Paper: Mascaró M, Amaral-Ruiz M, Huipe-Zamora I, Martínez-Moreno G, Simões N, Rosas C (2016) Thermal tolerance and phenotypic plasticity in juvenile Hippocampus erectus Perry, 1810: Effect of acute and chronic exposure to contrasting temperatures. J Exp Mar Bio Ecol 483:112–119 (Full text!!)
It’s no secret that our oceans are warming up. It stands to reason that some animals will be able to handle the temperature increase, and others may not do so well under different conditions. If animals exhibit phenotypic plasticity – the idea that animals have some “leeway room” in the conditions that they can inhabit, due to different physiological tradeoffs that they can make – they’re likely to successfully survive environmental changes.
Animals that live in variable environments like tidepools exhibit a lot of phenotypic plasticity – they can survive large temperature and salinity ranges caused by high and low tides – whereas animals that live in stable environments like the deep sea do not, since their environment never changes. Animals used to changing conditions are more likely to do well with climate change, since they’re used to those changes.
However, animals do have what’s called an upper thermal limit. Much like people can’t survive for very long in ridiculous heat, animals can’t either. At some point, the water will get too hot for them to handle, and processes that keep their body moving (circulation, respiration, etc) won’t be able to function. The researchers here designed their study to figure out what the thermal limits are for seahorses, but with a twist – first, they were acclimated to 18°C and 30°C. They wanted to find out if the temperature a seahorse was “used to” affected their thermal range. They hypothesized that the seahorses that were “used to” a higher temperature would have a smaller thermal range, since they’re already living at high temperatures.
Methods & Results
To test that hypothesis, the researchers first needed some seahorses. They collected seahorses from Mexico, let them hatch out, and grew them up to the juvenile size. They then put the juveniles in two groups and raised them up in 18°C and 30°C aquariums for 30 days. Once that 30 day acclimation period was over, they tested the seahorses’ acute temperature tolerance. Testing the acute tolerance is important because it simulates what happens with an isolated event, like a seahorse swimming into a very high temperature area to get prey.
Then, they put one seahorse from either the 18°C or the 30°C treatment in a small chamber and increased the temperature (approximately +1.5°C per minute), noting its behavior throughout the entire trial. After that, they put another seahorse in the chamber and decreased the temperature (approximately -1°C per minute) and observed its behavior. They repeated the process until they had enough seahorses to make conclusions.
The seahorses from both treatments went through the same sequence of behavior throughout the temperature increase trial. At the start, the seahorses were stable, showing normal breathing and swimming. After that, the seahorses were stressed, showing a lot of unusual tail movement. After that, they showed disorganized locomotive activity, showing increased breathing and swimming upside down. Finally, they showed muscle spasms and hyperventilation. The difference between the treatments is evident in the temperature at which each stage happened (Table 1). The seahorses acclimated to the lower temperature showed a larger range – just what our researchers expected – even though the seahorses acclimated to the higher temperature could withstand higher temperatures.
When the temperatures decreased, the seahorses showed a different sequence of behavioral changes, but the pattern was the same of the increases: the seahorses acclimated to a higher temperature had a lesser thermal range (Table 2).
Why should we care about this? Why does it matter if seahorses have a narrow thermal range? Two reasons:
First, seahorses that have a wider thermal range can exploit more resources. Imagine you’re in the desert, and the only place with food is the hottest place. You can go, because you can tolerate the heat, but maybe your friend can’t, and they won’t be able to find food. As temperatures rise, seahorses who can handle the heat will be able to consume more food (or even escape more predators) than other fish that may not have that thermal tolerance.
Second, climate change may push the temperature to conditions that seahorses can no longer handle. This study also measured seahorse growth at 18°C and 30°C; seahorses at the higher temperature did not grow as much per day (Figure 1).
That suggests seahorses that live in tropical environments are already living at the edge of their thermal range – the higher growth rates at the lower temperature suggest that the optimal range would be closer to 18°C. Seahorses living in the Caribbean and the Gulf of Mexico will likely have to shift north to avoid the heat, or else their growth may be stunted.
Engage: What other animals grow better at cooler or warmer temperatures? What will happen to their populations as temperatures change?