Anthomastus mirabilis Li & Xu sp. nov.

Figs 5, 6, 7, 8

Material examined.

Holotype • FIO-ATL 83 - JLBEN 27711, collected by Qinzeng Xu using the HOV Jiaolong from the R/V Shenhai Yihao on 28 February 2024 at station JL 277 (14°3'28"S, 14°20'53"W), Southern Mid-Atlantic Ridge, at a depth of 1,553 m.

Paratypes • FIO-ATL 83 - JLBEN 27712 and FIO-ATL 83 - JLBEN 27713 were collected together with holotype.

Diagnosis.

Mushroom-shaped, with a spherical capitulum and a short and inconspicuous stalk. Polyps are dimorphic. Autozooids are large, retractile, and evenly arranged over the capitulum. Siphonozooids are invisible, numerous, densely scattered among autozooids. The sclerite profile is diverse, but characterized by the absence of spindles in all parts of the colony. Sclerites of the capitulum surface and stalk predominantly include multiradiates and plates. Tentacular sclerites are composed predominantly of dumbbells and plates; anthocodial walls contain clubs and dumbbells. Pharyngeal sclerites are platelets with a distinct median waist and crosses.

Description of holotype.

Colony form and size. The specimen FIO-ATL 83 - JLBEN 27711 has a prominent capitulum supported by an inconspicuous stalk, with an approximate 1: 1 height ratio between the capitulum and stalk, and a total height of 42 mm (Fig. 6 A, G). The capitulum is dome-shaped, with a diameter of 33 mm and a height of 20 mm. The stalk is short, thick, and cylindrical, with a diameter of 20 mm and a height of 22 mm.

Polyps. There are 22 large autozooids evenly arranged over the capitulum, typically spaced about 8 mm apart. The autozooids are retractile, with some being fully retracted while others are contracted with their tentacles curved inward, forming small spherical protrusions on the surface of capitulum (Fig. 5 C). The autozooids are large, with a maximum length of 51 mm (including tentacles, which are 17 mm long). The pinnules are arranged either oppositely or alternately, with 12–18 pairs. Siphonozooids are small, numerous, invisible, and scattered among the autozooids at intervals of 1.6–2.0 mm.

Sclerites. Tentacular sclerites (Fig. 6 A) are plates (0.12–0.21 mm long), rods (0.13–0.23 mm long), dumbbells (0.11–0.22 mm long), and a few rods with a short transverse axis (0.14–0.23 mm long). Pharyngeal sclerites (Fig. 6 B) are platelets with a median waist (0.05–0.08 mm long) and a few crosses derived from small plates. Sclerites of the anthocodial wall (Fig. 7 A) are rods (0.17–0.35 mm long), clubs (0.14–0.19 mm long), dumbbells (0.13–0.18 mm long), multiradiates (0.10–0.14 mm long), and a few crosses. Sclerites from the surface of capitulum (Fig. 7 B) are rods (0.18–0.37 mm long), clubs (0.17–0.23 mm long), multiradiates (0.11–0.14 mm long), and a few crosses. Sclerites from interior of capitulum (Fig. 7 C) are long rods (0.27–0.49 mm long). Sclerites from the surface of stalk (Fig. 8 A) are multiradiates (0.10–0.14 mm long), rods (0.15–0.23 mm long), and a few crosses. Sclerites from the interior of the stalk (Fig. 8 B) are long rods (0.32–0.40 mm long). Sclerites of the holdfast (Fig. 8 C) are rods (0.17–0.31 mm long), multiradiates (0.09–0.14 mm long), and a few crosses.

Color. Live specimens display a bright red color, while appearing vermilion in ethanol. The distal ends of the tentacles are yellowish-white.

Variation. Paratype FIO-ATL 83 - JLBEN 27712 was attached to a dead coral skeleton and has 20 autozooids. The capitulum is 16 mm high with a diameter of 23 mm. The stalk is short and inconspicuous, with a length of 6 mm. Paratype FIO-ATL 83 - JLBEN 27713 was also attached to a dead coral skeleton and has 28 autozooids. The capitulum is 22 mm high with a diameter of 33 mm, and the stalk is 16 mm high.

Etymology.

The Latin mirabilis, meaning “ wonderful ” and “ astonishing, ” concisely reflects how this deep-sea coral’s striking red, spherical form with long autozooids captivates observers with its beauty.

Distribution and habitat.

Known only from the type locality. Southern Mid-Atlantic Ridge, 1,553 –1,649 m.

Remarks.

The morphological characteristics of Anthomastus mirabilis sp. nov. match well with the generic definition of Anthomastus. Morphologically, A. mirabilis sp. nov. is most closely allied with A. grandiflorus Verrill, 1878, showing no obvious differences in the external morphology of the capitulum, stalk, and polyps. Both species are grouped by having platelets as the predominant pharyngeal sclerites and by having autozooids distributed across the whole capitulum. However, they are fundamentally separated by their sclerite composition: A. mirabilis sp. nov. possesses dumbbells and more plates in its tentacles, and clubs and dumbbells in its anthocodial wall. Crucially, A. mirabilis sp. nov. lacks spindles throughout the colony (capitulum, stalk, and holdfast), which is a key distinction from A. grandiflorus, for which spindles are a central diagnostic feature.

The new species also closely resembles A. gyratus. While both share similar sclerite morphology, A. mirabilis sp. nov. is separated because its autozooids are distributed across the entire capitulum, whereas in A. gyratus, they are arranged only over the distal three-quarters of the capitulum; furthermore, A. mirabilis sp. nov. lacks spindles and has dumbbells present in the anthocodial tentacles and wall, a combination not found in A. gyratus. The unique traits of A. mirabilis sp. nov. also exclude it from all remaining Anthomastus species. It differs from A. bayeri by the presence of pharyngeal sclerites. It is separated from A. nanhaiensis and A. tongi because its tentacular sclerites are more diverse and include plates, unlike the rods-exclusive composition of the latter two. Finally, its composition of predominantly platelet-type pharyngeal sclerites and its unique sclerite combination (dumbbells present, spindles absent) clearly distinguish it from all other Anthomastus species that either have different pharyngeal sclerite types (e. g., A. megacephalus) or different overall sclerite profiles (e. g., A. globosus and A. antarcticus). The combination of having dumbbells and plates in the tentacles while lacking spindles throughout the colony provides robust and comprehensive morphological evidence to support the designation of A. mirabilis as a distinct new species.