Temperature Effects on Growth Performance, Fecundity and Survival of Hippocampus guttulatus

This experiment aimed to determine the temperature limits beyond which seahorse growth and reproduction become suboptimal due to climate change. Four temperatures (16, 20, 24, and 28 °C) were tested to evaluate their effect on juvenile (1–56 days post-parturition (DPP)) and adult (one year old) long-snout seahorses, Hippocampus guttulatus. Additionally, the reproductive performance of adults was observed. Another experiment measured oxygen consumption (MO2) in the same age groups and temperatures. Adults showed significantly higher growth rates at 20 and 24 °C compared to 16 and 28 °C. Adult mortality rates were 0%, 0%, 6.2%, and 62.5% at the respective temperatures. Juvenile growth performance was higher at 20 °C and 24 °C but significantly lower at 16 °C and null at 28 °C, with survival rates of 8%, 62%, 10%, and 0%, respectively. Oxygen consumption increased with temperature, ranging from 106.3 ± 3.1 to 203.3 ± 3.1 μmol O2/g BW/h at 16 °C, and from 127.6 ± 3.5 to 273.3 ± 3.1 μmol O2/g BW/h at 28 °C for adults and 1 DPP juveniles, respectively. The study highlights that juvenile and adult H. guttulatus have narrow thermal boundaries, beyond which reproduction, growth, and survival are seriously affected. Under climate change, the species appears unable to cope, potentially leading to their rapid disappearance.