Pseudocalanus in Svalbard waters : identification and distribution patterns of two sibling copepod species

Abstract

Marine copepods of the genus Pseudocalanus (Calanoida) are common in Svalbard waters as well as throughout the northern hemisphere. They contribute considerably to plankton biomass in addition to Calanus spp. and play an important role in the marine food web as a link between planktivores, such as fish larvae, and microalgae. Pseudocalanus consists of several sibling species, and two of them, Pseudocalanus minutus and P. acuspes, are known to inhabit the Svalbard area. A third species, P. major, is listed as potentially present in this area, but has rarely been reported. As for the other members of the genus, discrimination between these species is time consuming and requires detailed microscopy. Most studies thus lump them into Pseudocalanus spp., ignoring differences in life history strategies and behaviour. In the current master’s project, a molecular species-specific polymerase chain reaction (PCR) was developed, using the mitochondrial gene cytochrome oxidase subunit I (mtCOI) to aid morphological and morphometrical discrimination. The DNA sequence variation confirmed the presence of both species in Svalbard waters. The statistical comparison of morphological identification with the PCR results gave ambiguous results: a Chi-square test yielded a significant difference between the groups, but with low power due to a low sample size. However, this indicates that microscopic species determination may be wrong. Moreover, morphometrical analysis revealed deviations from expected length ratios in both species, and the PCR sometimes failed to produce results. This may suggest the presence of a third species, presumably P. major, for which no molecular information is available.