Latitudinal Clines in Temperature and Salinity Tolerance in Tidepool Copepods

Abstract

Local adaptation has been understudied in marine systems, but might be expected to be pronounced in the tidepool copepod Tigriopus californicus, which has a broad geographic range and extremely restricted dispersal. Tolerance to temperature and salinity was assessed in 14 populations over a 20° latitudinal range. Adaptive differentiation to temperature and salinity was found at scales as low as 5.6 km. Latitudinal clines were significant, with northern populations being more tolerant of low salinity and less tolerant of high temperature and high salinity. Both temperature and salinity tolerance were more closely associated with long-term thermal maxima than with long-term precipitation data. Hyperthermal and hyposmotic tolerance were inversely correlated, a pattern that could potentially slow adaptation to future conditions. Together, these studies of intraspecific geographic patterns in resistance to multiple stressors are important in predicting how environmental change may effect range shifts and local extinctions.

Phenotypic and transcriptomic responses to salinity stress across genetically and geographically divergent Tigriopus californicus populations

“…Our results complement work by Leong et al. () and allow us to make further inferences about intraspecific variation in salinity tolerance in T. californicus . Using an approach similar to our knock‐down exposures, Leong et al.…”

“…In low salinity exposures, our treatments (0–5 psu) largely overlapped those of Leong et al. () (1–25 psu) and both our results and theirs show the northern populations are more tolerant to low salinity stress (Table ; Figure ). Previous work in Fundulus heteroclitus killifish (Brennan, Galvez, & Whitehead, ) and Eurytemora affinis copepods (Lee, Remfert, & Gelembiuk, ) showed that gains in low salinity tolerance came at the expense of high salinity tolerance.…”

“…Using an approach similar to our knock‐down exposures, Leong et al. () assessed mortality in T. californicus copepods exposed to low and high salinities for 1 hr, finding that southern populations were more tolerant than northern populations to high salinities ranging from 100 to 135 psu. Intriguingly, over a range of salinities from 78 to 95 psu, we observed no significant population differences in knock‐down (Table ; Figure ).…”

“…Leong et al. () used only male copepods in their salinity exposures based on evidence that males are more sensitive to environmental stress (Kelly et al., ; Willett, ). While our data support sex‐based difference in tolerance to low salinity stress (Figure ), we found juvenile females had a higher probability of being knocked down than males, suggesting salinity tolerance in T. californicus varies by life history stage in addition to sex.…”

“…Populations of T. californicus are genetically structured, particularly in the mid-to southern portion of its range (Burton, Feldman, & Curtsinger, 1979;Burton & Lee, 1994;Edmands, 2001;Ganz & Burton, 1995;Willett & Ladner, 2009), which may foster divergence in physiological tolerance among populations that experience differing abiotic conditions (i.e., local adaptation) (Sanford & Kelly, 2011). Tigriopus californicus populations have previously been shown to differ strongly in their phenotypic (Kelly, Sanford, & Grosberg, 2012;Leong, Sun, & Edmands, 2017;Willett, 2010) and transcriptomic (Schoville, Barreto, Moy, Wolff, & Burton, 2012) response to environmental gradients, most notably gradients in temperature, with southern populations more tolerant to heat stress. In addition to temperature, Tigriopus populations span a north to south gradient in which precipitation, humidity and wave height increase with latitude (Schoch et al, 2006) and likely influence the frequency and severity of low and high salinity events.…”