Do tropical specialist sea urchins have higher thermal tolerances and optimal temperatures than their more widely distributed relatives?

Abstract

Warming tolerance (WT), thermal safety margins (TSMs), and thermal performance curves have been documented empirically for few tropical marine invertebrates, although calculations of realized niches from biogeographic data suggest that WT is lower for tropical organisms than for temperate organisms. To determine these characteristics for 8 species of Caribbean sea urchins, we documented the effects of acute heat stress and cold stress on righting time (a measure of performance) and survival. The upper lethal limit occurred between 35.1 and 37.1°C for all of the species, and the upper limit for righting occurred very close to this, between 34.0 and 36.9°C. The lower lethal limit occurred between 4.8 and 7.6°C for all species except for Tripneustes ventricosus, for which it was 14.6°C. The lower limit for righting was significantly warmer than the lower lethal limit and occurred between 13.4 and 14.6°C for all species except for T. ventricosus (19.1°C). Within these critical limits, the thermal performance curves are broad, and optimal performance windows range from 26 to 32°C. Environmental data show that in Bocas del Toro, Panama, urchins have warming tolerances of 6 to 8°C for 2 h exposures but that TSMs range from -4 to 2°C, highlighting that these species are vulnerable to moderate environmental warming. Species with exclusively tropical ranges did not show higher optimal temperatures than species with ranges that extend into the subtropics or temperate regions.