Paramolgula gigantea (Cunningham, 1871)

Turon, Xavier, Cañete, Juan I., Sellanes, Javier, Rocha, Rosana M. & López-Legentil, Susanna, 2016, Ascidian fauna (Tunicata, Ascidiacea) of subantarctic and temperate regions of Chile, Zootaxa 4093 (2), pp. 151-180 : 176-177

Paramolgula gigantea (Cunningham, 1871)

Fig. 15

References and synonymy: Molgula gigantea Herdman (1882) p. 69; Paramolgula gigantea f typica Ärnbäck-Christie-Linde (1938) p. 15.

Localities: 1S, 2S, 7S.

Individuals up to 4 cm in diameter, body globular with whitish to brownish coloration. Tunic smooth or with small protuberances. There were no well-developed rhizoids, but the ventral part of the tunic often agglutinates some sediment and, occasionally, epibionts cover the tunic surface. The quadrangular siphons are short and relatively close together.

The mantle has abundant muscular fibres. The buccal siphon has ca. 15 heavily branched (up to 4 orders of branching) tentacles. The aperture of the neural gland forms an “U” tilted towards the right side and with the horns rolled inward. There is a smooth dorsal lamina. The branchial sac has 7 false folds, each formed by an elevated longitudinal vessel, on each side. Five conspicuous transverse vessels cross the branchial sac. In the meshes formed by the longitudinal and transverse vessels there is a network of smaller vessels underneath which lie the irregular stigmata, often arranged in spiral shapes.

The digestive has a stomach with a hepatic gland forming papillae (not arranged in rows) and a thin-walled intestine describing a double loop, the secondary loop completely closed. The renal sac on the right hand side is curved. The left gonad is adjacent to the distal part of the primary intestinal loop, in contact with the intestine and clearly outside the secondary loop. The right gonad is in contact with the renal sac. The gonads are elongated or globular, with a basal female part and testicular lobes occupying the sides and posterior part of the gonad. The female apertures are single, and there can be from one to four male papillae in each gonad.

Remarks. We have assigned the specimens of Paramolgula found in abundance in the Magellanic region to P. gigantea , a species previously synonymized with P. gregaria by Van Name (1945). This author noted the great variability of this taxon and decided to “lump” all species with similar appearance under P. gregaria . Later authors (e.g. Millar 1960, 1970, Kott 1969) have adopted that same list of synonyms, although some (e.g., Monniot & Monniot 1983) pointed out that the taxonomic status of this species needs revision.

Several authors have remarked the position of the left gonad in Paramolgula gregaria as being dorsal to, and partly in, the secondary loop of the digestive system (Van Name 1945, Kott 1969, Monniot & Monniot 1983). P. gigantea was described (as Cynthia gigantea ) by Cunningham (1871) with little morphological information. Later on, Herdman (1882) described it in more detail (as Molgula gigantea ) and defined the left gonad as lying “anteriourly to the intestine” (and so it has to be outside the secondary loop). In the same work, this author redescribed P. g re g ar i a (as Molgula gregaria ), a species described by Lesson (1830a), again with only superficial external description. According to Herdman, in P. gregaria the left gonad lies “in front of the intestine on the ventral side of the rectum” (thus it should be in front of the aperture of the secondary loop). There are, therefore, two forms differing in the position of the left gonad. This difference is likely to be of taxonomic value, all other characters being in general similar (or at least with overlapping variability).

Ärnbäck-Christie-Linde (1938) reported Paramolgula gigantea from Tierra del Fuego and Falkland Islands, with the left gonad “above both primary and secondary intestinal loops”. She also described a new form, P. gigantea f. capax, with the left gonad “in the concavity between the intestinal loop and the rectum, partly above the loop”. Apparently she didn’t realize that her P. gigantea f. capax was identical with P. gregaria , and indeed her drawings were used by Van Name (1945) to illustrate P. g re g ar i a. For some reason, the distinction between species with the gonad outside the secondary loop and those with the gonads (partly) inside the secondary loop was not maintained and everything was considered as P. gregaria since Van Name (1945).

We have found only specimens with the left gonad totally outside the secondary loop, anterior to its aperture. We placed them in P. gigantea , which we consider a valid species. We have not found any “true” P. gregaria according to this character. Recent papers on the Chilean coasts report P. gregaria (Lagger et al. 2009, Tatián & Lagger 2010) and both refer that the left gonad is included in the secondary loop. This may be, however, a misinterpretation, as the figure in p. 904 of Tatián & Lagger (2010) shows a gonad outside and anterior to the secondary loop. Clearly, all citations of P. gregaria need to be revised. P. c h i l en s i s Hartmeyer, 1914, a species with the left gonad outside the secondary loop may be the same as P. gigantea . Another described species, P. canioi Monniot & Monniot, 1983 , also has the left gonad outside the secondary loop, but it seems to be consistently of smaller size (up to 1.5 cm). It is likely that there are several species of Paramolgula with the left gonad in either position. The taxonomy of this genus still requires clarification, but this task is outside the scope of the present work. We note that our specimens have the left gonad outside the secondary loop, name them as P. gigantea , and call for caution when considering previous citations of the genus in the subantarctic area.

Figures

FIGURE 15. Paramolgula gigantea. A, underwater image of an specimen from Porvenir Bay; B, dissected individual; C, same individual without branchial sac; D, close-up of the aperture of the left gonad (stained), showing one female (♀) and three male openings (short tubes around ♀); E, image of the branchial sac (stained). Scale bars: A, B, C, 2 cm, D, 2 mm, E, 0.2 mm.