Limited plasticity in thermally tolerant ectotherm populations: evidence for a trade-off.

Many species face extinction risks owing to climate change, and there is an urgent need to identify which species' populations will be most vulnerable. Plasticity in heat tolerance, which includes acclimation or hardening, occurs when prior exposure to a warmer temperature changes an organism's upper thermal limit. The capacity for thermal acclimation could provide protection against warming, but prior work has found few generalizable patterns to explain variation in this trait. Here, we report the results of, to our knowledge, the first meta-analysis to examine within-species variation in thermal plasticity, using results from 20 studies (19 species) that quantified thermal acclimation capacities across 78 populations. We used meta-regression to evaluate two leading hypotheses. The climate variability hypothesis predicts that populations from more thermally variable habitats will have greater plasticity, while the trade-off hypothesis predicts that populations with the lowest heat tolerance will have the greatest plasticity. Our analysis indicates strong support for the trade-off hypothesis because populations with greater thermal tolerance had reduced plasticity. These results advance our understanding of variation in populations' susceptibility to climate change and imply that populations with the highest thermal tolerance may have limited phenotypic plasticity to adjust to ongoing climate warming.

Differential toxic effects of Ulva lactuca (Chlorophyta) on the herbivorous gastropods, Littorina littorea and L. obtusata (Mollusca).

Members of the genus Ulva are widespread and abundant in intertidal and shallow subtidal areas but there are conflicting data regarding susceptibility to herbivory. While some studies have documented that Ulva spp. were favored by a diversity of marine herbivores, other work has revealed herbivore deterrence. We investigated grazing and growth rates of the littorinid species, Littorina littorea and L. obtusata, when offered Fucus vesiculosus, Ascophyllum nodosum, Ulva lactuca, and Chondrus crispus, highlighting distinctive vulnerabilities to toxic effects of U. lactuca. Ulva lactuca was the preferred food of L. littorea, while L. obtusata showed no grazing on this ephemeral algal species. In contrast, F. vesiculosus was highly preferred by L. obtusata. Although L. littorea demonstrated a grazing preference for U. lactuca, growth rate of this gastropod species was nearly 3× greater when fed F. vesiculosus, suggesting a non-lethal, negative effect of U. lactuca on L. littorea with long-term exposure. Mortality of L. obtusata ranged from 0% to 100% when held in the presence of various Ulva densities for 1 week, and Ulva exudate depressed herbivory of this gastropod. We conclude that the water-soluble, toxic exudate produced by U. lactuca in response to herbivory had allelochemical properties, and may contain a cleavage product (acrylic acid) of dimethylsulfoniopropionate or reactive oxygen species (i.e., H2 O2 ). Observed differences in susceptibility to Ulva toxicity by the littorinid species may be related to generalist versus specialist feeding and habitat strategies.