Rhopalaea Philippi, 1843

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 295

Genus Rhopalaea Philippi, 1843

REMARKS

This genus units phlebobranch species, solitary or colonial, with the gut located under the branchial sac. It is closely allied to the genus Diazona , in which all species are colonial and have at least the abdomen included in a common tunic. Rhopalaea species are known in all warm and temperate seas, from shallow waters to the bathyal area. Their mode of budding is not completely known. In some solitary species a regeneration of the thorax starting from the abdomen has been demonstrated ( R. cloneyi Vasquez & Young, 1996 ). In other species the regeneration has not been directly observed but can be supposed, as some specimens have rings of hard tunic around the abdominal region which correspond to the successive accumulation of tunics from regressed thoraces; examples of the latter are R. neapolitana Philippi, 1843 and R h o p a l a e a p e r l u c i d a Monniot C., 1997. Colony formation has been observed two times by Millar (1975) in Rhopalaea fusca ( R. crassa ), where buds arise from vascular ampullae, and in R. piru Monniot C. & Monniot F., 1987 . In the latter case, there is a multiple regeneration phenomenon.

The first descriptions of Rhopalaea were established from badly preserved specimens, often not complete. All the Rhopalaea have the same kind of thorax, and the gut and gonads are embedded in an opaque tissue, characters which were unavailable to the authors to distinguish the species. The type specimens and those from old collections are in poor condition, and the criteria now used to distinguish the species in this genus cannot be seen in them. In these circumstances, it is understandable that some authors have concluded that there might be only one species in the genus: “the information that is presently available suggests that R. neapolitana , R. crassa , R. birkelandi and R. abdominalis could represent populations of a single diverse circumglobal species” ( Kott & Goodbody 1982: 510). In that publication the name R. crassa covers all described species from the western Pacific Ocean. The last descriptions of R. crassa from Kott & Goodbody (1982), Nishikawa (1991), and Kott (1992b) are difficult to work with, as characters of several species are mixed up.

For some years the species of Rhopalaea have been collected in excellent condition and photographed in situ. Precise descriptions have been made and have shown that morphological differences exist between the species. The genus is diversified but few species are yet identified. The large variety of shapes and colours is well-illustrated by Gosliner, Behrens & Williams (1996: 293) in “Coral Reef Animals of the Indo Pacific”. In the five photographs we recognise Rhopalaea fusca and Rhopalaea circula n. sp. The three others have distinctly different appearances and probably belong to other species, but they were not collected. It is also probable that the species figured in Kott (1990: pl. 1b, c), which has the extremity of the sperm duct marked with a yellow point, is undescribed.