Ototyphlonemertes longissima, Liu & Sun, 2018

Liu, Hai-Long & Sun, Shi-Chun, 2018, Ototyphlonemertes longissima sp. nov. (Hoplonemertea: Monostilifera: Ototyphlonemertidae), a new interstitial nemertean from the South China Sea, Zootaxa 4527 (4), pp. 581-587 : 582-585

Ototyphlonemertes longissima sp. nov.

Etymology. The specific name is from the Latin “ longissimus ” (very long), to indicate the new species is much longer than its congeners.

Material examined. Holotype: Immature , 158 mm in length; collected from a crevice of volcanic rock in the intertidal zone, Diaolou (19.91° N, 109.52° E), Lingao , Hainan, China, 31 October 2011; series of transverse sections of anterior body region and fragments of posterior body region, deposited at the Marine Biological Museum ( MBM286540 ), Chinese Academy of Sciences, Qingdao, China; COI sequence (GenBank accession number: MK 045324 ) obtained from a small fragment.

Voucher materials: One complete specimen, immature, 138 mm long; collected from intertidal sand, Fangchenggang (21.51° N, 108.23° E), Guangxi, China, 18 November 2017; series of transverse sections of anterior body region and fragments of posterior body region, deposited at the Marine Biological Museum (MBM286541); COI sequence ( GenBank accession number: MK 045322 ) obtained from a small fragment. An incomplete specimen, immature; from intertidal sand, Fangchenggang , 18 November 2017; consisting of series of transverse sections of anterior body region and fragments of posterior body region (20171118 I3, deposited at Institute of Evolution and Marine Biodiversity, Ocean University of China) and COI sequence (GENBANK accession number: MK 045321 ) . One incomplete specimen, immature; from a rock crevice at Guangcun (19.87° N, 109.44° E), Danzhou , Hainan, China, 9 May 2018; consisting of body fragments (20180509 I5, deposited at Institute of Evolution and Marine Biodiversity, Ocean University of China) and COI sequence (GENBANK accession number: MK 045323 ) .

External features. The worm is filiform, slightly flattened dorso-ventrally. Relaxed specimens are 138–158 mm long and 0.5–0.6 mm wide. The anterior end is rounded. The color is orange in anterior body region, becomes lighter toward the tail. The brain region is reddish ( Fig. 1A ). Numerous white spots can be seen through the whole of body length. The head possesses a pair of cephalic furrows that are located behind the brain, and meet dorsally to form a posteriorly pointed V, and ventrally an anteriorly pointed V ( Fig. 1B, C ). Sensory cirri and caudal adhesive plate are absent.

Body wall and musculature. The ciliated glandular epidermis is mostly about 10–20 µm thick. The dermis is about 1–2 µm thick. The body wall musculature consists of an outer circular and an inner longitudinal muscle layers, which are about 1–2 µm and 5–30 µm thick, respectively ( Fig. 2E ).

Proboscis apparatus. The rhynchodaeum opens ventrally near the anterior tip. It is lined by a thin epithelium without glands or cilia. The rhynchocoel accounts for about 1/8 of the body length, contains an outer circular and an inner longitudinal muscle layers. The anterior chamber of proboscis is about 3–6 mm long. The diaphragm is bulbous, about 160–230 µm in length. The middle chamber is tubular, 400–500 µm long ( Fig. 1D ). The posterior chamber is 4-8 mm in length. The central stylet, which is 54–60 µm long, is composed of three twined strands. The stylet basis is cylindrical, and measures 50–52 µm long and 12 µm wide ( Fig. 1E ). The ratio of stylet/basis is 1.06– 1.17. Two accessory stylet pouches are situated lateral to the central stylet, each contains 4–6 accessory stylets that point either forward or backward.

