Arthropoma magniporosum, 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 : 479-481

Arthropoma magniporosum n. sp.

( Figs 10–13 )

Arthropoma cecilii: Okada & Mawatari 1936: 62 ; Rho & Seo 1986: 39, pl. 8; Seo 2005: 452, pls 181, 182A; Seo 2011: 77, fig. 36, pl. 27.

Arthropoma n. sp.: Hirose 2010: 111, pl. 190A–C.

Material examined. Holotype: MBRBKSP033, 34.5366° N, 128.7314° E, Hong Island (Hongdo), Gyeongsang Province, collected 1 June 1987 by J.E. Seo and J.G. Je . Paratype: MBRBKSP034, 34.4292° N, 127.2280° E, Dok Island (Dokdo), South Jeolla Province, collected 24 June 2008 by J.E. Seo, B.S. Min and H.J. Yang, 10– 15 m . Other material: Woosuk University Collection—specimens from Chuja Islands , Daepo , Gapa Island , Gu Island , Mijo , Moseulpo and Seogwipo , collected by J.E. Seo, H.J. Kil and J.H. Yoo.

Etymology. Latin magnus, large, and porosus, full of holes ( Brown 1956), alluding to the relatively large size and distribution of the pseudopores. Arthropoma is neuter in gender, hence the species epithet is also neuter.

Description. Colony encrusting, unilaminar, up to 20 mm in diameter. Zooids regularly quincuncial, the distal margin rounded or angular, more rarely truncated, the lateral margins more or less parallel-sided in many zooids. Orifice proportionately large, the anter high-arched, curving proximally to horizontal ‘shoulders’ that bear thin condylar surfaces that project slightly across the entrance to the small U-shaped sinus, constricting it. Operculum with jointed sinus tab. Frontal shield convex, perforated by c. 35–46 relatively large, round pseudopores that are distributed over the most of the surface in the majority of zooids, except for an irregular, smooth narrow area midfrontally that is subumbonate or weakly carina-like in some zooids. Additional pseudopores are distributed around the orifice in an arc of 10–19 pores in an inner series, sometimes with a few other pores in an incomplete second series. No oral spines. No avicularia. Ooecium prominent, hyperstomial, apparently cleithral (subcleithral), with the zooidal operculum able to close both the ooecial orifice and the lower zooidal orifice; ooecial orifice nearly at right angles to the maternal zooidal orifice. Ectooecium wholly membranous except for a thin peripheral rim around the base; endooecium minutely and densely pitted and textured, the pits less obvious frontally. Multiporous mural septula present in lateral and distal walls. Ancestrula not seen.

Measurements. ZL 480–636 (540) µm, ZW 287–403 (360) µm; OL 165–197 (177) µm, OW 134–175 (158) µm; OoL 223–310 (277) µm, OoW 295–371 (342) µm.

Remarks. Arthropoma magniporosum n. sp. has in the past been identified as Arthropoma cecilii Audouin, 1826 , a species from the Mediterranean/ Red Sea that has been accorded a wide distribution in cool-temperate to warm-temperate and tropical waters of the Atlantic and Indo-Pacific, from 25–1235 m depth (summarized by Dick & Grischenko 2017). If the species illustrated by Hayward & Ryland (1999) were to be taken as representing the species in the absence of actual type material, then it differs from A. magniporosum in several features. For example, A. cecilii has more than twice the number of frontal-shield pseudopores (c. 120–130), which are accordingly smaller and denser than in A. magniporosum . The ooecium of A. cecilii has a smoothly calcified rim around the orifice, with a mid-distal extension, and the sides of the ooecium taper more closely towards to proximal orificial rim than in A. magniporosum . Dick & Grischenko’s (2017) illustration of A. cecilii from the Mediterranean Sea area of Marseille resembles that of Hayward & Ryland (1999) from Britain (presumed, but not stated as such), but has fewer frontal-shield pores (81–96), and there are minor differences in frontal ectooecial sculpturing and placement of the proximolateral corners of the ooecium. Their conspecificity is uncertain. In either case, whether from the Mediterranean or elsewhere, the material from Europe is not the same as A. magniporosum n. sp. It is likely that A. cecilii was originally illustrated and described from Red Sea specimens (at least in part), but, in the absence of type specimens, no further conclusions can be drawn; Savigny’s drawings (d’Hondt 2006, pp. 31, 60) are inconclusive.

