Page:Popular Science Monthly Volume 85.djvu/225

Rh but now called Acropora, are killed at 97.5°, and cease to take food at about 90.5° while the more resistant forms such as various species of Siderastrea, and Porites and some of the brain corals, survive to 100°. or even 102° F., but cease to feed at from 95° to 97.5° F. Now, in general, those corals which are most sensitive to high temperature are correspondingly so to the smothering effects of silt. The more delicate forms such as Acropora, Pocillopora and Seriatopora are killed by being buried for only 10 to 1-i hours beneath the mud, whereas, those corals which die at 100° can withstand as much as 40 to 50 hours of burial and Siderastrea radians of the Atlantic, which dies at 102°, can survive being buried 72 hours without apparent injury. It seems, therefore, that high temperature may produce death by causing asphyxiation, and thus those corals which can withstand the highest temperature are usually those which are best able to resist being covered by mud and silt, and these are the very corals which live in the hot, muddy shallows near the shore, while corals which require pure water live far out upon the reef, where the temperature is lower.

It will be recalled that Winterstein who studied frogs decided that the nervous paralysis that results from high temperature was caused by asphyxiation; but later Babàk, and Amerling showed that some of the frogs and toads are very resistant to lack of oxygen but easily paralyzed by heat, while the reverse is the case with others. Becht also casts doubt upon the asphyxiation theory by showing that recovery from heat paralysis can take place in the absence of oxygen in the water surrounding the nerves of the frog or of the horse-shoe crab (Limulus). Oxygen may however have been derived from the tissues themselves.

Among corals we find that Favia fragum and Mæandra areolata are more resistant to the effects of CO2 or of silt than one would expect from their death temperatures, which are fairly low. If however they be buried in the sand and then heated they are still nearly as resistant as if in the open water, whereas sensitive corals such as Acropora or Orbicella are killed at lower temperatures if buried than if heated in the open water.

The explanation may be that Favia and M. areolata can survive at a low as well as at a high rate of metabolism, in other words "hibernate" under the mud; and thus be almost as well able to resist heat in this condition as when living at a higher rate of oxygen-consumption in the free water of the ocean.

We still incline to the belief, therefore, that high temperature may produce death in corals by asphyxiation, although other factors may complicate the matter, as is so often the case in physiological reactions.

Vaughan, indeed, has shown that those corals which live in muddy regions are quite able to free themselves from silt by means of the cilia which cover their surfaces, but those forms which live in the pure water of the outer parts of the reef are not so efficient in this respect.