Distaplia matua, Sanamyan, Karen & Sanamyan, Nadya, 2017

Sanamyan, Karen & Sanamyan, Nadya, 2017, Shallow-water Ascidians from Matua Island (central Kuril Islands, NW Pacific), Zootaxa 4232 (3), pp. 301-321 : 318-320

Distaplia matua n. sp.

( Figure 13 )

Material examined. Holotype: KBPGI 1451 /1, Kuril Islands , Matua Island, Point Kluv, 15 m, 25.08.2016 . Paratypes: KBPGI 1452 /2, Matua Island, Point Kluv , 15 m , 22.08.2016, one colony and KBPGI 1453 /3, same locality , 26.08.2016, one colony.

Description. The holotype (in formaline) is a dirty-brown, dark and an almost flat crust with glossy shinning surface, about 6 cm in diameter and up to 10 mm thick ( Figure 13 D). The test is firm, cartilaginous, not spongy in consistence, opaque, free from foreign particles on surface and inside. Position of each zooid is marked by branchial openings, but zooids are not visible through the test and the systems are not recognizable. In live the colonies look completely different ( Figures 12 A–C). They form low, wide, reddish or, sometimes, carmin red cushions, attached to substratum by the whole wide lower surface. The branchial openings of zooids are large and crowded, six-lobed, distributed evenly along the whole surface of the colony. Common cloacal openings are on the tops of short and wide siphons, situated, sometimes, on the slightly raised parts of the colony ( Figure 12 A). Common cloacal siphons have lobed margin.

The zooids, with strongly contracted thoraces, are about 3.5 mm long. They are deep-brown, or sometimes black-brown, opaque. The branchial siphon, when not contracted, is conspicuous, with six short lobes. Atrial opening is wide, in live all four rows of stigmata are exposed to the cloacal cavity, in contracted zooids it may be slit like. The longitudinal thoracic muscles are very numerous, crowded and difficult to count, approximately about 30 on each side. The circular muscles on the branchial siphons are well developed. The branchial tentacles usually, but probably not always, are distributed in a following way: three tentacles of the first size order are the longest (one dorsal and two ventro-lateral), between them are inserted three medium sized tentacles of the second size order, and there are several minute tentacles of the third size order. The stigmata are in four rows grouped by two (as in Sycozoa ). We counted about 14–17 stigmata in the middle rows but about 18–19 in the first row. The parastigmatic vessels are not discernible and probably absent. The oesophagus is long, its distal end bent at almost right angle to enter the stomach. The compact asymmetric obliquely oriented stomach is in the posterior half of the abdomen, on some distance from its posterior end. Stomach wall has numerous (about 20 or more) rather regular longitudinal folds ( Figure 12 E). The subdivision of the intestine is not pronounced and obscured by opaque wall of zooid. The gastric reservoir is present. The gonad is in the abdomen on the right side of the gut loop, not in the protruding sac, contains several (up to 10 were counted) oval male follicles, and one to three ova. Several zooids have minute brood pouch which just start to develop and contains no ova yet. The colonies contain no larvae.

Remarks. The species of the genus Distaplia , known from the region include: one species, D. alaidi , known from Kurile Islands only, two species, D. rzhavskii Sanamyan, 1993 and a species recorded as Distaplia sp. aff. clavata (Sars, 1851) by Sanamyan (1993) known from Kamchatka waters, and one species, D. dubia (Oka, 1927) from Sea of Japan.

Distaplia dubia is very common on sea algae in the vicinity of Vladivostok. Distaplia unigermis Ivanova- Kazas, 1965 is probably its synonym. We had chance to examine many colonies of D. dubia , they are either whitish or greenish, small, often composed of several separated systems (exactly as figured by Nishikawa, 1990, Figure 9 ) and in overall are very different from the present species. In the preservative they remain whitish and not become deep-brown as D. matua n. sp.

Distaplia rzhavskii has a colony composed of several closely placed upright lobes. Sanamyan (1993) pointed to transverse thoracic muscles as to a significant taxonomic feature of this species, but this may be connected with a way thoraces contracted during fixation and should be confirmed. The shape of the colony, however seems to be distinctive. Also, D. rzhavskii has areolated stomach, rather than longitudinally folded in D. matua n. sp.

Distaplia sp. aff. clavata: Sanamyan, 1993 has massive colony attached by a small point. Most probably it has no relation with D. clavata (Sars, 1851) and the colony, as well as the zooids with areolated stomach, differ distinctly from D. matua n. sp.

The species from Alaska, D. alaskensis Lambert et Sanamyan, 2001 , also has different colony, composed of club shaped lobes and different zooids with finely areolated stomach.

Distaplia matua n. sp. appears to be most closely related to Distaplia colligans Sluiter, 1932 , which also inhabits cold waters but in a geographically distant region, Magellan Islands and Antarctic Peninsula. The colonies of this species are flat, encrusting and dramatically change colour from bright yellow in live to deep-brown or almost black in preservative (see Sanamyan, Schories, 2003). The zooids have fine longitudinal plications on the stomach wall.

Figures

FIGURE 9. Aplidiopsis pannosum. A, zooids; B, preserved colony; C, larva.

FIGURE 12. Distaplia alaidi. A, preserved colonies; B, colony underwater; C and D, the same colony, enlarged to show details of cloacal opening and orientation of branchial siphons; E, zooids.

FIGURE 13. Distaplia matua n. sp. A, holotype KBPGI 1451 / 1; B, paratype KBPGI 1452 / 2; C, the same colony, enlarged view of branchial siphons; D, holotype, preserved; E, zooids.