Synoicum polyzoinum, Sanamyan, 2017

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

Synoicum polyzoinum n. sp.

( Figures 1 , 2 )

Material examined. Matua Island , Point Kluv, 14 m, 16.08.2017, one colony (#400). Holotype KBPGI 1454 /1.

Description. The species forms extensive (about 10 см in extent) colonies composed of several groups of short, sometimes partially fused, sometimes completely separated from each other lobes of various shape and size ( Figure 1D and 2A, B ). Smaller lobes, containing only one system of zooids, may be described as short upright cylinders fused with each other at their basal part and along about a half or more of their length. Larger lobes are cushion-like, low, and contain several systems of zooids. They still have depressions between the systems on their upper surface giving an impression that these larger lobes are results of fusion of several smaller ones. The test is leathery, firm, reddish and opaque. Upper surface is in part covered by filamentous green algae and diatoms. Sides and basal parts of the colony are impregnated by some amount of sand.

Zooids within the colony stand vertically and parallel to each other, they extend whole length from the top to the bottom of the colony. Zoods are arranged in regular circular systems composed by few (usually five to seven) zooids ( Figure 2B ). Common cloacal openings and openings of the branchial siphons of individual zooids on the surface of the colony are similar in shape and size, they are large, almost round, on short siphons, and at first glance barely shown any traces of lobes. On macro photographs is evident that the margin of the common cloacal openings has undulating rim—the wall of common atrial siphons is supported by tridentate atrial languets of zooids and therefore the number of small lobes on the margin of common cloacal siphons is three times larger than the number of zooids composing the system.

Zooids in somewhat contracted state are about 10–15 mm long, with the thorax about 3 mm and the abdomen somewhat shorter than the thorax ( Figure 1A, B ). Branchial siphon is short and widely open, its margin is almost smooth, with weak hints of six lobes characteristic for the family. Atrial aperture is small, situated on a rather distinct siphon. Upper rim of this siphon is drawn into a wide tridentate atrial languet. Longitudinal thoracic muscles are very fine, equally spaced, about 15 in number. Branchial tentacles cannot be counted precisely, about 10 or 12 were seen in some zooids. Branchial sac has 12 rows of stigmata with 25–27 stigmata per row on each side.

The stomach is absolutely smooth, no any traces of areolation or folds are present ( Figure 1C ). It is situated in the middle of abdomen. The cardiac end of the stomach is rounded and somewhat narrower than the pyloric and its dorsal border (along the typhlosole) is somewhat shorter than the ventral border. The orientation of the stomach is nearly straight, the oesophagus enters its cardiac end nearly vertically. The post pyloric subdivision of the intestine is not clear. There is no gastric reservoir. Posterior abdomen is filled with parenchymatous tissue and no gonads are developed.

Remarks. In live, the colour, consistency, widely open round siphons and overall habitus of the colony resembles colonial styelid of the subfamily Polyzoinae (e.g. Syncarpa oviformis ) rather than a member of Polyclinidae , hence suggested specific name " polyzoinum ". The structure of zooids is, of course, very different.

The shape of the stomach reminiscent the stomach of Aplidiopsis spp. but the orientation of the stomach (straight) and a whole configuration of the gut loop is different and characteristic for Synoicum but not for Aplidiopsis . In Aplidiopsis (and in related Polyclinum ) the stomach is always obliquely oriented with the oesophagus bent at right or sharp angle to enter its cardiac end (see photo of zooids of Aplidiopsis pannosum in Sanamyan & Sanamyan, 2017 , Figure 9A).

The genus Synoicum in NW Pacific is represented by many nominal species and taxonomic status some of them is not clear. In order to compare described here S. polyzoinum n. sp. with other member of the genus, and in order to systematize our knowledge on species inhabiting NW Pacific waters, we give below an overview of all known nominal species reported in the region (Commander Islands, Kamchatka, Sea of Okhotsk and Kuril Islands). For convenience they may be divided into the following groups:

1. The first group comprises three nominal species with colonies composed of upright lobes, each with one or several (but generally few) circular systems, and zooids with very clear and prominent areolation of the stomach wall. This group includes Synoicum cymosum Redikorzev, 1927 , S. solidum Redikorzev, 1937 and S. irregulare Ritter, 1899 . The latter species was synonymized with S. turgens Phipps, 1774 by several authors (including Van Name, 1945 and Sanamyan, 1998), but actually the conspecificity between European S. turgens and Pacific species S. irregulare is not firmly established—existing descriptions of both species are old and rather superficial. Synoicum cymosum differs from S. irregulare only by possessing prominent papillae on the test. They are often arranged in longitudinal rows on the sides of the lobes composing the colony. Sanamyan (1998) examined several such colonies with longitudinally arranged test papillae and preferred to keep this species as distinct from S. irregulare . After publication of that paper we had chance to examine several other specimens from pacific coasts of Kamchatka referable to S. irregulare and S. cymosum . The presence and the degree of development of test papillae vary significantly from colony to colony and now we believe these species are conspecific, S. cymosum becomes a junior subjective synonym of S. irregulare .

Synoicum solidum also synonymized here with S. irregulare —the reexamination of the holotype of S. solidum revealed that Redikorzev's figure of zooid ( Redikorzev, 1937, Figure 8) is precise and correct, the zooids indeed have very prominent areolation of the stomach wall, and the shape of colony corresponds to that of S. irregulare .

