Variation of thermal tolerance during northward range expansion in the invasive golden star tunicate, Botryllus schlosseri.

Abstract

Populations within a species can differ with respect to their thermal physiology, with variation often observed across gradients in environmental temperature with latitude or elevation. The tempo at which phenotypic plasticity and/or local adaptation are able to shape variation in thermal tolerance has implications for species persistence in an increasingly volatile climate. Having encountered novel environments during introduction and subsequent range expansion, non-indigenous species present useful case studies for examining thermal tolerance differentiation on contemporary time scales. Here we test for differentiation of heat and cold tolerance among three populations of the invasive golden star tunicate, Botryllus schlosseri (Pallas), spanning a 24.3° latitudinal gradient in the Northeast Pacific. We observed differentiation of post-larval heat tolerance among our sites, with our southern, putatively warm-adapted population exhibiting a significantly higher LT₅₀ than the two more northern populations. We also found that adult cardiac performance at cold temperatures is progressively greater in colder, higher latitude populations. This pattern may suggest compensatory genetic adaptation to colder environmental temperatures. By examining both heat tolerance and cold performance simultaneously among populations of an invasive ascidian, we document how this marine ectotherm is capable of shifting its physiology to novel environmental conditions over compressed time scales, with implications for the spread of this invasive species and, more broadly, for species' responses to temperature in an era of global change.