Arthropoma minus, Min, Bum Sik, Seo, Ji Eun, Grischenko, Andrei V., Lee, Sang-Kyu & Gordon, Dennis P., 2017

Min, Bum Sik, Seo, Ji Eun, Grischenko, Andrei V., Lee, Sang-Kyu & Gordon, Dennis P., 2017, Systematics of some calloporid and lacernid Cheilostomata (Bryozoa) from coastal South Korean waters, with the description of new taxa, Zootaxa 4226 (4), pp. 471-486 : 481-482

Arthropoma minus n. sp.

( Figs 14–16 )

Material examined. Holotype: MBRBKSP035, Kuklipgongwon-yeoungu 1 station I3, 34.1585° N, 126.7689° E, 29 July 2016, 42 m, west of Cheongsan Island (Cheongsando). Paratype: MBRBKSP036, same data as holotype .

Etymology. Latin minus , neuter, less, alluding to the relatively small size of the zooids.

Description. Colony encrusting, unilaminar, up to 35 mm in diameter. Zooids variably somewhat hexagonal, regularly quincuncial, the distal margin rounded or angular, often widest midlength but this is variable. Orifice with moderately high-arched anter, barely curving proximally to horizontal ‘shoulders’ that bear thin condylar surfaces projecting somewhat across the entrance to the small oval sinus, constricting it. Operculum with jointed sinus tab. Frontal shield convex, perforated by c. 66–95 pseudopores that are distributed over most of the surface in the majority of zooids, except for an irregular, smooth narrow area midfrontally. 29–36 additional pseudopores, distributed around the orifice distad of the level of the proximal orificial rim, in mostly two rows. No oral spines. No avicularia. Ooecium prominent, hyperstomial, apparently cleithral (subcleithral), able to close both the ooecial orifice and the lower zooidal orifice. Ectooecium wholly membranous except for a thin peripheral rim around the base; endooecium minutely and densely pitted and knobbly. Multiporous mural septula present in lateral and distal walls. Ancestrula not seen.

Measurements. ZL 433–611 (515) µm, ZW 328–445 (372) µm; OL 144–159 (151) µm, OW 125–145 (134) µm; OoL 222–228 (225) µm, OoW 257–265 (261) µm.

Remarks. Arthropoma minus n. sp. superficially resembles A. cecilii but differs in having non-overlapping zooidal dimensions. Dick & Grischenko (2017) cite mean zooid length as 715 µm and mean zooid width as 493 mm in A. cecilii from the Mediterranean Sea, with A. cecilii from southern Britain being slightly larger; in contrast, zooids of A. minus n. sp. have a mean length of only 515 µm and mean width of 372 µm. Further, A. minus has two rows of pseudopores distal to the orifice, whereas there is only a single row in A. cecilii from Marseille and southern Britain. On the other hand, yet another ‘variant’ of A. cecilii from the Mediterranean (which looks like a separate species), is that illustrated by Chimenz Gusso et al. (2014). It overlaps with A. minus n. sp. in zooid length but not in width, has a double row of distal-oral pseudopores (but fewer in number), and a centrally imperforate area. It differs most obviously in having a coarse, nodular-tubercular ectooecium in which no pores are visible.

Distribution. Korea: South Sea coast; 42 m depth on hard substrata.

Figures

FIGURES 14 – 16. Arthropoma minus n. sp., Stn I 3 near Cheongsan Island. 14, Part of a colony with two ovicellate zooids; scalebar 500 µm. 15, Autozooidal orifice; scalebar 50 µm. 16, Autozooidal orifice with operculum; scalebar 50 µm. FIGURES 17, 18. Phonicosia crena n. sp., Stn I 2 near Wan Island. 17, Part of a colony with an ovicellate zooid; scalebar 200 µm. 18, Close-up of orifice and spine bases; scalebar 50 µm.