Molgula tzetlini Sanamyan, 1993

Sanamyan, Karen & Sanamyan, Nadya, 2007, Poorly known Ascidiacea collected in the vicinity of the Commander Islands and East Kamchatka, NW Pacific, Zootaxa 1579 (1), pp. 55-68 : 66-68

Molgula tzetlini Sanamyan, 1993

( Figure 6 )

Molgula tzetlini Sanamyan, 1993: 129 .

Material examined: Four specimens collected during July and August 1995 on Medny Island ( Commander Islands ), 8–20 m, KBPIG 700 / 3, 723/5, 725/6, 741/7; Kamchatka, Avacha Bay , 5 m, one specimen, KBPIG 1066 /8.

Previous records: NW Pacific, Commander Islands ( Sanamyan, 1993).

Description. All specimens are small, 5.5–11 mm in diameter and heavily incrusted with sand. Sand grains are attached directly to the test and hair-like processes are not present. The shape of the body removed from the test is characteristic ( Fig. 6B ). Both siphons are long and muscular and have well marked pointed lobules fringing the rim of the apertures. Body muscles are present only on the anterior half of the body and consist of 1) crowded thick circular siphonal muscles present only on the siphons and not extending outside the bases of the siphons; 2) spaced thick muscle bands radiating from siphons to about the middle of the body; 3) compact field of short muscle fibers crossing the intersiphonal area and completely covering the region of neural ganglion and dorsal tubercle; 4) rows of short transverse muscles running along mid-ventral and middorsal lines in the anterior half of the body. The branchial sac has a regular structure of seven longitudinal rows of 12 (in the most ventral row) or six (in all other rows) square spirals separated by five transverse vessels. There are seven branchial folds, all except the most ventral one being well developed with 4–6 internal longitudinal vessels. The most ventral fold is represented by only one longitudinal vessel in all the specimens. The typical formula is: (1)(5)(5)(6)(6)(6)(6)DL(5)(6)(5)(6)(4)(4)(1). Longitudinal vessels are not present between the folds or, rarely, a single vessel is between the dorsal lamina and adjacent fold on the right side of the branchial sac. Radial and rather irregular parastigmatic vessels are present and bear minute papillae (recognizable only after staining). The dorsal lamina has a plain margin. The gut forms a long, narrow loop, usually but not always open at the pole, and curved to varying degrees, from a slight arc to a J-shaped curve ( Fig. 6D ). The shape of the gonads varies with maturity. Commonly they are of moderate length, almost straight ( Fig. 6C ) or only slightly curved ( Fig. 6A ). The central tubular ovary ends in a wide, short, almost sessile oviduct, which may be slightly longer only in smaller specimens with immature gonads. A wide ring of minute papillae on the body wall surrounds the opening of the oviduct. Large testis follicles surrounding the ovary may be simple, pear or sausage-shaped or become branched terminally, especially in larger specimens. One or two male papillae are on the mesial surface of the ovary. Eggs were found being incubated in the peribranchial cavity in all examined specimens, but tailed larvae were not present.

Remarks. The species is based on two small specimens collected in the intertidal zone of the Commander Islands. Both specimens agree well with the original description, the only difference is that the holotype has two vessels in the most ventral fold on the right side of the branchial sac, while only one vessel is present in all newly examined specimens. Also, oviducts are described as "relatively long" ( Sanamyan, 1993: 130), but actually they are not longer than those of the specimen KBPIG 723/5 ( Fig. 6D ). The following features are characteristic and appear to be stable in this species: 1) the body covered by sand but without hair-like processes; 2) seven branchial folds, the most ventral represented by one (exceptionally two) vessels; 3) one or two male papilla on the mesial surface of the ovary; 4) eggs being incubated in the peribranchial cavity and, probably associated with this feature, a ring of papillae around the opening of the oviduct.

In brooding its eggs in the peribranchial cavity M. tzetlini resembles M. cooperi , but they differ in many features including the number of the branchial folds, the structure of the gonads and especially the gonoducts. Molgula tzetlini is found in depths of 0– 20m.

Figures

FIGURE 6. Molgula tzetlini. A, specimen opened along ventral midline, branchial sac removed (KBPIG 1066/8); B, body removed from the test (KBPIG 725/6); C, right gonad and renal sac (KBPIG 700/3); D, gut loop and left gonad of juvenile specimen (KBPIG 723/5).