Arthropoma subarensis, Jain & Gordon & Huang & Kuklinski & Liow, 2022

Jain, Sudhanshi S., Gordon, Dennis P., Huang, Danwei, Kuklinski, Piotr & Liow, Lee Hsiang, 2022, Targeted collections reveal new species and records of Bryozoa and the discovery of Pterobranchia in Singapore, Raffles Bulletin of Zoology 70, pp. 257-274 : 266-268

Arthropoma subarensis , new species

( Fig. 5B‒D )

Material examined. Holotype: ZRC.BRY.0908 (Zoological Reference Collection, Lee Kong Chian Natural History Museum; collected as SG2019 No. 51; sequenced as BLEED No. 1466B), Big Sister’s Island (1.2125°N, 103.8362°E), Singapore, 15‒17 m, coll. S.S. Jain, D. Huang & P. Kuklinski, 3 May 2019.

Etymology. Named for the Malay name of the island, Pulau Subar Laut, where this species was discovered. The island is part of the Sisters’ Islands Marine Park ( Jaafar et al., 2018). Diagnosis. Colony encrusting unilaminar, small. Zooidal frontal shield with numerous simple pseudopores either side of an imperforate median strip; three rows of pseudopores continue around orifice. Orificial sinus longitudinally oval, the distal part constricted by condyle tips. Four short stumpy unjointed oral spines present. Endooecial surface evenly and minutely reticulated with a cluster of tiny pores at the bottom of each dimple. Ovicell opening non-cleithral.

Description. Colony encrusting, unilaminar, multiserial, up to 4–6 mm diameter. Colour in life translucent pale orange. Zooids arranged regularly in quincunx, separated by a thin boundary line. Zooids variably subhexagonal, some parallelsided [ZL 500‒743 (606), n = 11; ZW 303‒599 (483), n = 11]. Frontal shield lepralioid, smooth overall, with 4‒6 series of simple funnel-shaped pseudopores on each side of a longitudinally imperforate central area; three rows of pseudopores continue distally around the orifice. Orifice comprising a near-semicircular anter, a little wider than long, and a longitudinally oval sinus [OL 147‒188 (164), n = 11; OrW 125‒177 (150), n = 11]. Shoulders of orifice straight, bearing very long and narrow smooth-edged condyles, the tips of which partially constrict the sinus opening. Orificial anter bordered by four oral spines, these short, tapering, non-articulated and hollow with a small apical pore. No avicularia . Ovicell conspicuous, hyperstomial; ooecium with almost wholly membranous ectooecium, excepted by a proximal ectooecial calcified border, thin, smooth; calcified endooecium with surface reticulated into a network of polygonal ridges defining dimples, with a cluster of microscopic perforations at the bottom of each dimple. Orifice non-cleithral, flanked on each side by a single spine [OvL 226‒272 (251), n = 7; OvW 223‒327 (283), n = 7]. Ancestrula not seen.

Remarks. Living Arthropoma used to be regarded as essentially monospecific, with the name Arthropoma cecilii ( Audouin, 1826) accorded to specimens from many parts of the world geographically very distant from that species’ Atlanto-Mediterranean range (see synonymies in Osburn, 1952; Harmer, 1957; Gordon, 1984). Subsequently, eight additional recent species have been named, five of them in the past decade, including two from the western Pacific. Six of these species are listed by Bock (2022) on the Bryozoa Home Page and two others are indicated in Harmer’s synonymy, i.e., Arthropoma gandyi ( Haswell, 1880) and Arthropoma mediolaevis ( Ortmann, 1890) . Arthropoma subarensis , new species, differs from A. cecilii in the combination of oral spines, a broad imperforate area of frontal shield, 2‒3 rows of pseudopores distal to the orifice and a reticulate ooecial surface (see Chimenz Gusso et al., 2014). The only other species with oral spines is Arthropoma occidua Winston & Jackson, 2021 from Jamaica, with 2‒3 spines, but these are entirely distal and the ooecial surface is only faintly textured.

The identity of Arthropoma cecilii (the type species of the genus) is itself problematic. The original material was probably collected from the Red Sea, and AtlantoMediterranean and Red Sea specimens attributed to this species differ. For example, that described from Britain by Hayward & Ryland (1999) has a minutely pitted and granulartessellated endooecial surface, whereas that illustrated from the Red Sea (Ostrovsky et al., undated) is likewise pitted but not granular, but that illustrated by Chimenz Gusso et al. (2014) is not pitted but has a highly rugose surface. The material illustrated by these authors probably constitutes three different species. The comprehensive molecular phylogeny presented by Orr et al. (2022) pairs nominal A. cecilii with A. subarensis , new species, and we have no hesitation including the latter in Arthropoma despite its moderately distinctive ooecium.

Arthropoma gandyi from the Great Barrier Reef was never illustrated, but Harmer (1957: 1002) examined a specimen from the Haswell collection in the Manchester Museum and included it in the synonymy of A. cecilii . Haswell (1880) had described zooids as centrally imperforate, with a deep orificial sinus constricted at its entrance and the ovicells as having a subgranular surface. These characters differ from the material from Heron Island erroneously attributed to A. cecilii by Ryland & Hayward (1992), which means there are two species of the genus in Queensland waters. Arthropoma mediolaevis from Japan was illustrated by Ortmann (1890) as having a centrally imperforate area, but with many more pseudopores proximal to this area than in A. subarensis , new species.

Distribution. The species is so far known only from Singapore.

Figures

Fig. 5. A, Pleurocodonellina microperforata Tilbrook, 2006, with one ovicellate zooid and a small avicularium adjacent to each orifice; B‒D, Arthropoma subarensis, new species, at three magnifications; note the four arrowed spine bases in C, with paired spine bases in ovicellate zooids; D shows the reticulate dimpled patterning of the ovicell endooecium, with minute perforations at the bottom of each dimple. Scale bars = 0.5 mm (A, B); 0.2 mm (C); 0. 1 mm (D).