No evidence for hybridization between Calanus finmarchicus and Calanus glacialis in a subarctic area of sympatry

Abstract

In the North Atlantic and the Arctic Ocean, four species of the copepod genus Calanus dominate the zooplankton biomass. Because of their morphological resemblance, knowledge of their respective distribution range has long been biased by misidentification, until the recent use of molecular tools uncovered numerous areas of sympatry. As hybridization between Calanus finmarchicus and Calanus glacialis has been claimed in the East-Canadian Arctic based on microsatellites, we investigated further the potential for interbreeding in newly uncovered areas of sympatry. Calanus species and stage composition were analyzed during winter in two Norwegian subarctic fjords, using molecular markers developed specifically for species identification and hybrid detection between C. finmarchicus and C. glacialis. Overall, C. glacialis were the most abundant throughout the winter, followed by C. finmarchicus and Calanus hyperboreus with only a few records of Calanus helgolandicus. The presence of C. glacialis, C. hyperboreus, and C. finmarchicus' nauplii was recorded, indicating that these species reproduce locally. In January and February, the simultaneous occurrence of males and females of both C. finmarchicus and C. glacialis suggested a potential for interspecies mating. However, genetic admixture tests performed on all 1126 individuals revealed no signal of hybridization, implying a strong reproductive isolation mechanism. We conclude that no evidence supports a potential for hybridization between C. finmarchicus and C. glacialis.

Contrasting Life Traits of Sympatric Calanus glacialis and C. finmarchicus in a Warming Arctic Revealed by a Year-Round Study in Isfjorden, Svalbard

“…Differences in the timing of appearance of adults between C. glacialis and C. finmarchicus similar to that observed in our study have been documented previously in other cold-water regions (Kjellerup et al, 2012). The early January appearance of C. glacialis males and females in Isfjorden is consistent with Daase et al (2018), and appearance of C. finmarchicus adults in February-March corresponds well with observations further south (Arashkevich et al, 2004;Choquet et al, 2021).…”

“…One additional reason for absence of hybridization could be the difference in timing of appearance of adults as observed in this study which would provide an efficient temporal barrier for inter-breeding. However, hybridization has also not been detected in nauplii or older copepodite stages in fjords located further south (66°N) where adult males and females of C. glacialis and C. finmarchicus co-occur, suggesting that hybridization is not occurring between these two Calanus species (Choquet et al, 2021).…”

“…In regions where C. glacialis and C. finmarchicus co-exist, it has been suggested that extensive hybridization occurs (Parent et al, 2012). However, recent studies show that this is very unlikely (Choquet et al, 2017), even when adult males and females of both species are present at the same time (Choquet et al, 2021). Calanus finmarchicus is considered to be a boreal expatriate in the Arctic (Hirche and Kosobokova, 2007;Kosobokova, 2012;Wassmann et al, 2015).…”

Element cycling and aquatic function in a changing Arctic

“…The succession of calanoid species due to ongoing climate change is complex and recent work has suggested that cross-species hybridization might occur (Parent et al 2012). However, this hypothesis has since been challenged and the debate is ongoing (Choquet et al 2021). Nevertheless, understanding the responses of zooplankton communities toward increasing temperature, changes in stratification, changes in sea-ice coverage, and changes in productivity is essential for projecting ecosystem responses at higher trophic levels and the effects of pelagic-benthic coupling.…”

Variable predator–prey relations in zooplankton overwintering in Subarctic fjords

“…Calanus hyperboreus reproduces in both fjord systems and proximate shelf waters mainly in January and February, producing a regional spring generation (Sømme 1934;Olsen 2002;Choquet et al 2020). In Saltfjord in 1997, C. hyperboreus remained a diapausing CV until July (Olsen 2002) when the spring generation was probably lost to the shelf together with other Calanus spp.…”