An echinoid that “gives birth”: morphology and systematics of a new Antarctic species,Urechinus mortenseni (Echinodermata, Holasteroida)

Summary

Male and female echinoids are almost always separate, and seldom exhibit pronounced sexual dimorphism. In the most extreme previously known examples of sexual dimorphism, ova, zygotes, and juveniles are housed in a shallow depression in the surface of the test, or in a deepened petaloid ambulacrum. A new species of urechinid holasteroid,Urechinus mortenseni, is described in which juveniles are brooded inside a deeply invaginated extension of the body wall suspended from the interior edges of the female's apical plates. Although technically not inside the test, the juveniles are completely invisible from the exterior, and are contained in a series of brood pouches that communicate with the environment only through a small opening at the apex. This is the first complete description of brooding in the Holasteroida. The morphology of the brooding system is described, and a new terminology is erected to refer to the completely novel features of the system: the apical aperture, birth canal, and brood pouches. The plating, spines, and pedicellariae of the brooding system are also described. The salient characteristics of the brooding system found inU. mortenseni are contrasted with those of brooding strategies in other echinoderms. The new species is compared with other holasteroids. For the first time, a brooding system is also described inPlexechinus nordenskjoldi, which implies that it is closely related toU. mortenseni. This casts doubt on the integrity of the two genera, and suggests that a phylogenetic revision is required to highlight the unique features found not only in these unusual urechinids, but in other holasteroids as well.