| Abstract: | The appearance, in situ activity and structure of the primary spines of the two deep sea echinothuriid sea urchins Phormosoma placenta and Araeosoma belli are described with particular reference to the unusual, fleshy, aboral spines. Oral primary spines of both species are clearly but differently adapted for movement over soft substrata. The aboral spines of both species bear fleshy extensions which are composed of gelatinous material in both species and are not poison sacs. Field experiments showed that the fleshy parts of the aboral spines of P. placenta 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 hypodermic 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 possible non-defensive functions are briefly discussed. |
