Calanus finmarchicus diel and seasonal rhythmicity in relation to endogenous timing under extreme polar photoperiods

Abstract

Changing environmental conditions cause poleward distribution shifts in many marine organisms including the northern Atlantic key zooplankton species Calanus finmarchicus. The copepod has diel cycles of vertical migration and feeding, a seasonal life cycle with diapause in winter and a functioning circadian clock. Endogenous clock mechanisms control various aspects of rhythmic life and are heavily influenced by environmental light conditions. Here we explore how the extreme seasonal change in photoperiod (day length) in a high Arctic fjord affects circadian clock functioning as well as diel and seasonal cycles in C. finmarchicus. Expression of clock genes was measured in the active life phase at the end of midnight sun, in early diapause when photoperiod was ~12 h, and in late diapause during the polar night. While the clock maintained diel rhythmicity under extremely long photoperiods, it became arrhythmic during diapause. This was probably not due to a lack of light but was related to the physiological state of diapause. Seasonal expression analyses of 35 genes show distinct patterns for each investigated life phase. C. finmarchicus is able to maintain diel clock rhythmicity at photoperiods close to 24 h, and it is discussed how this may be related to the nature of the marine environment. The work also evaluates the potential negative consequences of rigid clock-based seasonal timing in a polar environment exposed to climate change and with high interannual variability.

Evidence for oscillating circadian clock genes in the copepodCalanus finmarchicusduring the summer solstice in the high Arctic

“…Copepods, specifically C. finmarchicus, are a dominant constituent of the zooplankton community and have been the focus of many DVM studies [2,12,23]. It has been shown that C. finmarchicus collected from a high-latitude Fjord (78°N) maintained circadian clock gene rhythmicity even under long photophases at the very end of the Midnight Sun period [24]. Our results go further, showing circadian clock gene oscillations within days of the summer solstice where daily changes in sun's altitude are at a minimum.…”

Biological Clocks and Rhythms in Polar Organisms

“…DVM of zooplankton has also been documented in the deep sea (as a proxy for another environment without overt solar signals) yet the mechanisms of entrainment are not understood (van Haren and Compton 2013). Noteworthy is that cycling clock gene expression in C. finmarchicus, has only been demonstrated during the Polar Day, but not the Polar Night, as shown in Figure 8.2 in animals collected from Kongsfjorden (Häfker et al 2018a). Copepods in early diapause (September; for details see Chapter 5) which were collected below 200m, where light levels were probably below their visual detection limit (Båtnes et al 2015), still showed clock gene cycling.…”

“…As copepods typically overwinter at depth where light or other seasonal cues are unavailable or reduced, an argument is made in favour of seasonal/circannual clocks in regulating diapause timing as has been described for various insects (Meuti and Denlinger 2013). The circadian clock of C. finmarchicus remains functional during extremely long photoperiods in summer (Häfker et al 2018a), and it is likely that the same is true for the polar-adapted species C. glacialis and C. hyperboreus. Thus, the circadian clock could be used to determine the seasons based on photoperiod and the use of a critical day length in diapause initiation.…”

“…Expression of per1 gene was analysed for 24 h rhythmicity with RAIN analysis with significant 24 h cycling (p < 0.05) indicated. Adapted fromHäfker et al 2018a…”

Rhythms during the polar night: evidence of clock-gene oscillations in the Arctic scallopChlamys islandica

“…Circadian clocks have been largely investigated in terrestrial species, but knowledge of marine clock systems is still scarce despite the ecological importance and complexity of marine ecosystems [17]. This is particularly true for polar ecosystems, with few studies devoted to understanding the behavioural or molecular rhythms during the polar day or night [15,[18][19][20].…”