Morphological diagnosis of the two genetic lineages ofAcrocnida brachiata(Echinodermata: Ophiuroidea), with description of a new species

Abstract

The burrowing brittle-starAcrocnida brachiatahas so far been regarded as a single, easily identifiable species. Recent studies showed habitat-related differences in maximum size, life span, breeding time and recruitment between intertidal and subtidal populations, which at first were attributed to environmental effects on individuals within the same species. Molecular data, however, strongly suggested the existence of two distinct lineages and ultimately two cryptic species with clear bathymetric segregation. Morphological evidence had so far not been presented, because any differences were interpreted as intraspecific variation. We collectedA. brachiatafrom intertidal and subtidal habitats at the coast of Brittany, France, and examined 15 specimens of each group externally by SEM. A key character ofA. brachiatais that the scales at the edge of the disc and on the ventral side are conically enlarged. Intertidal individuals showed a sparser disc scalation, more spine-like than conical ventral disc scales and spatulate, distally widened arm spines. In addition, we dissected several specimens of different size and examined the internal skeleton by SEM. The oral plates showed a rib-like structure on their abradial face that differs between individuals from either habitat. Subtidal specimens have fewer ribs than intertidal ones. These consistent differences support the existence of two species withinA. brachiata. To describe the second species, we needed to establish the identity ofA. brachiata. We describe a neotype, because no type material has been preserved since it was first described; it corresponds mainly to subtidal samples. The new species is described asAcrocnida spatulispinasp. nov. The taxonomic status ofAcrocnidahas been debated over the years with reference to its close affinities withAmphiura chiajei. We compared the species ofAcrocnidawithA. chiajeiandAmphiura filiformisand found thatAcrocnidais indeed morphologically similar toA. chiajei, among other characters by a similar oral plate structure, whereasA. filiformisdiffers greatly fromAcrocnidaas well asA. chiajei. Most strikingly, it has a different type of oral plate. These findings indicate that fundamental taxonomic changes may need to be made in the family Amphiuridae in the future.