Aplidium dissectum, Sanamyan, Karen & Sanamyan, Nadya, 2011

Sanamyan, Karen & Sanamyan, Nadya, 2011, Shallow-water species of the genus Aplidium (Ascidiacea) from Kamchatka and Commander Islands, Zootaxa 2922, pp. 41-50 : 43-46

Aplidium dissectum n. sp.

(Figures 2,3)

Material examined. Kamchatka, Avacha Bay: Starichkov Island, 10m, 15.09.2002 (Holotype KBPIG 1439/1); 13m, 20.06.2006, two colonies (Paratypes KBPIG 1440/2, 2006-06-20-01); Vilucha Bay, Laperusa Rock, 12m (Paratype KBPIG 1441/3, 2006-07-25-04).

Description. Each colony is a group of low cushions or lobes closely adherent to each other. Each lobe is about 2 or 3 cm in the greatest diameter, but occasionally up to 8 cm. On preserved material the upper surface of the lobes is concave but more rounded on living specimens ( Figure 3 D). Basally the lobes are joined and a colony is attached to substratum by not very large area. Sand grains may present on the sides of the lobes and at the base of the colony, but generally foreign particles are sparse and completely absent inside, and the upper surface of the colony is always clear. Living specimens are dull yellow-reddish, preserved in formaline are brownish with slight reddish tint. The common test of the living specimens is slightly translucent, but not transparent, and zooids are not visible through the test on underwater photographs. In preservative it becomes more clear, but still dull and foggy. Zooids arranged along branched but rather short cloacal canals converging to the cloacal openings. Short transparent cloacal siphons, several on each lobe, are visible on living specimens but retract completely in preservative and the surface become quite smooth, without any kind of depressions indicating systems.

Zooids are up to 15–20mm long with rather short thorax and abdomen and long postabdomen. They are mostly parallel to each other. Short branchial aperture has usual six small pointed lobes. Upper rim of small atrial aperture is drawn into occasionally simple but usually three-lobed atrial languet. Zooids with simple and three-lobed languets may occur together within the same lobe of colony and the shape probably depends on the position of a particular zooid in the system. About dozen of fine longitudinal muscles are in the thoracic wall. The branchial sac has usually 11, sometimes 10 or 12 rows of about 12–14 stigmata.

The abdomen is shorter than thorax. A short cylindrical stomach located in the middle of abdomen has 24–28 rather irregular and shallow folds which can be counted with reasonable degree of certainty only on transverse cross-sections. Subdivision of the post pyloric part of the gut loop on typical for Aplidium regions is quite clear: there is rather long duodenum, mid-intestine with oval posterior stomach and noticeable rectal valves.

The posterior abdomen is long and in many zooids looks empty. Several large ova may be located just below the gut loop or some distance from it, in the upper third of the posterior abdomen.

Numerous larva and embryos incubated in the atrial cavity of many zooids. The trunk of the larva is 0.7– 0.75mm long. Three adhesive organs alternate with four median ampullae (which often have forked ampullaceous ends) and four large bulbous lateral ampullae on each side of the trunk. Several small epidermal vesicles are along mid-dorsal and mid-ventral lines but not around the anterior end of the trunk.

Remarks. The species belongs to a difficult group of Aplidium species with numerous stomach folds and relatively few rows of stigmata. Initially it was identified as Aplidium translucidum Ritter, 1901 originally described from "Orca, Prince William Sound, Alaska, on reefs at low tide" ( Ritter, 1901: 251). According to Ritter (1901) A. translucidum has very regular inverted cone shaped colonies attached by a short peduncle. Ritter (1901) especially mentions unusually transparent test through which the zooids are seen very clearly. Van Name (1945) also mentioned great transparence of the test. Zooids, according to Ritter's (1901) description are similar to those of the present species, although the thoraces are slightly wider, he counted 18–20 stigmata in each half row. Material identified as A. translucidum by Sanamyan (1998) is a mixture of several species and certainly contains at least several specimens of the present species, although the larva on his figure 7E differs distinctly from larvae of the present species.

Aplidium strandi Redikorzev, 1937 was regarded as a probable synonym of A. translucidum by Van Name (1945). This species is known from only one inverted cone shaped cormidium collected at the north of the Sea of Okhotsk. The zooid has triffid atrial languet and numerous stomach folds, but only seven rows of stigmata. Although it is hard to identify the species based on a single cormidium and without larvae we do not think A. strandi is conspecific with A. translucidum or with the present species.

Several species having numerous stomach folds and relatively few rows of stigmata were described by Beniaminson (1974) from Kurile Islands. Aplidium oculatum is valid species (see above). It differs from the present species by the shape of systems and the presence of white spots along the endostyle. Aplidium tenuicaudum ( Beniaminson, 1974) has different larva with about 50 small epidermal vesicles dorsally and ventrally on each side (see Sanamyan, 1998), and taxonomic relationship of A. kurilense ( Beniaminson, 1974) and A. vinogradivae ( Beniaminson, 1974) is hard to access, both species are based on a single and remarkably featureless colonies.

Aplidium confusum Sanamyan, 2000 known from several lots of specimens collected in the intertidal zone of South Kurile Islands has very similar zooids and similar, although somewhat smaller, larva. The shape of system in this species is not known and not possible to access from the existing material, but zooids with constantly simple atrial languet suggest that the systems possibly differ from those of the present species. We reexamined available material and confirm that atrial languet is always simple in specimens from South Kurile Islands assigned to A. confusum .

Figures

FIGURE 3. Aplidium dissectum n. sp. A, lava (holotype); B, colony, preserved (holotype); C, D, colony underwater (2006 - 07 - 25 - 04 and 2006 - 07 - 25 - 01 respectively).