Vertical distribution, life cycle, and developmental characteristics of the mesopelagic calanoid copepod Gaidius variabilis (Aetideidae) in the Oyashio region, western North Pacific Ocean

Vertical distribution, life cycle, and developmental characteristics of the mesopelagic copepod Gaidius variabilis in the Oyashio region were investigated by combining the analyses of field copepodite populations with laboratory-rearing data of egg hatching and naupliar development. Field samplings from five discrete depths between the surface and ≤ 2000 m were made approximately every month for one year. Most populations of G. variabilis occurred at 600-1000 m depth. A modest degree of reversed diel vertical migration behavior and some stage-specific depth-distribution patterns were noted. All copepodite stages were observed throughout the year, suggesting a year-round spawning of G. variabilis. From a prominent abundance peak of copepodite stage 1 (C1) seen in June-August, together with development times of eggs and nauplii obtained in laboratory-rearing experiments, the major spawning season was extrapolated to April-June, the phytoplankton bloom season. Tracing the peak abundance of each copepodite stage (distinguishing males and females for C4-C6), the generation lengths of males and females were deduced as 2 and 1 yr, respectively. All between-stage increments in terms of wet-, dry-, and ash-free dry weights were greatest in C3/C4, and least in C5/C6 for both males and females. The increments in C3/C4 and C4/C5 were greater for males than for females, reflecting a longer stage duration of the males. These weights did not increase in C5/C6 males, possibly because feeding ceased in C6 males. These results of G. variabilis are compared with those of some mesopelagic copepods previously reported from other regions