Larval development of the tropical deep-sea echinoid Aspidodiadema jacobyi: Phylogenetic implications

The complete larval development of an echinoid in the family Aspidodiademat idae is described for the first time from in vitro cultures of Aspidodiadem a jacobyi, a bathyal species from the Bahamian Slope. Over a period of 5 mo nths, embryos grew from small (98-mu m) eggs to very large (3071-mu m) and complex planktotrophic echinopluteus larvae. The fully developed larva has five pairs of red-pigmented arms (preoral, anterolateral, postoral, postero dorsal, and posterolateral); fenestrated triangular plates at the bases of fenestrated postoral and posterodorsal arms; a complex dorsal arch; postero dorsal vibratile lobes; a ring of cilia around the region of the preoral an d anterolateral arms; and a long, unpaired posterior process containing a f enestrated rod. The presence of a posterior process and posterodorsal arms makes the larva of Aspidodiadema jacobyi much more similar to larvae of irr egular urchins in the order Spatangoidea than to other families of the orde r Diadematoida, to which the family is normally assigned. This unexpected l arval form lends support to a recommendation that the Aspidodiadematidae sh ould be either elevated to ordinal status as a sister group of the order Di adematoida, or split off as a sister group of the other families within the order. In either case, if we accept the parsimonious hypothesis that the a boral process and posterodorsal arms were derived only once in the evolutio nary history of euechinoids, then the larval data suggest that the Aspidodi adematidae may be very near the node where the irregular and regular euechi noids first diverged.