Latitudinal variation in larval development of coral reef fishes: implications of a warming ocean

Latitudinal gradients in water temperature may be useful for predicting the likely responses of marine species to global warming. The ranges of coral reef fishes extend into the warmest oceanic waters on the planet, but the comparative life-history traits across their full latitudinal range are unknown. Here, we examined differences in early life-history traits of 2 coral reef fishes, the damselfish Pomacentrus moluccensis and the wrasse Halichoeres melanurus, among 8 locations across 21 degrees of latitude, from northern Papua New Guinea (2.3 degrees S) to the southern Great Barrier Reef (23.3 degrees S). Water temperature during larval development ranged between 25.6 and 29.8 degrees C among sites, with the warmest sites closest to the equator. Recently settled juveniles were collected and otolith microstructure was analysed to estimate pelagic larval duration (PLD), daily growth, and size at settlement. Latitudinal comparisons revealed a non-linear relationship between temperature and each of PLD, larval growth and size at settlement. PLD declined with increasing temperature up to approx. 28 to 29 degrees C, above which it stabilised in P. moluccensis and increased in H. melanurus. Larval growth increased with increasing temperature up to approx. 28 to 29 degrees C before stabilising in P. moluccensis and decreasing in H. melanurus. Size at settlement tended to be highest at mid-latitudes, but overall declined with increasing temperature above 28.5 degrees C in both species. These results indicate that the thermal optima for growth and development is reached or surpassed at low latitudes, such that populations at these latitudes may be particularly vulnerable to global warming.