Laetmogone multiradiolus, Xiao & Zhang, 2025

Laetmogone multiradiolus sp. nov.

Figs 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4

Material examined.

Holotype South Pacific • 1 specimen; from the abyssal plain adjacent to the Kermadec Trench ; 31°55.54'S, 176°57.09'W; depth 5735 m; 3 Nov 2022; preserved in 99 % high grade absolute ethanol; NIWA 164015 View Materials . GoogleMaps

Type locality.

The abyssal plain adjacent to the Kermadec Trench, the South Pacific, depth 5735 m.

Diagnosis.

A member of the genus Laetmogone with the following features: Colour uniformly dark violet; tentacles 17, with rounded terminal discs, slightly lobed; calcareous ring absent; papillae conspicuous, nine in each dorsal radius; tube feet 12 pairs, placed in single rows along ventrolateral radii. Body-wall ossicles in form of elasipodid wheels, circular in outline, with 4–5 (rarely 6) central rays, 8–17 spokes, nave covered by a calcareous membrane; rods and few irregular ossicles in papillae, tube feet and tentacles; cross-shaped ossicles absent.

Description.

Body long, cylindrical, slightly pointed anteriorly (Fig. 2 A, B View Figure 2 ), 31 cm long, 7.5 cm wide in situ (Fig. 2 C, D View Figure 2 ), 11 cm long, 1.5 cm wide in preserved state. Colour uniformly dark violet in both vivo and preserved states (Fig. 2 A – F View Figure 2 ). Tentacles 17, large, 0.6–1 cm in length after several days of fixation, slightly lobed (Fig. 2 E, F View Figure 2 ). Dorsal papillae conspicuous and nine pairs, arranged in single rows along dorsal radii, 0.7–3 cm long after preservation. Ventrolateral tube feet large, robust and conical, up to 12 pairs, 0.4–0.9 cm in a state of preservation. Mouth ventral, anus terminal. Ventrolateral brim absent. Calcareous ring absent; gonads with numerous branched tubules, arranged in several clusters. Only wheel ossicles in dorsal and ventral body walls (Fig. 3 A – G View Figure 3 ), with no sharp distinction between the two types. The nave of every wheel covered by a calcareous membrane (Fig. 3 D View Figure 3 ). Wheels in body wall 0.04–0.21 mm in diameter, central rays 4–5, rarely 6, 8–17 spokes. Rods and wheels in tube feet, papillae and tentacles, wheels similar to those of the body wall. Papillae wheels, rods and few irregular ossicles (Fig. 4 A View Figure 4 ), rods smooth and stout, curved or straight, occasionally possessing a few little spines on rod ends, 0.2–0.5 mm long. Wheels and curved rods in tentacles (Fig. 4 B View Figure 4 ), rods up to 0.8 mm in length, with small spines on both ends, some rods terminally bifurcated (Fig. 4 B View Figure 4 ). Ossicles in tube feet (Fig. 4 C View Figure 4 ) wheels, rods and irregular ossicles, rods similar in size and shape to those in papillae, irregular ossicles may represent a developmental stage of wheels.

More details of the hub, rim, central rays, spoke spaces and orientation of wheels from L. multiradiolus sp. nov. were shown in Fig. 5 View Figure 5 .

We counted the spokes and central rays and the diameter was measured in the specimen collected from the Kermadec Trench (Table 1 View Table 1 ). A sample of 100 wheels taken from the body wall, papillae, tube feet and tentacles (Table 1 View Table 1 ) revealed the following data:

A total of 44 % of wheels had four central rays, 55 % had five and only 1 % had six rays. The number of spokes per wheel varied from 8–17 with the majority (84 %) having 9–14 spokes (11 % with 9, 23 % with 10, 11 % with 11, 15 % with 12, 14 % with 13 and 10 % with 14 spokes). The largest wheel (0.21 mm) had only 11 spokes. Only wheels with a diameter of 0.06 mm had the maximum spokes (17). A higher spoke number (15–17) is typical of smaller (<0.1 mm) wheels.

Etymology.

The specific name was derived from the Latin words multi (many) and radius (ray or beam), which refers to the large number of the spokes of wheel ossicles.

Distribution and habitat.

Known only from the type locality so far. In this field, the holotype was found on flat sedimentary terrains. Benthic species, no swimming behaviour was observed.

Remarks.

Laetmogone multiradiolus sp. nov. clearly belongs to the genus Laetmogone and possesses 12 pairs of tube feet, which makes Laetmogone multiradiolus sp. nov. unique amongst the known Laetmogone species. The new species was characterised by a single type of wheel, with 4–5 (rarely 6) central rays and 8–17 spokes. These features place it most similar to Laetmogone wyvillethomsoni Théel, 1879 . However, L. multiradiolus sp. nov. can be differentiated from L. wyvillethomsoni by the following features: (1) The number of tentacles and tube feet were different. L. multiradiolus sp. nov. had 17 tentacles and 12 tube feet on each side, whereas L. wyvillethomsoni had 15 tentacles and at least 15 tube feet on each side. (2) In the new species, larger wheels reached 0.21 mm in diameter, whereas in L. wyvillethomsoni , wheels> 0.16 mm in diameter were not found. (3) Wheels in L. wyvillethomsoni had 8–14 spokes and some wheels in L. multiradiolus sp. nov. had spokes> 14 (8–17).

The new species was also quite different from other Laetmogone species. Laetmogone multiradiolus sp. nov. differed from L. maculatus by the absence of rosettes, from L. violacea Théel, 1879 by the absence of crosses, from L. scotoeides , L. maculata , L. fimbriata , L. billetti , L. ijimai , L. biserialis and L. pervipedata by the absence of two distinct types of wheels. The number of tentacles and tube feet makes L. multiradiolus sp. nov. and L. theeli different; the latter species had numerous tube feet and 20 tentacles, whereas the new species had relatively few tube feet and 17 tentacles. The difference between L. interjacens , L. perplexa and L. multiradiolus sp. nov. is that the new species had large, conspicuous papillae and the papillae of the former two species were small or minute. The dorsal papillae were arranged in two rows along the dorsal radii (four rows along dorsal radii in L. parvipedata ), which distinguished L. multiradiolus sp. nov. from L. parvipedata .