Aplidium matua, Sanamyan, 2017

Sanamyan, Nadya, 2017, Shallow-water Ascidians from Matua Island (central Kuril Islands, NW Pacific), part 2, Zootaxa 4337 (1), pp. 121-131 : 126-129

Aplidium matua n. sp.

( Figures 3 , 4 )

Material examined. Matua Island , Point Kluv, 14 m, 16.08.2017, one colony (#409). Holotype KBPGI 1455 /1.

Description. The species forms cushions several centimeters across ( Figures 4A, B ). The colony of examined specimen consists of two such cushions, the larger is 2.5 cm in diameter and 3 cm in height. Sides of colony are in part covered by sponge and sand, upper exposed surface also contains fine sand grains but here they are much sparser. Inner layers of the colony, especially at lower (basal) parts, also contains embedded sand grains. Apart from embedded sand the tunic is transparent, light-red in live. In preserved colony the upper half of the colony is deep brownish-red.

Zooids open on a whole exposed surface of the colony. They are arranged in small circular systems which are clearly visible on live inflated colonies and on colonies preserved in formalin. Each system is composed of seven to ten zooids arranged in a circle around large common cloacal opening through which atrial openings of some zooids may be visible ( Figure 4C ). Common cloacal cavity is limited to a small shallow space beneath common cloacal opening, its roof and margins of common cloacal opening are supported by three-lobed atrial languets of zooids ( Figure 4C ).

Zooids in contracted state are about 10–15 mm long in average, with thorax of about 2 mm and abdomen, in less contracted zooids, up to 5 mm long ( Figures 4A–C ). Branchial siphon has usual six lobes, which are short, wide and inconspicuous (they are obscurely visible on surface of the colony on macro photographs of inflated live colonies). Atrial opening of preserved zooids is small, on short siphon upper rim of which is drawn into short languet with tridentate margin.

Longitudinal thoracic muscles are thin and numerous (about 20 on each side of the thorax were counted, but this number is rather approximate). Longitudinal muscles continue along the whole length of abdomen where they and continue Branchial sac has 17 rows of stigmata. About 25 stigmata per row were counted in the middle part of the right side of the branchial sac.

Barrel shaped stomach is in the middle of abdomen. It has seven or eight narrow and rather regular longitudinal folds with wide flat depressed spaces between them ( Figure 4B,C ). Subdivision of the post pyloric part of the gut loop is typical and clear, there is rather long duodenum, mid-intestine with oval posterior stomach and noticeable rectal valves.

Posterior abdomen is long and filled with parenchyma. No gonads or larvae were found in examined zooids.

Remarks. Aplidium matua n. sp. can be distinguished by combination of the small circular systems, zooids with few (seven or eight) stomach folds and rather numerous (17) rows of stigmata.

Although the number of Aplidium species reported in Far East Seas of Russia in various published sources and lists (e.g. Sirenko, 2013) is rather high, many of these listed species are wrongly identified or were reassigned to other genera (e.g. to Synoicum ). Below we provide a list of all species of the genus known from Commander Islands, Kamchatka, sea of Okhotsk and from Kurile Islands. For easier identification they may be divided in a following way:

1. The species with five prominent and sharply defined longitudinal stomach folds: Aplidium spitzbergense Hartmeyer, 1903 , A. redikorzevi Sanamyan et Sanamyan, 2011 and A. macrenteron Sanamyan et Sanamyan, 2017 . These three species are easy for identification and cannot be confused with other Aplidium species: A. spitzbergense has only four rows of stigmata, A. redikorzevi and A. macrenteron have very characteristic sandy colonies and characteristically looking zooids (see Sanamyan & Sanamyan, 2011, 2017).

2. The species with seven to 15 stomach folds: Aplidium dubium ( Ritter, 1899) , A. vinogradovae (Beniaminson, 1974) , A. disiphonium (Beniaminson, 1975) , A. lebedi Sanamyan, 1998 and A. eborinum Sanamyan et Sanamyan, 2011 .

Above described Aplidium matua n. sp. also belongs to this group and is more closely related to A. dubium . Aplidium dubium was originally described from Commander Islands and not known from other places. It is most common colonial ascidian species there, we have numerous (more than 50) colonies identified as A. dubium from Medny and Behring Islands. This material was briefly described by Sanamyan (1998) who reported 11–15 rows of stigmata and 8–10 longitudinal stomach folds for this species. These numbers differ slightly but are rather close to number of stomach folds and rows of stigmata of A. matua n. sp. We tried to reexamine the material from Commander Islands but it was found that all numerous colonies of this species contain so strongly contracted zooids that stomach folds and rows of stigmata cannot be counted correctly and the structure of systems cannot be revealed. The species certainly has strong thoracic and abdominal musculature, much stronger than in A. matua n. sp., and the different degree of contraction of zooids is not an artefact of different handling—all these colonies were collected and fixed in a same way. We believe they belong to different species and describe here a specimen from Matua Island as a new species. Other species of this group differ quite distinctly from A. matua n. sp and cannot be confused with it.

3. The species with 20 or more stomach folds include: A. strandi ( Redikorzev, 1937) , A. oculatum (Beniaminson, 1974) , A. kurilense (Beniaminson, 1974) , A. tenuicaudatum (Beniaminson, 1974) , A. confusum Sanamyan, 2000 and A. dissectum Sanamyan et Sanamyan, 2011 . Of these species A. kurilense , A. tenuicaudatum and A. confusum are known only from southern group of Kuril Islands, while other three known from North Kuril Islands and more northern localities. Unfortunately in this group the appearance of live, fully inflated colonies, and the details of the structure of the systems, the features which constitute very reliable species specific characters, are known only for A. oculatum and A. dissectum . Anyway, high number of stomach folds clearly distinguish all these species from A. matua n. sp.

Finally it is necessary to mention Aplidium species reported in our earlier papers for Far East Seas of Russia which were erroneously identified:

Aplidium glabrum (Verrill, 1871) was reported by Sanamyan (1998, 2000) and Sanamyan & Sanamyan (2010) from Kamchatka and South Kuril Islands. All these specimens are distinct from European A. glabrum . True A. glabrum does not occurs in NE Pacific. The specimens from Kamchatka recorded under this name belong to A. eborinum (see Sanamyan & Sanamyan, 2011) and the taxonomic status of specimens from southern group of Kuril Islands is not clear.

Aplidium pliciferum (Redikorzev, 1927) reported from Kamchatka in several papers of Sanamyan (e.g. Sanamyan, 1998, Sanamyan & Sanamyan, 2010). As it was shown by Sanamyan & Sanamyan, 2011 this material belongs to A. oculatum . Original description of A. pliciferum is based on a specimen from warmer waters (Sagami Bay) and true A. pliciferum does not occurs in Kuril Islands, Kamchatka and more northern regions.

Aplidium sagamiense (Tokioka, 1967) reported by Sanamyan (1998) from Kamchatka is also wrongly identified. Aplidium sagamiense is known from much warmer waters and it is hard to believe it may occur in cold waters around Kamchatka and Kuril Islands. The taxonomic status of specimens from Kamchatka and north and central Kuril Islands is not clear. Most probably this is an undescribed species. However all colonies we examined have small, strongly contracted zooids, and we still have no good underwater photos of inflated colonies of this species to give adequate description of structure of systems. Therefore we refrain from describing of a new taxon.

Figures

FIGURE 3. Aplidium matua n. sp. A, zooid; B, C, gut loop and stomach of two zooids; D, preserved colony (Holotype).

FIGURE 4. Aplidium matua n. sp. A, holotype; B, another colony (not collected); C, details of one system of zooids.