Thermal tolerance and phenotypic plasticity in juvenile Hippocampus erectus Perry, 1810: Effect of acute and chronic exposure to contrasting temperatures

The seahorse Hippocampus erectus inhabits estuaries and coastal areas from southern Canada to northern Brazil, including the Yucatan Peninsula (YP), Mexico. Fish species living in thermally heterogeneous environments have developed physiological and behavioural adjustments that provide individuals with the phenotypic plasticity to respond to environmental changes within certain limits. When thermal changes exceed the ability of fish to adjust, energy supply is predominantly directed to fuel essential maintenance costs at the expense of growth and reproduction. In such circumstances, not only temperature, but also time of exposure, are elements that determine the adaptive value of phenotypic plasticity. The present study was designed to examine the consequences of acclimation of juvenile H. erectus to 18 and 30 °C on the phenotypic plasticity associated with exposure to acute thermal events (measured through the thermal limits) and chronic exposure during longer periods (measured through growth). Similar survival and growth was found in all seahorses that experienced acute exposure (~ 1 min) to temperatures ranging from 7 to 33 °C and 12 to 36 °C in both acclimation treatments, respectively. Chronic exposure to 30 °C compared to 18 °C, however, significantly reduced seahorse growth. A thermal tolerance polygon calculated as 302 °C2 suggests that H. erectus has a wide tolerance to the extreme conditions commonly present in estuarine environments. Given that the amplitude in the thermal tolerance polygon was mainly due to an increase in the lower limits of the interval and that no minimum temperature threshold could be determined, thus suggesting that the populations from YP are near their upper thermal limit.