Early life history of Neocalanus plumchrus (Calanoida: Copepoda) in the western subarctic Pacific

The life history of copepods during nauplius stages has rarely been studied, because of difficulties in identification. This study describes the development of an identification method that uses integrated microscopic analysis and real-time PCR based method to distinguish and estimate the abundance of pelagic calanoida nauplii, Neocalanus plumchrus, and to determine their phenology and ontogenetic vertical migration (OVM) in the Oyashio region. The overall approach of identification included: classify field samples of calanoida nauplii into six developmental stages by microscopic analysis, determining the Cq values from each representative nauplii by real-time PCR using specie specific probe and primer pair for N. plumchrus, calculating the |ΔCq| values from the standard curve, distinguish N. plumchrus nauplii by the criterion of |ΔCq| ⩽ 2, and calculate the abundance of N. plumchrus nauplii using results obtained from microscopic analysis and real-time PCR. Validation of the method showed that our real-time PCR based approach could identify N. plumchrus nauplii in field samples with as low as 9% of underestimation, the effect of which is negligibly low on the investigation. The results showed that early nauplius stages (first and second nauplius stages: N1 and N2, respectively) were observed during the period from October 2009 to May 2010. Spawning females were also observed during the same period, and these results confirm that the spawning period of N. plumchrus is at least 8 months long. N3 was the most dominant stage in October 2009, January, and March 2010, the distribution depth of N3 was between 500 m and 2000 m in October 2009 and their vertical distribution range was extended until April 2010. Late stages (N4-N6) were rarely observed during the period from October 2009 to March 2010, and they occurred abundantly from the surface layer (0-20 m) in April 2010. The dominance of N3 from October to March and the sudden occurrence of later stages in April indicate arrested development of N3 in the mesopelagic layer.