Synallactes tenuibrachius, Xiao & Xiao, 2025

Synallactes tenuibrachius sp. nov.

Figs 2 View Figure 2 , 3 View Figure 3

Material examined.

Holotype • West Pacific, the Jiaolong Seamount in the SCS , depth 3610 m, 5 Jul. 2013, preserved in 95 % ethanol, CN: III, RN: MBM 286921 View Materials .

Diagnosis.

Body subcylindrical, convex dorsally, flattened ventrally. Mouth and anus terminal. Dorsal papillae conical, scattered on the bivium, with alternating large and small papillae. Ventrolateral papillae placed in a single row on each body side. Midventral radius with one or two small tube feet at the anterior and posterior ends. Ventrolateral tube feet arranged in double alternate rows. Dorsal body wall perforated plates and tables, tables with four or five slender arms. Papillae with tables, spires of tables higher than those of tables in the dorsal body wall. Ventral body wall tables with four or five arms. Tables with 4–6 arms in ventral tube feet, the distal ends of the arms enlarged, branched, rarely connected, and possessing a number of perforations.

Description.

Body subcylindrical, slightly convex dorsally, flattened ventrally. Skin soft and gelatinous. Color red in life (Fig. 2 A, B View Figure 2 ). After 95 % ethanol fixation, color white, body length 9 cm and width 4 cm (Fig. 2 C, D View Figure 2 ). Mouth and anus terminal. Tentacles, calcareous ring, and polian vesicles lost. Dorsal surface with numerous large and small papillae, scattered all over the bivium (Fig. 2 A – C View Figure 2 ). Large papillae arranged in longitudinal rows along dorsal radii, ~ 10 in each row; small papillae mainly distributed in both dorsal interradii, ~ 20 on each body side (Fig. 2 C View Figure 2 ). Ventrolateral papillae large, forming a simple row around the margin of the brim (Fig. 2 A, B View Figure 2 ). Ventrolateral tube feet conical, arranged in a double alternate row, ~ 22 on each body side (Fig. 2 D View Figure 2 ). Midventral tube feet radius with one or two small tube feet at the anterior and posterior ends of the body (Fig. 2 D View Figure 2 ). Ossicles. Dorsal body wall tables with four or five arms (Fig. 3 A 1 – A 9 View Figure 3 ), arms slightly broader at the tip with one or two perforations, 50–70 μm in length, spires with simple tips, often bearing several spines on the side, 82–100 μm in height; simple concave perforated plates (Fig. 3 A 10), 75 μm in diameter. Papillae with four-armed tables (Fig. 3 B 1, B 2 View Figure 3 ), the spires significantly higher than those in the dorsal body wall, 110–155 μm in height. Ventral body wall with tables similar to those in the dorsal body wall (Fig. 3 C 1 – C 5 View Figure 3 ), except that the end of arms bifurcated or bearing more perforations (up to 5) in some tables. Tables with 4–6 arms in ventral tube feet (Fig. 3 D 1 – D 10 View Figure 3 ), irregular in shape, the distal ends of the arms enlarged, branched, and possessing several perforations, sometimes the enlarged ends of the arms connected with one another (Fig. 3 D 1 View Figure 3 ); rarely a four-armed table with two parallel pillars (Fig. 3 D 8 View Figure 3 ), ~ 68 μm in height, without transverse beam.

Type locality.

The Jiaolong Seamount in the South China Sea, tropical Western Pacific, depth 3610 m.

Etymology.

The name of the new species is a combination of the Latin words tenuis meaning ‘ slender’ and brachium meaning ‘ arm’. It refers to the very slender arms of tables in the body wall.

Distribution.

Only known from its type locality.

Remarks.

The new species, Synallactes tenuibrachius sp. nov., is characterized by the tube feet arranged in a double alternate row along each ventrolateral radius and all tables possessing very slender arms. Among the five species of Synallactes with single-pillared tables featuring pointed tops (i. e., S. aenigma , S. horridus , S. robertsoni , S. profundus , and S. laguardai ; Solís-Marín 2003; Solís-Marín 2005), the new species most resembles S. aenigma Ludwig, 1894 , S. horridus Koehler & Vaney, 1905 , and S. profundus ( Koehler & Vaney, 1905) based on the ossicle features of the body wall. Specifically, all have single-pillared tables with pointed tops and four- or five-armed discs, with the distal ends of the arms exhibiting only a few perforations. However, there are some differences between them. Compared with S. aenigma : 1) In the new species, the midventral ambulacrum bears only one or two small tube feet at the anterior and posterior ends of the body. In contrast, Ludwig (1894) noted that S. aenigma possesses a few tube feet organized into two long rows on the midventral radius. 2) The anus in S. tenuibrachius sp. nov. is terminal, whereas in S. aenigma , it is dorsal. 3) The spires in S. tenuibrachius sp. nov., are generally unbranched at the end, with some arms having enlarged ends that may connect to each other, while in S. aenigma , the spires of tables are divided into 2–10 elongated tips at the end, and the arms of tables never seem to be connected. 4) Synallactes tenuibrachius sp. nov. differs from S. aenigma by the types of ossicles in the tube feet. The tube feet of S. tenuibrachius sp. nov. have only tables, whereas those of S. aenigma have spiny, curved support rods, and well-developed mesh-like discs in addition to tables.

Compared with S. horridus : 1) S. tenuibrachius sp. nov. exhibits numerous large and small papillae scattered on the dorsal surface and only one or two small tube feet placed on the midventral ambulacrum at the anterior and posterior ends of the body. By contrast, S. horridus has large conical papillae on all radii except midventral radius, and some tube feet are confined to the anterior region of midventral radius. 2) The body wall of S. tenuibrachius sp. nov. contains tables with cross-shaped discs and small perforated plates, but no rods, while the body wall of S. horridus has many cruciform bodies and rods, with perforated plates absent. 3) All the arms and spires of tables in the body wall of S. tenuibrachius sp. nov. are more slender and fragile, compared to the solid and high spires of the cruciform bodies in the body wall of S. horridus .

Compared with S. profundus : The new species differs from S. profundus by the arrangement and number of the ventral tube feet. The tube feet in S. tenuibrachius sp. nov. are arranged in a double alternate row on each ventrolateral radius, while the midventral radius is completely naked. In S. profundus , the tube feet are arranged in a single row on each ventrolateral radius, with the midventral radius hosting a few tube feet near the anterior and posterior ends.