On the surprising lack of differences between two congeneric calanoid copepod species, Calanus finmarchicus and C. helgolandicus

Abstract

The important calanoid copepod species Calanus finmarchicus and C. helgolandicus have distinct geographic ranges which are changing under the influence of climate change. Understanding the mechanisms underlying their distributions is becoming increasingly important as a result of the possible ecological impacts of these range shifts. Here we review inter-species differences in key life cycle traits that influence each species’ geographic distribution, in particular development and growth, fecundity, feeding behaviour, vertical migration and overwintering behaviour. The distinct temperature niche of each species leads to an a priori assumption that the response of life cycle traits to temperature is a key determinant of their contrasting geographic distributions. A new development model was created to reconcile published experimental development times for each species. Model output indicates that at temperatures below approximately 12-13 ˚C, C. finmarchicus is the faster developing species, but above these temperatures C. helgolandicus develops more quickly. Conventionally Calanus development time is assumed to decrease monotonically with temperature; however our model indicates that the response of development time to temperature is instead U-shaped. Differences in life cycle aspects such as seasonality and vertical structuring are interpreted in light of this development model. Body size and lipid accumulation abilities could be significant influences on each species’ geographic distribution; however evidence is consistent with inter-species differences not existing for these traits. Published evidence shows that inter-species differences in egg production may exist, but do not follow a clear pattern. Diapause is an important and well studied life cycle adaptation of C. finmarchicus, but has received little attention in C. helgolandicus. We synthesised knowledge of diapause and suggest the hypothesis that C. helgolandicus is restricted to continental shelf regions as a result of an inability to diapause for significant periods. This synthesised view of each species’ respective life cycle traits is that response of growth and development to temperature is the only known difference between each species, which indicates a promising direction for the extension of population models of C. finmarchicus to C. helgolandicus

Copepod Life Strategy and Population Viability in Response to Prey Timing and Temperature: Testing a New Model across Latitude, Time, and the Size Spectrum

“…Field observations of size in relation to temperature in C. finmarchicus and C. helgolandicus across the North Atlantic show a similar relationship (Q 10 = 1.65, Wilson et al, 2015, with prosome length converted to carbon weight based on Runge et al, 2006). Somewhat surprisingly, even wide variation in prey conditions (clusters of gray dots, Figure 4) has only minor effects on this slope.…”

“…C. finmarchicus, for example, is fit well by u 0 = 0.007 d −1 at higher temperatures (4-12 • C), whereas near 0 • C in Disko Bay, it has been observed to be considerably smaller than extrapolation along the u 0 = 0.007 d −1 power law would predict. Past studies have also found C. finmarchicus growth and ingestion FIGURE 4 | Relationship between adult size W a and mean surface temperature T 0 in the "global" model experiment, for three values of relative development rate u 0 , in comparison with observations (Peterson, 1986;Swalethorp et al, 2011;Wilson et al, 2015;Campbell et al, in press) and laboratory results (Campbell et al, 2001;Rey-Rassat et al, 2002). Model results (gray dots) represent structural biomass S, as do observations marked with a ⋆; observations marked with a • represent total biomass R + S. Clusters of gray dots indicate families of model cases varying productive season length (horizontal axis in Figure 5).…”

The North Atlantic Spring-Bloom System—Where the Changing Climate Meets the Winter Dark

“…However, while there is a comprehensive amount of literature on the lipid deposition and overwintering dynamics of the arcto-boreal C. finmarchicus, the spares literature on the similar issues for the temperate C. helgolandicus indicates that lipid deposition is much less (Rey-Rassat et al, 2002) and too low for bringing the latest copepodite stages through the winter dark with a positive energy balance. Moreover, Wilson et al (2015) found "no direct evidence that C. helgolandicus undergoes a period of diapause." We do not know whether future life cycle adaptation (i.e., large lipid deposition and overwintering) would be possible for temperate species like C. helgolandicus.…”

“…We hypothesize that the latitudinal change in bloom dynamics determined by the destined and stable latitudinal-dependent seasonal light cycle represents a fundamental barrier at high latitudes to the onward climate change-induced migration of temperate-adapted planktivorous from lower latitudes, unless they are able to adapt by depositing lipids and undertake overwintering. C. helgolandicus with its core production area to the south of the North Atlantic Current is a copepod species that is known to deposit only limited amounts of lipids and with no clear evidence of overwintering response (Wilson et al, 2015). There is no need since this temperate species can feed during winter, although less so than during spring/summer/autumn.…”

“…Hence, we define a latitudinal transition range as indicated in Figure 2. Wilson et al (2015) reviewed the literature on the two congeneric calanoid copepods C. finmarchicus and C. helgolandicus. They concluded that there were surprisingly small differences between the two species.…”