Styela hadalis, 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 : 314-317

Styela hadalis sp. nov.

( Figures 3 , 4A )

Material examined

St. 7391, 6330 m, one specimen. Holotype KIE 1/1132.

Description

The small hemispherical specimen (about 6 mm diameter and 2.5 mm high) is attached by its wide ventral surface to a small stone. The rigid and almost naked test with only sparse sand grains and without hair-like processes expands basally to a thin flat membrane. The small sessile apertures are well spaced on the upper surface. The body wall is thin and transparent. Body muscles are confined to a few thin circular siphonal muscles. Muscles were not detected on the remainder of the body. Six short branchial tentacles arise from the margin of the high branchial velum. The prepharyngeal band is a single high lamella close to the branchial velum ventrally and making a deep, wide V dorsally. The neural ganglion is halfway between the siphons and the small round dorsal tubercle is just anterior to it. The dorsal lamina has a plain edge. The branchial sac is almost flat, with one poorly defined fold on each side. The distribution of the longitudinal vessels is: 6(5)3DL3(6)4. Longitudinal stigmata are in 10 or 11 transverse rows crossed by parastigmatic vessels (about two per mesh). Several short oblique or transverse stigmata are at the posterior end of the left side of the branchial sac.

The gut forms a short, narrow primary loop, and a relatively deep open secondary loop. The short stomach has six or seven well-formed longitudinal folds. There is a long gastrointestinal connective issued from the distal end of the stomach and branching into thin vessels on the intestine. A gastric caecum is not present. The anal margin is divided into five distinct rounded lobes. One long narrow immature gonad is on each side of the body. Few, very small male follicles are closely applied to the proximal end of the ovary and the male duct runs along the mesial surface of the ovary to open close to the female opening. The atrial aperture is surrounded by a ring of short clavate atrial papillae, but a velum is not present. The atrial papillae are different from the atrial tentacles known in many deep-water Styelidae , and reminiscent of the papillae of Pyurella hernia Monniot and Monniot, 1973 (see Sanamyan and Sanamyan, 2005).

Remarks

The main distinguishing characters of the species are the absence of the endocarps, the few longitudinal vessels, only one fold on each side of the branchial sac and the body shape. In the absence of the endocarps and in the general form of gonads and gut loop, the species resembles S. calυa Monniot, Monniot and Millar, 1976. The latter species has three or four branchial folds and more numerous vessels, smooth anal margin, hair-like processes on the test, and different branchial and atrial tentacles. Cnemidocarpa sericata (Herdman 1888) also lacks endocarps, but has much more numerous branchial vessels and a different body shape. The absence of hair-like test processes and the body firmly attached by wide area to a solid object are unusual though not unique features for abyssal Styelidae , most of which are anchored in sediment by test processes. The presence and distribution of the test processes and foreign particles attached to the test, as well as the shape of the body, are remarkably stable characters in many abyssal ascidians and usually these features (often rather variable in shallow-water species) can be used safely to distinguish deep-water species. Among abyssal Styelidae Cnemidocarpa bythia ( Herdman, 1881) and Styela similis Monniot, 1970 are always attached by a wide ventral area, but they have large endocarps.

This is the deepest known record of a Styela spp. Only two other Styelidae species are recorded at depths exceeding 6000 m: C. bythia recorded by Millar (1959) at 7000 m and Bathystyeloides enderbyanus recorded in the present work at 7450 m.

Figures

Figure 3. Styela hadalis sp. nov. (A) Specimen opened along ventral mid line; branchial sac removed; (B) gut loop and distal part of the left gonad; (C) branchial velum, prepharyngeal band, dorsal tubercle, and ganglion; (D) ring of atrial papillae.

Figure 4. (A) Styela hadalis sp. nov.; (B) Styela multitentaculata sp. nov. External view.