Tetrastemma sundbergi Sagorny & Döhren & Rouse & Tilic 2022, sp. nov

Tetrastemma sundbergi sp. nov. urn:lsid:zoobank.org:act: 10E0A1CA-5CC4-40D6-879D-2861A91821C9 Figs 8J–L, 11A–G Diagnosis As for the genus (compare Sundberg & Gibson 1995). Tetrastemma sundbergi sp. nov. has a bright orange yet translucent body. The head is clearly demarcated from the rest of the body and is slenderer than the rest of the body. On dorsal surface with two red spots at lateral sides of the head. Dorsal and ventral ganglia are fused. Cerebral organ alongside brain. Etymology For Dr Per Sundberg in recognition of his contributions to nemertean taxonomy. Material examined Holotype COSTA RICA • ♀ (fixed in formaldehyde and prepared as 33 slides with transverse sections, see cybertype); methane seep Mound 12; 8.93° N, 84.31° W; depth 996–999 m; 24 Oct. 2018; Jorge Cortés and Oliver Ashford leg.; collected by HOV Alvin, Dive 4978; GenBank: ON186255, ON036054, ON021855, ON182052; SIO-BIC N256. Cybertype Images of serial sections of holotype: https://doi.org/10.5281/zenodo.5346538. Description Specimens 15 mm long and 0.7 mm in diameter. Body flattened (Fig. 8J). Two pairs of cephalic furrows. Paired anterior cephalic furrows V-shaped, dorsally pointing posteriorly (Fig. 8K). Posterior furrows U-shaped, located ventrally, pointing anteriorly (Fig. 8L). Head clearly demarcated from rest of body, not wider than trunk, tapered. Body coloration orange. Two dark brown spots on the lateral sides of the head in place of eyes. Cerebral ganglia, gut and gonads well visible through body wall (Fig. 8J). Epidermis in foregut region 30 µm thick. Body wall composed of outer circular and inner longitudinal muscle layers (Fig. 11B). Esophagus opening subterminal, ventral; stomach with lateral pouches; pylorus shorter than stomach; intestinal cecum with paired lateral, anterior pouches along brain lobes (Fig.11D); intestine with deep lateral pouches. Excretory system confined to foregut region, canals inside musculature (Fig. 11F). Precerebral septum closed. Rhynchocoel wall with two very thin muscle layers (outer circular, inner longitudinal), rhynchocoel almost extending to posterior end of body. Proboscis with outer circular, middle longitudinal, and inner circular muscle layers (Fig. 11C). Proboscis with 10 nerves (Fig. 11C); central stylet plus 2 accessory stylet pouches with 2 stylets each; stylet basis circular in cross section. Proboscis pore subterminal, ventral (Fig. 11A). Nervous system with thin outer neurilemma only, without neurochord cells; dorsal and ventral ganglia fused (Fig. 11B); broad ventral commissure, narrow dorsal commissure. Precerebral nerves in two groups with 3–4 nerves each. Lateral nerve cords without accessory nerve (Fig. 11F–G). Cerebral ganglia and anterior part of lateral nerve cords internal to longitudinal muscle layer (Fig. 11B, F); posteriorly, lateral nerve cords within body wall musculature (Fig. 11G). Eyes absent. Cerebral organs less than half the size of brain lobes, alongside brain. Paired cerebral canal simple, unforked, opens ventro-laterally into cephalic furrows (Fig. 11B). Apical organ present, unpaired; cephalic gland extending to brain, scattered between muscle fibres in anterior part of head, forming distinct lobules further back (Fig. 11A–B). Blood system consisting of cephalic loop, mid dorsal vessel with vascular plug (Fig. 11C–D). Ovaries lying above each other, alternating with intestinal diverticula (Fig. 11D–E). Remarks Tetrastemma sundbergi sp. nov. is attributed to the genus Tetrastemma based on the executed phylogenetic analyses. Furthermore, this assignment is supported by morphological characteristics such as the presence of a vascular plug, small cerebral organs, and the amount of muscle layers. Tetrastemma sundbergi is sister to a clade formed by the two other newly described species of Tetrastemma. These three species are sister to three yet undescribed specimens from the Kuril Islands. Together, this clade is sister to the two former species of Quasitetrastemma Chernyshev, 2004, Tetrastemma stimpsoni Chernyshev, 1992 and Tetrastemma nigrifrons (Coe, 1904). This clade is firmly nested within the genus Tetrastemma (Fig. 7). The three species differ morphologically from the two closely related, former species of Quasitetrastemma by the absence of lateral accessory nerves.
Published as part of Sagorny, Christina, Döhren, Jörn von, Rouse, Greg W. & Tilic, Ekin, 2022, Cutting the ribbon: bathyal Nemertea from seeps along the Costa Rica margin, with descriptions of 2 new genera and 9 new species, pp. 132-174 in European Journal of Taxonomy 845 (1) on pages 159-161, DOI: 10.5852/ejt.2022.845.1959, http://zenodo.org/record/7258852
{"references":["Sundberg P. & Gibson R. 1995. The nemerteans (Nemertea) of Rottnest Island, Western Australia. Zoologica Scripta 24: 101 - 141. https: // doi. org / 10.1111 / j. 1463 - 6409.1995. tb 00395. x","Chernyshev A. V. 2004. Two new genera of nemertean worms of the family Tetrastemmatidae (Nemertea: Monostilifera). Zoosystematica Rossica 12 (2): 151 - 156."]}