 3–9 days.
• Females with hydrated ovaries occurred during the daytime in the mesopelagic layer.
• Egg size at hydration and batch fecundity was largest and highest in D. watasei.
• D. garmani was characterized by maturation at a smaller body size.
• D. watasei used greater energy for reproduction as larger-sized mature adults.

We examined the reproductive biology of three dominant pseudoceanic myctophids, Diaphus garmani, D. chrysorhynchus, and D. watasei, on the slope region of the East China Sea. Overall, females were more abundant in D. garmani and D. chrysorhynchus, while males were more abundant in D. watasei. The gonadosomatic index (GSI) at 50% sexual maturity of females of D. garmani, D. chrysorhynchus, and D. watasei was 3.65, 3.92, and 6.27, respectively. Based on these criteria, females of each species can mature at sizes larger than 42, 71, and 120 mm standard length (SL), respectively. In all three species, GSI of females showed high values during summer, i.e. the primary spawning season, and occurrence of females with hydrated ovaries was restricted to the daytime (0600–1530 h) in the mesopelagic layer of 270–450 m depth. Oocytes at various sizes were found in mature ovaries, indicating that they are multiple spawners. Spawning frequencies were estimated to be approximately 3–9 days. Mean egg size at hydration and batch fecundity was respectively smallest and lowest in D. garmani (0.60 mm and 612–2200 eggs) and largest and highest in D. watasei (0.65 mm and 10,356–44,879 eggs). Batch fecundity increased linearly with SL in all species, with a steeper slope in D. watasei. Diaphus garmani is characterized by early maturation at a smaller size, while D. watasei channels a larger part of the energy budget into somatic growth during the juvenile and later immature stages (<120 mm SL) than the other two species and subsequently reproduces at a larger size as mature adults.