Tetrastemma sundbergi, Sagorny & Döhren & Rouse & Tilic, 2022

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, European Journal of Taxonomy 845 (1), pp. 132-174 : 159-161

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.

Figures

Fig. 8. Live images of eumonostiliferous hoplonemerteans collected along the Costa Rica margin and the NE Pacific. A. Chernyshevia escarpiaphila sp. nov., paratype (SIO-BIC N112). B. Chernyshevia escarpiaphila sp. nov., paratype (SIO-BIC N266). C. Octocoral Paragorgia sp. (SIO-BIC Co3054) with several specimens of Alvinonemertes dariae sp. nov., holotype (arrows; ex SIO-BIC N262). D. Alvinonemertesdariaesp.nov.,holotype(exSIO-BICN262).E.Antipathariancoral(SIO-BICCo3031) with specimens of Alvinonemertes dagmarae sp. nov., holotype (SIO-BIC N260). F. Alvinonemertes christianeae sp. nov., holotype (SIO-BIC N253). G. Alvinonemertes claudiae sp. nov., holotype (ex SIO-BIC N231). H. Alvinonemertes tatjanae sp. nov., holotype (SIO-BIC N119). I. Eumonostilifera (SIO-BIC N259). J–L. Tetrastemma sundbergi sp. nov., holotype (SIO-BIC N256) whole animal (J), dorsal view of head (K), and ventral view of head (L). M. Tetrastemma polyakovae sp. nov. (MZUCR Nemertea Collection MZUCR-101-01).N. Tetrastemma polyakovae sp. nov., holotype (SIO-BIC N258), with everted proboscis. O. Eumonostilifera (SIO-BIC N109).

Fig. 11. A–G. Tetrastemma sundbergi sp. nov., holotype (SIO-BIC N256), transverse histological sections. H–M. Tetrastemma strandae sp. nov., holotype (SIO-BIC N285), horizontal histological sections (H–I) and transverse histological sections (J–M). A. Anterior tip of head with proboscis pore. B. Brain region with well-visible cerebral canal (arrowhead). C. Proboscis with 10 proboscis nerves (arrowheads) in stomach region. D. Pyloric region. E. Intestinal region with ovaries. F. Anterior part of lateral nerve cord. G. Posterior part of lateral nerve cord located in body wall musculature. H. Longitudinal section through anterior tip with proboscis pore. I. Longitudinal section through the ventral part of the anterior tip. J. Intestinal region with testes. K. Proboscis in stomach region with 10 proboscis nerves (arrowheads). L. Proboscis armature. M. Lateral nerve cord in intestinal region. Abbreviations: ap = accessory stylet pouch; as = accessory stylet; br = brain; cc = cerebral canal; cg = cephalic glands; cm = circular muscle layer; co = cerebral organ; dl = dorsal brain lobe; ec = excretory canal; i = intestine; ic = intestinal cecum; lb = lateral blood vessel; lm = longitudinal muscle layer; ln = lateral nerve cord; mv = mid-dorsal-vessel; o = ovary; p = proboscis; pe = proboscis epithelium; pp = proboscis pore; py = pylorus; rc = rhynchocoel; sb = stylet basis; t = testis; vc = ventral commissure; vl = ventral brain lobe. Scale: 100 µm.

Fig. 7. Maximum likelihood (ML) tree for the monostiliferan supermatrix based on the concatenated four gene dataset (16S, COI, 18S, H3). Black circles indicate nodal support of 100/100/1.0; asterisks indicate nodal support of either 100/ or /1.0. Alternative branches found in the BI analysis are given as blue dotted lines. Sonnenemertes cantelli Chernyshev, Abukawa & Kajihara, 2015 and Micrura callima Sundberg & Gibson, 1995 were used for outgroup rooting. Specimens belonging to new species are given in bold. Specimen-ID, sampling locality, and depth (for deep-sea species) are provided.