Hood Coral

The corals at the northern end of this sea survive summers that would kill the same species almost anywhere else, and the reason is a bottleneck they passed through thousands of years ago.

After the last ice age the basin was recolonized from the south, and everything moving north had to cross the hot, shallow strait at its mouth. That crossing worked as a filter: only heat-tolerant genotypes got through. Their descendants now live in the cooler north, several degrees below the temperature they can actually stand - colonies in the Gulf of Aqaba have been held at 34 C, some 7 C above their local summer maximum, without bleaching. It is the main reason this sea is discussed as a refuge while reefs elsewhere are dying, and this coral is the animal most of that work was done on.

It is the laboratory rat of coral biology. Its genome has been sequenced, and it is the standard species for studies of calcification, of the algae living inside coral tissue, and of how a colony tells its own tissue from a neighbor's. One reason is convenience: it broods its young rather than releasing eggs to the current, so a colony in a tank delivers swimming larvae on a lunar schedule instead of one night a year.

On the reef it is a low, rounded colony of thick blunt branches, forked and often swollen at the ends, cream to pink to blue depending on the light. The branch surface is finely beaded, each polyp sitting in a shallow cup with a hooded rim, which is where the English name comes from.

One caution for anyone matching photographs: the name covers more than one animal. Barcoding has shown that Stylophora pistillata as usually applied is a complex of several distinct lineages, and the regional guide notes that a second described species from here is probably the same thing again.