Hemistyela pacifica, Sanamyan & Sanamyan, 2006

Sanamyan, K. E. & Sanamyan, N. P., 2006, Deep-water ascidians (Tunicata: Ascidiacea) from the northern and western Pacific, Journal of Natural History 40 (5 - 6), pp. 307-344 : 335-338

Hemistyela pacifica sp. nov.

( Figures 15 , 16A )

Material examined

St. 6109, 3460 m, two specimens. Holotype: KIE 1/1130. Paratype: KIE 2/1131.

Description

Rounded specimens are 6 mm (holotype) and 5 mm diameter. Sparse, unbranched, and relatively long, hair-like test processes mainly on the proximal (ventral) half of the body, are less crowded on the sides, and the almost whole anterior half of the body, including a wide area between and around the siphons, is bare. The test and the test processes are almost smooth or have sparse sediment attached. Apertures are widely separated on the upper surface.

Body musculature consists of an external layer of regularly spaced bands of circular muscles around the whole body (especially crowded around the siphons) and an internal layer of longitudinal muscles radiating from the siphons. Fifteen short, flattened, simple branchial tentacles arise from the margin of the short branchial velum. The prepharyngeal band of two lamellae makes deep undulations (associated with the branchial folds as in Molguloides ) and has a long narrow dorsal indentation. A long neural ganglion, about 1 mm in the holotype, is on the right side of the dorsal V. A short, thick neural duct extends transversely from the gland at the anterior end of the neural ganglion to a small round opening at the top of the peritubercular V-shaped area ( Figure 15B ). The dorsal lamina is represented by at least 10 long triangular languets. The branchial sac has four folds on the left, and (apparently) five on the right. A group of the most dorsal vessels on the right may constitute one fold or two. The formula for the anterior part of the branchial sac is: (6)(10)(12)(4)(6)DL(11)(8)(10)(12). The middle longitudinal vessels on each fold do not reach the bottom of the branchial sac and the number of longitudinal vessels significantly decreases toward the posterior end of the branchial sac. Only transverse stigmata are present, the stigmata under the branchial folds are longer than the stigmata on the sides of the folds (as in H. hirta and H. pilosa , see Monniot and Monniot 1974, Figure 16B ; 1977, Figure 6E, F ).

One thick gonad is on each side, the left one completely enclosed in the gut loop and the right some distance from, and parallel to, the endostyle. Two or three large, lobed testis follicles are between the ovary and the body wall, their rounded lobes projecting slightly from the sides and posterior end of the ovary. The male duct runs along the mesial surface of the ovary from the posterior testis follicles terminates near the almost sessile oviductal opening. One or two lateral branches join it along its length. The gut forms a wide, closed loop. A long oesophagus is clearly demarcated from the short cylindrical stomach. The stomach has five or six deep longitudinal folds but a gastric caecum was not detected. The anal border is smooth. Endocarps are not present.

Remarks

This is the first record of the genus in the north Pacific. Although very similar to the two already known Hemistyela species, the present one appears to be distinct from both. The type specimen of H. hirta ( Monniot and Monniot, 1974) has several male openings on each gonad. Originally described from the south Indian Ocean, H. hirta was subsequently recorded in the Weddell Sea and from New Caledonia, but these specimens have not been described and it is not known if they also have multiple male openings. Hemistyela hirta has about 50 branchial tentacles (many more than the present species) and a characteristic flattened pouch on the ventral side of the intestine (similar to those of Pyurella ) that is not present in H. pacifica sp. nov. Hemistyela pilosa Millar, 1955 a species known from several specimens from the North and Central Atlantic, and one dubious record from West Indian Ocean appears to have different body muscles (see Monniot and Monniot 1974).

Originally described by Millar (1955) as a genus belonging to Styelidae , Hemistyela was synonymized with the genus Boltenia by Monniot and Monniot (1974). Although certainly belonging to Pyuridae , Hemistyela cannot be congeneric with Boltenia , all species of which (even very small specimens) have the branched tentacles.

Figures

Figure 6. Styela sigma Hartmeyer, 1906. (A) External view; (B) specimen opened along ventral mid line; branchial sac removed.

Figure 15. Hemistyela pacifica sp. nov. (A) Specimens opened along ventral mid line; branchial sac removed; (B) internal view of intersiphonal area (prepharyngeal groove, undulating line of the anterior margin of the branchial sac, ganglion, and dorsal tubercle; (C) right gonad, lateral view; (D) right gonad, external view through the transparent body wall (male follicles).

Figure 16. (A) Hemistyela pacifica sp. nov.; (B) Pyura haustor (Stimpson, 1864). External view.