Mass mortalities of echinoids and other reef flat organisms coincident with midday, low water exposures in Puerto Rico

Echinoids, notably Lytechinus variegatus and Tripneustes ventricosus, and other reef flat animals (brachyuran crabs, chitons and ophiuroids) were observed to suffer heavy mortality in Puerto Rico during extreme, midday low water stands which occur in the spring and summer seasons. Death often resulted from prolonged exposure to intense heating (up to 40°C) in pools and slowly circulating bodies of water over periods of clear and calm weather; desiccation caused death in echinoids which subsequently floated away from the reef with the rising tide. The tolerance limits to exsiccation and high thermal stress were determined and found to lie within the lethal range realized on the reef. Unprotected Tripneustes may also be killed from exposure to the shorter radiations of sunlight. Severe mortalities were observed in populations of Lytechinus (64%) and Tripneustes (86%) located at a shallow depth toward the lee side of the reef. Less affected were the echinoids Echinometra lucunter, Diadema antillarum and Brissus unicolor. Similar, tide-related echinoid kills are expected to occur at other localities in the Caribbean region.