FORM AND FUNCTION OF THE PRIMARY SPINES OF 2 BATHYAL ECHINOTHURIID SEA-URCHINS

The appearance, in situ activity and structure of the primary spines o f the two deep sea echinothuriid sea urchins Phormosoma placenta and A raeosoma belli are described with particular reference to the unusual, fleshy, aboral spines. Oral primary spines of both species are clearl y but differently adapted for movement over soft substrata. The aboral spines of both species bear fleshy extensions which are composed of g elatinous material in both species and are not poison sacs. Field expe riments showed that the fleshy parts of the aboral spines of P. placen ta are palatable to shallow-water fish. However, the aboral spines are shown to have stored within the hollow ossicle of the spine secretory material likely to be irritant in function. The structure of the tip of the spine ossicle of both species suggests that they may act like h ypodermic needles. We conclude that the aboral spines of both species are probably defensive in function but remain equivocal over the exact role of the massive fleshy extensions found in P. placenta. Other pos sible non-defensive functions are briefly discussed. (C) 1998 The Roya l Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved.