Can morphology reliably distinguish between the copepods Calanus finmarchicus and C. glacialis, or is DNA the only way?

Abstract

Copepods of the genus Calanus play a key role in marine food webs as consumers of primary producers and as prey for many commercially important marine species. Within the genus, Calanus glacialis and Calanus finmarchicus are considered indicator species for Arctic and Atlantic waters, respectively, and changes in their distributions are frequently used as a tool to track climate change effects in the marine ecosystems of the northern hemisphere. Despite the extensive literature available, discrimination between these two species remains challenging. Using genetically identified individuals, we simultaneously checked the morphological characters in use for C. glacialis and C. finmarchicus identification to compare the results of molecular and morphological identification. We studied the prosome length (1); the antennules and the genital somite pigmentation (2); the morphology of the fifth pair of swimming legs and of the mandible (3). Our results show that none of these morphological criteria can reliably distinguish between C. glacialis and C. finmarchicus. This has severe implications for our current understanding of plankton ecology as a large part of our knowledge of Calanus may be biased due to species misidentification and may subsequently require reinvestigation with the systematic use of molecular tools.

The intriguing co-distribution of the copepods Calanus hyperboreus and Calanus glacialis in the subsurface chlorophyll maximum of Arctic seas

“…Based on the same criteria, C. finmarchicus was absent at stations further north. Given the overlap in size of C. finmarchicus and C. glacialis, especially in the southern reaches of their sympatric distribution, the reliability of discriminating the two species based on size and morphometrics has been questioned (e.g., Parent et al, 2011;Choquet et al, 2018). However, consistent with our identification, Parent et al 2011found none and few C. finmarchicus, respectively, at two stations in NW Baffin Bay and the Canadian Archipelago using molecular techniques.…”

Gradients of season length and mortality risk cause shifts in body size, reserves and reproductive strategies of determinate growers

“…4–5 times heavier than a C. glacialis maturing in its 2nd year of life, and more than 10 times heavier than bivoltine C . finmarchicus (Falk‐Petersen et al., ), although intraspecific variability in body size exists (Choquet et al., ). The Bergmann cline observed in Calanus spp.…”

Borealization of Arctic zooplankton—smaller and less fat zooplankton species in Disko Bay, Western Greenland

“…In Disko Bay, the average lengths of the Calanus spp. females differed significantly (and the distinction between the female Calanus could be further supported based on differences in antennules and genital somite pigmentation (Nielsen et al ; Choquet et al ). However, C. finmarchicus and C. glacialis overlap in size, which may have led to an underestimation of C. glacialis by 5% when using size as the sole criterion (Nielsen et al ).…”