<P> Pedicellaria are a good means of defense against ectoparasites, but not a panacea as some of them actually feed on it . The hemal system defends against endoparasites . </P> <P> Sea urchins are established in most seabed habitats from the intertidal downwards, at an extremely wide range of depths . Some species, such as Cidaris abyssicola, can live at depths of several thousands metres . Many genera are only found in the abyssal zone, including many cidaroids, most of the genera in the Echinothuriidae family, and the "cactus urchins" Dermechinus . One of the deepest - living families is the Pourtalesiidae, strange bottle - shaped irregular sea urchins that live only in the hadal zone and have been collected as deep as 6850 metres beneath the surface in the Sunda Trench . Nevertheless, this makes sea urchin the class of echinoderms living the least deep, compared to brittle stars, starfish and crinoids that remain abundant below 8,000 m (26,250 ft) and sea cucumbers which have been recorded from 10,687 m (35,100 ft). </P> <P> Population densities vary by habitat, with more dense populations in barren areas as compared to kelp stands . Even in these barren areas, greatest densities are found in shallow water . Populations are generally found in deeper water if wave action is present . Densities decrease in winter when storms cause them to seek protection in cracks and around larger underwater structures . The shingle urchin (Colobocentrotus atratus), which lives on exposed shorelines, is particularly resistant to wave action . It is one of the few sea urchin that can survive many hours out of water . </P> <P> Sea urchins can be found in all climates, from warm seas to polar oceans . The larvae of the polar sea urchin Sterechinus neumayeri have been found to use energy in metabolic processes twenty - five times more efficiently than do most other organisms . Despite their presence in nearly all the marine ecosystems, most species are found on temperate and tropical coasts, between the surface and some tens of meters deep, close to photosynthetic food sources . </P>

How long can a sea urchin live out of water
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