Living On The Edge: Small Thermal Safety Factors For Fertilization And Development In The Tropical Sea Urchin Lytechinus variegates

Thermal tolerance is particularly relevant to understanding and predicting organisms' responses to changing environmental conditions because it influences organisms' performance and in turn limits their biogeographic ranges. Tropical terrestrial organisms are thought to live near their thermal tolerance limits, and small thermal safety factors put them at risk from global warming. However, little is known about the thermal tolerance of tropical marine invertebrates, and how this limit relates to the ambient temperatures they may experience. We used embryos and larvae of the tropical sea urchin Lytechinus variegatus from Bocas del Toro, Panama to document the effects of chronic elevated temperatures and acute heat stress on development of this common shallow−water species. Both fertilization success and short−term larval survival after 2−hour exposure to elevated temperatures were high between 28−32°C, with a rapid drop in survival and a median lethal temperature (LT₅₀) of ~34.5°C. Long−term rearing showed good (over 50%) survival to metamorphosis from 23−33.5°C, with a small percentage of larvae surviving slightly higher temperatures. Larvae grown at the highest temperatures were smaller, metamorphosed slightly later and at smaller sizes than those reared at 26−30°C. Environmental data show that larval L. variegatus do not currently experience temperatures over their thermal limit in Bocas del Toro. Published predictive models suggest they will be begin to experience negative impacts at shallow sites by 2054 and throughout much of the Bahia Almirante by 2084. Our results highlight that tropical marine invertebrates could have small thermal safety factors during some stages in their life cycles, and they may be vulnerable to climate change in the near future.