Alimentary canal. The esophagus, without glands and cilia ( Fig. 2C ), opens into the ventral side of posterior rhynchodaeum. The stomach, which is about 1.3–2.8 mm in length, is divided into two regions. The short anterior region is about 10% of the full stomach length, and is characterized by a larger number of red- staining and smaller number of blue-staining glands ( Fig. 2F ). At the long posterior stomach region, the wall becomes thinner, the number of gland cells decreases markedly. The intestine possesses distinct unbranched diverticula ( Fig. 1G ) but no caecum.

Nervous system. The brain is composed of paired ventral and dorsal ganglia. The dorsal and the ventral cerebral commissures are about 10 and 25 µm thick, respectively ( Fig. 2C ). The lateral nerve cord is about 25 µm in diameter ( Fig. 2D ); there is no accessory lateral nerve.

Sense organs. Eyes and cerebral organs are absent. A pair of statocysts, which are ovoid and 20-23 µm in diameter, are situated in the posterior part of the ventral lobes of cerebral ganglia. Statoliths are polygranular and 10–12 µm in diameter, each contains approximately 12 granules (unavailable to make accurate count) ( Fig. 1F ).

Frontal organ and cephalic glands. The frontal organ, opening in front of proboscis pore, is a simple epithelial pit about 6 µm in diameter ( Fig. 2A ). Cephalic glands ( Fig. 2B ), which are scarcely stained by Mallory, extend back to the foregut region. Parenchyma is scarcely developed.

Blood vascular system. As in most hoplonemerteans, vascular system consists of a pre-cerebral loop, a pair of lateral vessels, and a mid-dorsal vessel situated between rhynchocoel and alimentary canal ( Fig. 2F ). The origin of mid-dorsal vessel could not be determined. Vascular plug is not detected.

Excretory system. Excretory tubules are small and observed only in stomach region. A single pair of efferent ducts are observed. They open dorsal-laterally at the anterior stomach region ( Fig. 2D ).

Molecular species delimitation. The 409 sequences comprised 161 haplotypes, and the TCS analysis of the haplotypes results in 58 networks (not shown). The 4 individuals studied in this paper share a haplotype and constitute a unique network. The average uncorrected p-distances between this network and the remaining 57 networks range from 0.135 to 0.185 (within network values range from 0 to 0.010). In the ML haplotype tree, O. longissima sp. nov. constitutes a clade sister to Ototyphlonemertes macintoshi Bürger, 1895 (with low support value), and the both are located in a large clade containing all available members of O. macintoshi species group ( Fig. 3 ).

Figures

FIGURE 1. Ototyphlonemertes longissima sp. nov. A, Photograph of live specimen; B, dorsal view of head; C, ventral view of head; D, photomicrograph of partial proboscis from squeezed specimen; E, central stylet and basis, from squeezed specimen; F, statocyst, from squeezed specimen; G, intestinal region, from squeezed specimen, showing diverticula. ba, basis; cf, cephalic furrow; cs, central stylet; mc, middle chamber of proboscis; pd, proboscis diaphragm; sl, statolith. Scale bars: A = 6 mm; B, C = 300 µm; D = 50 µm; E, F = 20 µm; G = 100 µm.

FIGURE 2. Ototyphlonemertes longissima sp. nov. A, Transverse section near the tip of head; B, transverse section of precerebral region; C, transverse section through anterior foregut region, longitudinal muscles covering brain are arrowed; D, transverse section through stomach region; E, body wall (foregut region); F, transverse section of stomach. bv, blood vessel; cg, cephalic glands; cm, circular muscle layer; d, dermis; e, epidermis; es, esophagus; ex, efferent duct of excretory system; fo, frontal organ; lm, longitudinal muscle layer; ln, lateral nerve; rd, rhynchodaeum; st, stomach; vc, ventral cerebral commissure. Scale bars = 20 µm.

FIGURE 3. The phylogenetic tree resulting from the Maximum-likehood (ML) analysis of the 161 haplotypes of 409 COI sequences. The clade of Ototyphlonemertes macintoshi species group is enlarged. Each species name is shown with the name in GenBank followed by that listed in Kajihara et al. (2018: fig. 3 and Appendix). Numbers near nodes are bootstrap support values for ML analysis.