Aboral depressions in the tests of the sea urchin Tripneustes cf. gratilla (L.) in the Gulf of Eilat, Red Sea

Abstract

Flattening and aboral depressions occurred in high proportions in Tripneustes cf. gratilla (L.) populations that inhabited an artificial lagoon in the northern Gulf of Eilat, Red Sea. The average test height to diameter ( H D ) ratio of the most affected population, near a small wastewater outlet, was 0.43 ± 0.01 (Se) and 76% showed aboral depressions. The H D ratio was 0.54 ± 0.01 in populations outside the lagoon and <1% exhibited aboral depressions. The exposure of normal T. cf. gratilla to industrial CaCO 3 precipitation inhibitors affected the growth and H D ratio in a similar manner within 30–45 days. The light-weight tests of the deformed urchins and the occurrence of skeletal resorption in the interior part of the test indicate that the unknown pollutant, possibly contained in laundry wastewater, reduced calcification by these animals and resulted in a mechanical collapse of the tests. A regular pattern of small pits at the aboral tip of the ambulacra suggests that intestinal mesenterial threads, attached to the test at the same points from within, facilitated this collapse by a mechanical pull. The normal test morphology of the regular echinoids may be regulated by the mechanical activity of various contractile and elastic tissue elements, among which mesenterial threads are probably included.