2. The second group of nominal species comprises several species forming large massive colonies and having smooth stomach. The better known species of this group, Synoicum jordani ( Ritter, 1899) is rather common in Sea of Okhotsk and Kamchatka, especially in the places where water is turbid and where hard bottom is covered by some amount of sediment. It forms spherical colonies up to 12 cm in diameter, each with numerous circular systems of zooids. The systems in this species are not separated by any depressions or furrows on the surface of colony and the species not forms separate lobes containing single (or few) systems as in S. irregulare and in S. polyzoinum n. sp. Sanamyan (1998) examined many specimens assignable to this species. They all had large and easily recognizable gastric reservoir in the gut loop, a feature which Sanamyan (1998) believed to be constant and characteristic for this species.

Three species described by Ritter (1899) from NW Pacific, Amaroucium kincaidi Ritter, 1899 , A. pribilovense Ritter, 1899 and A. snodgrassii Sanamyan (1998) were synonymized under the questions with S. jordani by Sanamyan (1998). Van Name (1945) also synonymized three mentioned species together under the name Synoicum kincaidi (spelled as " kinkaidi ") but preferred to keep them separate from S. jordani basing on the structure of the stomach wall. He believed that in S. jordani the stomach is always smooth (that is in agreement with our data) while stomach being not smooth in S. kincaidi . It is not known if the irregular folds or areolation of the stomach in S. kincaidi are artefacts of fixation and are not caused by contraction of zooids and the status of S. kincaidi and its synonyms is uncertain. It has not been recorded in NW Pacific.

Two species described by Redikorzev (1927a, 1927b) from Sea of Japan and Sea of Okhotsk, Aplidiopsis knipowitschi Redikorzev, 1927 and Synoicum jacobsoni Redikorzev, 1927 were also synonymized with S. jordani by Sanamyan (1998). While the synomization of A. knipowitschi with S. jordani is certainly correct, the conspecificity of S. jacobsoni and S. jordani is less evident. Almost spherical of colony of S. jacobsoni with numerous round system is identical with colonies of S. jordani . Redikorzev's figure (1927a, Figure 16) shows rather long gut loop below the stomach and no gastric reservoir in it. Reexamination of the type material of this species revealed that the stomach is quite smooth ( Redikorzev, 1927a, figured sparse fine areoles on it), as in S. jordani , while in all other respects Redikorzev's figure is very precise and corresponds exactly to real material. We failed to find gastric reservoir in the gut loop in type specimen. Nevertheless, due to overall similarity in other features and the geographic locality (Sea of Okhotsk) we prefer to keep it as a doubtful synonym of S. jordani .

Thus, currently in the region including Commander Islands, Kamchatka, north and middle Kuril Islands we recognize only one species of the genus Synoicum which forms large massive colonies and has smooth stomach wall— S. jordani .

3. The rest Synoicum species recorded in the Far East Seas of Russia, which cannot be assigned to groups described above, are:

Synoicum derjugini Redikorzev, 1927 known only from original description based on three specimens collected in the Sea of Japan near Vladivostok. This species most closely resembles S. polyzoinum n. sp. We reexamined type material of S. derjugini . It is represented by shapeless pieces which are probably composed by several lobes. The test is hard and clear, without any sand on surface or in internal layers. The zooids correspond exactly to original figure, they have only 10 rows of stigmata (i.e. slightly fewer than we counted in S. polyzoinum n. sp.) and lack gastric vesicle. Original figure ( Redikorzev, 1927a, Figure 19) shows oval solid colony with typical for the genus circular systems. Despite the reexamination of the type material the obtained information is not sufficient and the taxonomic status of this species cannot be revealed without examination of additional colonies from the type locality. We prefer to keep specimen from Matua Island as distinct from S. derjugini .

Amaroucium polybunum Redikorzev, 1927 and Amaroucium soldatovi Redikorzev, 1937 , both from the Sea of Okhotsk, were synonymized with each other and placed to the genus Synoicum by Sanamyan (1998). Both have numerous rows of stigmata (20–21) and stomach wall with distinct swelling arranged in longitudinal rows. We had no chance to examine any colonies assignable to these species and cannot currently comment their taxonomic position and generic assignment.

Synoicum sabuliferum Redikorzev, 1937 is known only from original description based on three specimens from Kamchatka . The species is certainly valid. Unlike other Synoicum species of the region it has colony composed of vertical sandy lobes. The waters around Kamchatka are rather well sampled for ascidians but we never encountered any specimens assignable to this species.

Synoicum pellucens Redikorzev, 1927 also is known from original description only. The species is based on one colony recorded in the vicinity of Vladivostok. The colony consists of several long upright lobes, each with only four of five zooids.

Synoicum parvum Redikorzev, 1937 is based on a small specimen from the Sea of Okhotsk. Most probably this is a juvenile colony of another species.

Synoicum clavatum: Sanamyan (1998) is certainly wrongly identified. Synoicum clavatum (Oka, 1927) is a warm water species and its occurrence in cold water around Kamchatka and North Kuril islands is highly dubious.

Figures

FIGURE 1. Synoicum polyzoinum n. sp. A, B, zooid; C, stomach, enlarged; D, parts of preserved colony (Holotype).

FIGURE 2. Synoicum polyzoinum n. sp. A, overall view of colony. (Solitary ascidians on this photograph are Dendrodoa aggregata and small whitish translucent colony in the central part of the image is Placentela crystallina); B, detail of colony composed of several cormidia, each with single system of zooids.