Coriella chernyshevi Borisanova & Potanina, 2016, n. sp.

Coriella chernyshevi n. sp.

(Figs. 1, 2)

Material examined. Holotype: ZMMU № UK-13 (fragment of colony, 6 mm in length, with 12 zooids and several buds), RV “ Akademik Oparin, Sea of Okhotsk, near the Iturup Island, western Pacific, 4502'7'' N, 14700'6'' E, depth 305–624 m, coll. 10 July 2011 by A.V. Chernyshev. Paratype: ZMMU № UK-14 (fragment of colony, 8 mm in length, with 18 zooids), RV “ Akademik Oparin, Sea of Okhotsk, near the Iturup Island, western Pacific, 4502'7'' N, 14700'6'' E, depth 305–624 m, coll. 10 July 2011 by A.V. Chernyshev.

Description. Two fragments of colonies were investigated. One of the fragments was 3 cm in length (a part of this fragment, 6 mm in length, was provided to ZMMU as holotype), and the other was 1.8 cm in length (a part of this fragment, 8 mm in length, was provided to ZMMU as paratype). Zooids departed from closely adjoined stolons that formed a central stem. The stem of the holotype colony is formed of six adjoining stolons (Fig. 1 A). The stolons are lying straight, almost parallel to each other. The diameter of the stem is 0.5–0.6 mm, and the thickness of each separate stolon is 0.17–0.18 mm. The stem of the paratype colony is formed by up to 12 stolons (the diameter of the stem is about 1 mm). The inner stolons are straight and fertile, whereas the peripherical stolons are curved and do not give rise to zooids (Fig. 1 B). The colonies branch out. At the branching area the stolons of the stem disjoin and form two thinner stems (Fig. 2 A, 2B). All stolons of the stem in both fragments have a thick cuticle, which gives them a considerable rigidity. Stolons with zooids are not subdivided into fertile and sterile zones. Cuticular septa divide stolons only on fertile zones, from which zooids depart. The distance between zooids is relatively constant (about 1.8 mm). All zooids of each fertile stolon in the stem sprout at the same angle. As stolons in a stem are arranged in a circle, zooids are situated all around a stem within a colony.

New zooids form at the tip of stolons. Buds and juvenile zooids are adjacent, with no septa between them.

Zooids are 5.4–7.3 mm in length (Table 1). Each zooid consists of a calyx and a stalk; the stalk is subdivided into a peduncle and a muscular base (Figs. 1 C, 2C). The calyx is slightly laterally compressed and asymmetric (more gibbose on the aboral side). The calyx length is 0.95–1.2 mm and the calyx width is 0.8–1 mm (Table 1). The upper part of the calyx is twice as wide as the lower part. The calyx bears 32–34 thin, long (0.8–0.9 mm) tentacles. The stomach is roundish. The calyx is separated from the stalk by a cuticular septum. Under the septum, the stalk is not covered by a thick cuticle and forms a small expansion, where several spikes are placed. Spikes are organized into two or three circles with 2–3 spikes per circle (Fig. 2 D). The length of a spike in the top circle is 110–130 µm, and in the lower circle is 50–60 µm. The stalk length is 4.4–6.1 mm (Table 1). The overall calyx length/stalk length ratio is 4.89 (Table 1). The peduncle length is 2.9–4 mm (Table 1), and the peduncle width is 0.115–0.12 mm. Numerous pore complexes slightly project above the surface of a peduncle and occupy about 30–40% of the peduncle’s surface (Fig. 2 E). The peduncle is separated from the muscular base by a well-pronounced septum, through which cells of the body cavity pass into the muscular base (Fig. 2 F). The muscular base length is 1.6–2.1 mm (Table 1), and the muscular base width is 0.45–0.48 mm. The overall peduncle length/muscle base length ratio is 1.86 (Table 1).

Etymology. Named after Alexey Viktorovich Chernyshev, a leading researcher at the A.V. Zhirmunsky Institute of Marine Biology, who collected the entoproct samples and kindly transferred them to the authors for this study.

