Syncarpa composita (Tokioka, 1951)

Hasegawa, Naohiro & Kajihara, Hiroshi, 2019, A redescription of Syncarpacomposita (Ascidiacea, Stolidobranchia) with an inference of its phylogenetic position within Styelidae, ZooKeys 857, pp. 1-15 : 1

Syncarpa composita (Tokioka, 1951)

Syndendrodoa composita Tokioka, 1951: 14-16, fig. 11.

? Syncarpa longicaudata Skallin, 1957: 297-298, figs a, b.

Material examined.

Thirteen specimens: SMBL 104 (syntypes, two colonies); ICHUM 5815-5825 (non-types, each represented by a single colony).

Comparative material examined.

ZIRAS 508-911, one of the syntypes of Syncarpa oviformis Redikorzev, 1913.

Description.

Colonies ca. 30-50 mm (40 mm and 50 mm in syntypes) in thickness and ca. 40-130 mm (45 mm and 100 mm in syntypes) in diameter. Tunic grayish violet to black or red in life, tough and leathery; zooids more or less protruded and thus externally discernible from each other (Fig. 1 A–C). Zooids 12-50 mm long (21 mm and 22 mm in syntypes) and ca. 8 mm wide (Fig. 1D). Posterior extension of zooids varying in length within the colony and among different colonies; while main zooid length (La) varied from 9 mm to 20 mm, posterior extension length (Lb) varied from 3 mm to 22 mm among 20 zooids from 11 colonies, with Lb/La ratio being 0.33-1.83 (Fig. 1E, Table 3). Siphons four-lobed, reddish in life, close together. Approximately 30 oral tentacles present (Fig. 2A), comprised of larger and smaller ones alternating almost regularly. Approximately 30 atrial tentacles present and ca. 0.3 mm long. Ciliated aperture of the dorsal tubercle C-shaped, with its interval directing leftward (Fig. 2B). Prepharyngeal band consisting of a single lamina running close to the ring of oral tentacles; prepharyngeal band V shaped around the dorsal tubercle. Neural ganglion close to dorsal tubercle. Dorsal lamina smoothly margined. One pharyngeal fold and one reduced pharyngeal fold present on each side of pharynx with formula:

L D. 0 (7-8) 2 (2) 3 V.

R D. 0 (7) 2 (3) 3 V.

Thirteen-twenty stigmata per mesh between endostyle and first longitudinal vessel from endostyle. Transverse vessels comprised of larger and smaller ones almost regularly alternating antero-posteriorly (Fig. 2C); when running across each pharyngeal fold (as well as reduced pharyngeal fold) on outer surface of pharynx, larger ones always taking a ‘shortcut’ and bridging over fold valley, while smaller ones ‘detour’ and go along valley (Fig. 2D, E). Parastigmatic vessels present. Stigmata straight. Gut located on left side (Fig. 3A). Alimentary system occupying approx. half of the left side of body; intestinal loop J-shaped. Esophagus short and slightly curved; its length being one-third of stomach (Fig. 3A). Stomach spindle-shaped, shorter than one-third of body length and has no plication or striation on its outer surface; stomach lying almost parallel to longitudinal axis of body (Fig. 3A), with its internal wall having at least 22 well-defined, regularly arranged, parallel, longitudinal folds (Fig. 3B). Intestine gently curving from pyloric part. Anus lying almost beneath atrial aperture. Diameter of intestine almost uniform from pylorus to anus. Anus without lobes. Gonad with 2-5 branches, situated only on right side of body (Fig. 3A). Ovaries spherical, occupying medial side of gonad; oviduct slightly bending at its end to peripharyngeal cavity before opening on right side of body at almost same level as pylorus. Male follicles located laterally within gonad, surrounding ovaries. Many endocarps present on inner surface of body wall (Fig. 3A).

Hatched tadpole larvae found in peripharyngeal cavity of ICHUM 5824 and 5825; trunk spindle-shaped, ca. 1 mm in length (Fig. 3C). Three adhesive papillae arranged in triangle. Approximately 35 elongated ampullae discerned on anterior half of trunk surface. Photolith present in cerebral vesicle but invisible from the outside (Fig. 4). Tail twice as long as trunk.

Remarks.

Syncarpa composita and S. oviformis are different in terms of the number of oral tentacles, the number of size-classes of transverse vessels, and the number of anal lobes (Table 4). In addition, the transverse vessels in S. composita alternate ‘shortcut’ and ‘detour’ when crossing the valley of pharyngeal folds, while all the transverse vessels in S. oviformis make a shortcut and bridge over the valley of pharyngeal folds (Fig. 5A, B). Based on the consistent, discontinuous differences discovered in the present study, we conclude to leave S. composita as a valid species as opposed to S. oviformis , until molecular data settle the issue of conspecificity.

Syncarpa composita and S. longicaudata were supposed to be differentiated by the ratio of the lengths of the zooid’s main body (La) to its posterior extension (Lb), expressed as Lb/La (Fig. 1E). The values of this character for S. composita and S. longicaudata , based on the original figures ( Tokioka 1951, figs 11.2, 11.3; Skalkin 1957, fig. a), are 0.40 and 1.00, respectively. In this study, however, we discovered that the Lb/La values could vary from 0.33 to 1.83 even intra-colonially in S. composita (Table 3), completely encompassing the character state of S. longicaudata . Although S. longcaudata has been considered a junior synonym of S. oviformis , we think that it is more similar to S. composita (Table 4). Extensive population genetic studies on potentially different populations of these species from the Northwest Pacific would help to improve our understanding of the taxonomy of this genus.

Phylogeny.

In the phylogenetic tree, Syncarpa formed a well-supported clade together with Dendrodoa (Fig. 6). These two genera have a single gonad positioned on the right side of the body. This feature is likely to represent a synapomorphy for this clade. The only difference between Syncarpa and Dendrodoa is that the former is colonial while the latter is solitary. The latter currently consists of eight species ( Shenkar et al. 2019). Future studies should ascertain the possible reciprocal monophyly of the two genera by analyses with expanded taxon sampling from Dendrodoa . If they turn out to be reciprocally non-monophyletic (e.g., Syncarpa completely nested within paraphyletic Dendrodoa ), these two genera can be synonymized so that it consists of both colonial and non-colonial species, just as the diazonid Rhopalaea Philippi, 1843.

A clade comprised of Dendrodoa , Polycarpa , and Polyandrocarpa zorritensis was recovered in Alié et al.'s (2018) phylogenomic analysis based on 4,908 genes, in which Polyandrocarpa zorritensis was sister to Polycarpa aurata , forming a clade sister to Dendrodoa grossularia .

Although the nodal support values were generally poor, our tree does not support the three-subfamily classification system: Styelinae consisting of solitary styelid species, Polyzoinae of colonial styelid species without system, and Botryllinae of colonial styelid species with system. Highly reliable molecular analyses and detailed morphological observations including Syncarpa would help understanding the systematics of Styelidae .