Variation in cold tolerance in escaped and farmed non-native green swordtails (Xiphophorus hellerii) revealed by laboratory trials and field introductions

Abstract

Distributions of non-native fish represent the outcome of a match between the habitat and the traits of a given species. Thermal tolerance, in particular, is highly variable among species and is one of the dominant organismal traits limiting the establishment and persistence of non-native species. Population and individual variation in thermal tolerance has implications for ecological processes such as the establishment of non-native species, especially when non-native species are introduced at the periphery of their thermal niche. Cold tolerance of wild-caught (escaped) and farm-raised (farmed) populations of green swordtail (Xiphophorus hellerii) was determined using field surveys and thermal tolerance methodology to examine whether populations exhibit predictable patterns of cold tolerance distributed across the landscape. The green swordtail, native to tropical Central America, is common in the ornamental trade with some evidence of establishment in subtropical Florida, USA. Seven escaped or farmed populations were examined: one import farmed (Thailand), two Florida farmed, two interior Florida escaped and two coastal Florida escaped. Highly variable chronic lethal minimum temperatures (CLmin) were found for individual fish, ranging from 5.5 to 11.3 °C. Significant population differences were also found where escaped populations (range 6.1–7.1 °C) exhibited lower CLmin than farmed populations (7.9–8.0 °C) and were nearly 50 % lower than those imported from Thailand (8.9 °C). The field exposure experiment revealed similar patterns; escaped populations exhibited greater survival during cold January conditions. Escaped populations were captured throughout the year, suggesting locally persistent populations. Whether of genetic or plastic origins, variation in thermal tolerance shown here has implications for the persistence and predicted range of introduced species.