Remarks. The only species of Coriella described prior to this study is Coriella stolonata Kluge, 1946, found in the Arctic Ocean at a depth of 45 m. Coriella stolonata is similar to C. chernyshevi in terms of stem structure formed by several adjoining stolons, and in zooid organization, with one muscular base and a peduncle with numerous pore complexes. However, zooids of C. stolonata are smaller (length up to 3.4 mm, whereas in C. chernyshevi they can reach 7 mm), and have fewer tentacles (22–24 in C. stolonata and 32–34 in C. chernyshevi). Coriella stolonata lacks spikes on the tip of the stalk when C. chernyshevi stalk bears several circles of spikes under the septum between stalk and calyx. In C. chernyshevi, the stem is thick and consists of 6 or 12 stolons when the stem of C. stolonata consists of 3-4 stolons. The stolons of C. stolonata are branched, and young zooids are present along the entire length of the stem, whereas in C. chernyshevi, stolons do not usually branch, and buds form only on the top growing end of a colony.

Holotype and paratype colonies of C. chernyshevi differ from each other by number of stolons (six stolons in the stem of holotype and 12 in stolons in the stem of paratype) and by stem thickness (about 0.5–0.6 mm in holotype and about 1 mm in paratype), and also by presence of sterile stolons in the paratype colony. These differences in stem structure can be accounted for by differences in the age of the fragments taken from different parts of colonies. The holotype colony with the thin stem without sterile stolons was probably taken from the top part of a colony at its growing edge. The paratype fragment with a more complex stem structure was likely taken closer to the base of the colony. Thickening of the stem by sterile stolons at the base of the colony during growth arises from the need to support the vertically expanding colony. The fragment with the sterile stolons is small enough so it remains unclear how these sterile stolons originate; however, we propose that these are most probably the result of branching of stolons at the base of a colony.

Hincks (1884) described another genus of Barentsiidae, Pedicellinopsis, with a colony structure similar to that of Coriella. Pedicellinopsis includes one species, P. fruticosa, found in the coastal waters of South and Western Australia (Hincks, 1884; Wasson, 2002). Pedicellinopsis fruticosa forms large colonies (up to 30 cm in length), with zooids spiraled around a rigid central stem. The zooids of P. fruticosa are similar to the zooids of Coriella, with a muscular base and a peduncle without additional muscular nodes. The peduncle of both Pedicellinopsis and Coriella is decorated by rows of numerous pores. However, the structure of the upper part of the stalk differs between genera. In P. fruticosa, a large cuticular spine extends past the stalk-calyx junction. Coriella does not have such a spine. Coriella chernyshevi bears several circles of spikes under the stalk-calyx junction. Coriella stolonata does not bear neither spines nor spikes. Pedicellinopsis zooids are up to 6 mm in length and the calyx bears about 20 tentacles (Hincks, 1884; Wasson, 2002). Coriella chernyshevi zooids are larger (up to 7.3 mm) and have more tentacles (32–34) than Pedicellinopsis, whereas C. stolonata zooids are smaller (about 3.4 mm) and with almost the same number of tentacles as in P. fruticosa (22–24 tentacles).

A rigid stem is a unique feature of Pedicellinopsis and Coriella that distinguishes them from all other entoproct colonial genera. Emergence of a tough central stem allows colonies of Coriella and Pedicellinopsis to shift from the creeping colonies typical for other barentsiids, to the bushy colonies rising upright over a substratum (Busk 1886, Wasson 2002). The stem structure of Pedicellinopsis and Coriella differs from each other. The stem of Pedicellinopsis is formed by a single thick hard tube, whereas the stem of Coriella consists of several adjoined stolons. The inner structure of the stem of Pedicellinopsis has not been described in detail, and it remains unclear how it is formed. However, we can assume that it is the result of accretion of separate stolons. Such a scenario is supported by the fact that the base of Pedicellinopsis colonies consists of a large number of free thin stolons, which serve for attachment to a substratum (Hincks, 1884; Wasson, 2002). Wasson (2002) noted that free stolons can line up the stem of P. fruticosa, intertwining with it. These free stolons resemble peripherical sterile stolons of the stem of Coriella chernyshevi.

Based on the similarity in zooids structure, general structure of the colonies, the presence of the stem and free stolons around the stem and at the base of the colony, we suggest a close relationship between the Coriella and Pedicellinopsis genera, which probably would form a single clade, with a central stem as a shared characteristic. An overview of the Coriella and Pedicellinopsis species is given below.