The synonymy given above does not cite all records of A. cecilii in the northeast Asian region, only those that definitely or almost certainly refer to A. magniporosum n. sp. The record of Okada & Mawatari (1936) is included as the authors stated that their unillustrated specimen from Hatsushima, Sagami Bay, Japan, had “very larger tremopores than the normal form”.

Only a few other Recent species of Arthropoma have been described. Arthropoma inarmata Gontar, 1992 from the Kurile Islands differs in having only 21–26 pseudopores in the proportionately short frontal shield, while there is a complete double row of pores in the relatively broad area distal to the orifice. Additionally, Gontar (1992) mentioned that the pores have a radial structure, which would be exceptional among Arthropoma species, suggesting that Cribellopora Gautier, 1957 might be a more appropriate genus to accommodate A. inarmata . Arthropoma lioneli Florence, 2016 from South Africa has distinctive reniform pores. A new species from the west coast of Okinawa ( Dick & Grischenko 2017) (East China Sea) has a crescentic suboral umbo that sometimes contributes to a continuous peristomial rim.

As Dick & Grischenko (2017) point out, three species, none of them A. cecilii , are illustrated by Hirose (2010) from Japan. One of them corresponds to A. magniporosum n. sp. (see synonymy above). A second ( Hirose 2010, pl. 189A, B) corresponds to Arthropoma mediolaevis ( Ortmann, 1890) . He described the taxon as a ‘variety’ of cecilii , noting that it differed in the “Glattes Feld in der Mitte der Zelle grosser”, i.e. the smooth field in the center of the zooid is greater than in cecilii . The sinus is also very narrow and the pseudopores are very small. The third species, presently undescribed (Hirose’s 2010, fig. 189C, D), differs again in having an almost subcircular sinus and in the small pseudopores occupying the entire frontal shield, or nearly so. Further, this species has a dimorphic zooid with a large orifice and a few distal-oral spines. The proportions of the dimorphic orifice differ from those illustrated by Kirkpatrick (1890) from the South China Sea, and Harmer (1957) from Indonesia, indicating that additional forms need naming as new species. Yet another unnamed species occurs in the Sagami Bay region; this was illustrated by Grischenko & Mawatari (2006, fig. 1G) as A. cecilii , but it differs from all of the forms mentioned above in having a smooth non-pitted endooecium and 3–4 rows of pseudopores between the maternal orifice and developing ooecium.

Based on ooecial morphology, and using the terminology of Ostrovsky (2013, pp. 130, 131), the ooecium appears to be the subcleithral form of a cleithral orifice, i.e. having two potential operculum closure positions, with the lower position closing the zooidal orifice. Observations on living material would be required to confirm if this is the case. It is known that the sinus tab of the operculum is hinged, as reflected in the genus name; if and when the operculum closes the ooecial orifice, the sinus tab is almost at right angles to the porta (rest of operculum).

Distribution. Korea: South Sea coasts, intertidal to more than 30 m. Japan: Sagami Bay and Sagami Sea, 47– 108 m.

Figures

FIGURES 10 – 13. Arthropoma magniporosum n. sp., Jeju Island. 10, Autozooids, showing a range of sizes and shapes, one ovicellate; scalebar 400 µm. 11, Interior view of autozooids showing variations in pseudopore diameter; scalebar 200 µm. 12, Ovicellate zooids in frontal view; scalebar 400 µm. 13, Ovicellate zooids in partial profile, showing the marked difference in angle of orificial and ooecial openings; scalebar 200 µm.