Next time you fly to some tropical beach resort and step barefoot in a couple of sea urchin spines, you’ll know that’s karma.

Cold-blooded paradox

One would expect species in the tropics to be well-adapted to high temperatures and to appreciate them. One would expect this to be especially the case with ectotherms, cold-blooded animals, as these require an external heat source to keep their metabolism at pace.

But last year we learned these creatures have evolved around a rather delicate temperature balance. It is right about good as it is, but don’t make it any warmer, tropical ectotherms told us in Nature.

The reason is that although indeed reptiles and insects need to warm up to start their daytime activity, it is also crucial for them to sufficiently cool at night, to lower their caloric requirements. When nighttime temperatures are elevated, the ectotherms need to find more food to compensate.

Now a new study published yesterday in PLoS ONE shows the same applies for marine tropical ectotherms.

A group of researchers of the University of Singapore, Cambrigde University and the University of Tasmania, have investigated the upper [lethal that is] temperature limits for ’34 tropical marine ectotherm species’ from 7 different phyla from intertidal and subtidal habitats. These included sea urchins, green mussels, different bivalve species and starfish.

Let’s heat the water and see if they die

‘Boiling the live frog experiments’ showed especially the subtidal species to be vulnerable to temperature rises. In a fast warming experiment [warming by 1 degree Celsius per hour] these species died at water temperatures between 37-41 degrees. If however the warming was slowed substantially to 1 degree per week or month, it turned out the subtidal ectotherms were even more vulnerable, with lethal temperatures monitored around 35.5 degrees Celsius.

And for a tropical lagoon or mangrove swamp that need not be exceptionally hot. The researchers therefore conclude survival margins under a warming climate are to be considered small for these species:

“Our data reinforce the suggestion that animals living in thermally stable environments have reduced acclimatory ability, and animals living constantly close to their upper limits in aseasonal environments are particularly susceptible to increases in temperature.”

“[…] in the long term, activity and survival of tropical marine organisms could be compromised just 2–3°C* above present seawater temperatures.”

[*) Two to three degrees. That should remind of the 450 target. But then unfortunately for many species that live in or around tidal zones ocean warming is not their only carbon concern – there is acidification too – and even CO2 levels of 390 ppm already prove to be damaging to such marine life.]

© Rolf Schuttenhelm | www.bitsofscience.org