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

The complete larval development of an echinoid in the family Aspidodiadematidae is described for the first time from in vitro cultures of Aspidodiadema jacobyi, a bathyal species from the Bahamian Slope. Over a period of 5 months, embryos grew from small (98-micron) eggs to very large (3071-micron) and complex planktotrophic echinopluteus larvae. The fully developed larva has five pairs of red-pigmented arms (preoral, anterolateral, postoral, posterodorsal, and posterolateral); fenestrated triangular plates at the bases of fenestrated postoral and posterodorsal arms; a complex dorsal arch; posterodorsal vibratile lobes; a ring of cilia around the region of the preoral and anterolateral arms; and a long, unpaired posterior process containing a fenestrated rod. The presence of a posterior process and posterodorsal arms makes the larva of Aspidodiadema jacobyi much more similar to larvae of irregular urchins in the order Spatangoidea than to other families of the order Diadematoida, to which the family is normally assigned. This unexpected larval form lends support to a recommendation that the Aspidodiadematidae should be either elevated to ordinal status as a sister group of the order Diadematoida, 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 aboral process and posterodorsal arms were derived only once in the evolutionary history of euechinoids, then the larval data suggest that the Aspidodiadematidae may be very near the node where the irregular and regular euechinoids first diverged.