Life histories of Eucalanus bungii and Neocalanus cristatus (Copepoda: Calanoida) in the western subarctic Pacific Ocean

Abstract

Life cycles of the large suspension-feeding copepods, Eucalanus bungii and Neocalanus cristatus were investigated by seasonal sampling in the western subarctic Pacific. Eucalanus bungii has a diapause from August to March at copepodite stages between copepodite 3 (C3) and C6 female. We propose that individuals with early birth dates are young of overwintering C5 and C6-females that develop to C4 in their birth year, while individuals with late birth dates are young of overwintering C4 that develop to C3 in their birth year. Thus, a majority of the population has annual generations alternating with biennial generations. Neocalanus cristatus showed life history almost identical to the population in the Alaskan gyre. Timing of the life cycle in N. cristatus is very close to that in the eastern subarctic gyre, but that of E. bungii is 2 months earlier than in the eastern subarctic. This difference is probably caused by the timing difference in the maximum primary production in the two areas and the plasticity of the life cycle strategy in E. bungii.

Lipid and fatty acid/alcohol compositions of the subarctic copepods Neocalanus cristatus and Eucalanus bungii from various depths in the Oyashio region, western North Pacific

“…On the other hand, the N. plumchrus development occurs in the peak or declining season of the spring bloom period when it progresses rapidly through the C1-C4 stages without lipid accumulation. Tsuda et al (2004) revealed that C2-C4 individuals of N. cristatus had an oil sac or only droplets, but it rarely exceeded 10% of the prosome length, and lipid storage was mainly observed in the C5 stage. C1 individuals showed higher lipid storage, however, this storage is considered to originate from egg yolk (Saito and Tsuda, 2000) although the lipid composition of egg yolk is still unknown.…”

“…During this bloom period, the main population of N. cristatus undergoes a rapid development to stage C5 at the surface, and then migrates in summer/autumn to deeper layers (below 500 m) to undergo diapause. Overwintering C5 specimens develop to adults, spawn during autumn to winter at depth and lipid-rich eggs and nauplii ascend to the surface along with their development in spring (Kobari and Ikeda, 1999;Tsuda et al, 2004). Thus, N. cristatus depends on the lipids stored during the previous spring bloom for somatic maintenance and reproduction as do other Neocalanus species in the subarctic Pacific Ocean (Fulton, 1973;Miller et al, 1984;Saito and Tsuda, 2000).…”

“…This was especially pronounced in the alcohol composition where the proportions of the 20:1 and 22:1 alcohols increased from 44 to 80%. These energy-rich compounds are strongly biosynthesized and accumulated during the development and are probably a prerequisite to sustain the metabolic activities for overwintering and reproduction since the spawning of N. cristatus in deep sea is completely dependent on lipid storage (Tsuda et al, 2004).…”

“…Kotani (2006) followed up on this hypothesis and suggested that copepods storing triacylglycerols as reserve lipids may overwinter without a period of dormancy. However, it is evident that E. bungii migrates deeper than 500 m and undergoes diapause from autumn until early spring (Miller et al, 1984;Tsuda et al, 2004;Shoden et al, 2005;Yamaguchi et al, 2011). All specimens from December and also some from the other seasons in our study were collected deeper than 500 m ( Table 1).…”

“…In contrast, E. bungii spawns in the surface layer during the spring bloom, develops to the C5 stage and descents gradually to the deeper layers to enter diapause. The individuals which overwintered as C5 molt to adults, then ascend to the surface for spawning, whereas younger individuals overwinter in diapause at C4 stage and molt to adults the following year (Tsuda et al, 2004;Shoden et al, 2005).…”