Life isn't easy for a sea star living in the intertidal zone. Daily tides leave them exposed and without access to cooling seawater for up to 6 h. As ectothermic animals, sea stars are unable to control their own body temperatures and if not careful they can find themselves precariously heating up during their daily aerial stints. But how hot is too hot? From the perspective of the sea star's central disc, which houses critical organs such as the stomach, 35°C is the upper limit before death – but what about the arms, do they have a different limit? Sylvain Pincebourde, currently a Centre National de la Recherche Scientifique researcher at the Institute of Research on Insect Biology, France, explains that until now most scientists have assumed that body temperature is even throughout the sea stars' bodies. However, with their characteristic spindly appendages, which should in theory allow a significant amount of heat to be lost via convection, their arms could be cooler. So, as part of his post doc at the University of South Carolina, USA, Pincebourde decided to investigate further. He wondered how sea star arms coped during low tide and whether arm temperature affected a sea star's decision to undergo arm abscission, a rare phenomena with an unknown trigger (p. 2183).
With the help of Eric Sanford and Pincebourde's post-doctoral mentor, Brian Helmuth, from the University of California Davis, USA, Pincebourde collected 70 sea stars off the coast of California. After acclimating them to the lab, the team grouped their aquatic testees into 10 groups, placing each group in an empty aquarium. Monitoring them with an infrared camera, the team found that, as expected, body temperature was indeed heterogeneous. However, in contrast to their initial expectations, the arms were warmer, not cooler, than the sea star's vital core. Perhaps convective heat loss played more of a role during their long periods out of water?
To mimic the hot conditions encountered during low tide, the team used overhead heat lamps to set and maintain the sea stars' core temperatures for 6 h. Each group was heated to a different core temperature, ranging from a bearable but warm 26°C to a deathly 42°C. Again, the team found that the sea stars' arms remained consistently higher than core body temperatures, ranging from 29 to 39°C when the core body temperatures were set to 26–35°C. As expected, these sea stars survived their warm spell, whereas sea stars with core temperatures set between 36 and 43°C weren't so fortunate and died within 24 h. Their deaths were expected due to their core temperatures being set so high, but the team were interested to find that these unlucky sea stars were also the only sea stars with arm temperatures lower than their core, ranging from 34 to 40°C.
The team think that the sea stars use their arms as heat sinks, actively drawing away heat from the essential core, to aid survival. While this may ensure a cooler core, it is not without its own disadvantages. When arms reached 33–39°C, after 2 days at least one arm would undergo abscission. Heating the arms to their own thermal limit is a sacrifice sea stars may make to ensure that core remains below 35°C. After all, it's better to lose an arm and save a life than die with five intact arms.