whereas in the cushion star Asterina gibbosa, oxygen consumption increases during emersion at any given temperature .
Intertidal echinoderms such as the purple urchin Strongylocentrotus purpuratus may be able to use a facultative lung to obtain oxygen , while for most echinoids and asteroids, aerial gas exchange occurs across the tube feet, aboral papulae or body surface. Because the tube feet collapse in air the effective area for gas exchange is reduced , resulting in a decrease in coelomic fluid PO2 ; nevertheless, these intertidal echinoderms appear to undergo very little anaerobic metabolism. Coupled with a decrease in PO2 there is a concomitant rise in coelomic fluid PCO2 levels during emersion . The urchins Psammechinus miliaris and Echinus esculentus buffer the resulting acidosis by mobilising carbonate
stores from the test , while the decrease in pH is uncompensated in the purple urchin S. purpuratus The ability to buffer the coelomic fluid appears to be limited in the Asteriodea, possibly because they possess more efficient gas exchange
mechanisms that allow CO2 to be excreted directly across the body surface, whereas retraction of the tube feet during emersion and the thicker harder test of urchins may hinder this process .