The biology of larvae from the reef coral Porites astreoides, and their response to temperature disturbances

Pelagic larvae play a fundamental role in the life history of virtually all scleractinian corals, yet much of their biology remains unexplored. One aspect of coral larvae - their response to temperature perturbations - has potentially important consequences for understanding the effects of global warming and El Niño -Southern Oscillation events on coral recruitment. The present study tests the effects of temperature perturbations on coral larvae using Porites astreoides larvae as a model system. In June 1999, larvae were collected from 18 m depth on Conch Reef, Florida, and incubated at ambient (28°C), depressed (26°C), and elevated (33°C) temperatures in outdoor tanks shaded from direct sunlight. Treatments were repeated with new larvae every 24 h, and treatment effects quantified as larval motility, mortality, metamorphosis and metabolism. Elevated temperature significantly increased mortality and metamorphosis, and a similar trend was observed at the reduced temperature, although this was not significant. Neither elevated nor reduced temperatures affected larval motility. Gross photosynthesis (P) was significantly depressed by elevated and reduced temperatures, and respiration (R) varied proportionately with temperature (Q₁₀≅2), although this effect was not statistically significant. At the highest temperature the P/R ratio declined to < 1, indicating that thermal stress reduces the potential for autotrophy. Together, these results suggest that elevated temperatures affect coral larvae by depressing photosynthesis and creating an energy shortage, which ultimately could reduce recruitment (by increasing mortality), shorten larval longevity and favor premature metamorphosis. An unexpected finding was that larvae differed physiologically among release dates. Although preliminary, this suggests that larval fitness in Porites spp. may vary depending on the day of release, a phenomenon that could have significant ramifications with respect to the population structure of adults.