Thermal Response of Behavior and Gene Expression of Heat Shock Proteins in the Lined Seahorse, Hippocampus erectus
The lined seahorse, Hippocampus erectus, is a dispersal-limited fish residing in the coastal and estuarine habitats of the Western Atlantic. With sea surface temperatures expected to rise +2°C in the next 85 years, these dispersal limited fish will experience varying degrees of short-term (acute) and long-term (chronic) heat stress events. Previous studies have indicated that these different durations of increased temperatures will elicit distinctive behavioral changes in seahorses, but the molecular mechanisms underlying these changes are yet unknown. Three-hundred captive-bred Hippocampus erectus were exposed to control, mild, severe, and lethal temperatures for different durations to assess how these animals respond on a molecular and physiological level to heat stress events. Expression of the genes coding for heat shock proteins (HSPs) HSP60 and HSP70 were quantified to represent the molecular stress response of seahorses exposed to 22°C (the acclimation temperature of these animals, and therefore the control), 26°C, 30°C, and 32°C for acute (2, 6, and 10 hours) and chronic (4 weeks) periods of time. Respiratory rates, measured as breaths per minute, and activity levels were recorded daily to show the physiological response of seahorses to increased temperature for chronic durations. The relative expression of HSP60 and HSP70 increased in seahorses exposed to higher temperatures for acute durations as expected, and also increased through the 26˚C treatment temperature for the chronic duration. There was, however, a noticeable lack of the heat shock response at 30°C for the chronic duration, coinciding with a higher mortality rate at this temperature and indicating an inefficiency of the molecular protein-repair mechanisms at this temperature. Respiratory rates also increased with exposure to higher temperature. Seahorses are often used as indicator species of the health of an ecosystem. This research indicates that the relative expression of HSPs are an efficient marker of temperature stress in seahorses only when a large proportion of a population can be sampled.