Metamicropora areolae ( Sakakura, 1935 )

Arakawa, Shinji, 2016, Taxonomy of Some Microporids (Bryozoa: Cheilostomata) from the Pacific Coast of Japan, Species Diversity 21 (1), pp. 9-30 : 25-27

Metamicropora areolae ( Sakakura, 1935) ( Figs 10–12 , 13A )

Verminaria areolae Sakakura, 1935: 12 , pl. 2, fig. 5, text-fig. 3; Hayami 1970: 324, pl. 36, fig. 4; Hayami 1975: 89 (only listed); Nishizawa and Sakagami 1986: 84, pl. 11, fig. 3; Arakawa 1995: 80 (only listed); Nishizawa 1997: 147, 152 (only listed).

Microporina areolae: Sakakura 1936: 264 .

Microporina elongata (not of Hincks, 1880): Silén 1942: 65, pl. 3, fig. 12, text-figs 76–78; Hirose 2010: 45, pl. 78A–C. Micropora areolae: Arakawa 1999: 56 .

Material examined. NMNS PA 16846 (two fragments, A, B), and SGBC-0105, Station 1748, Hakurei-Maru cruise GH80-2; NMNS PA 16845 (two colonies, A, B, the latter with ancestrula, on mollusc shell), 16847 (on mullusc shell), 16848 (on mullusc shell), 18158 (inner surface of frontal shield) and 18159, and SGBC-0002 (on mullusc shell), -0407 (on mullusc shell, not coated with metal), Jizodo Formation, Pleistocene, Nishiyatsu, Chiba Prefecture, Japan; NMNS PA 16849 and SGBC-0370 (on mullusc shell), Jizodo Formation, Pleistocene, Oi, Chiba Prefecture, Japan; NMNS PA 18157 (lateral and basal walls), Yabu Formation , Pleistocene , Neriki , Chiba Prefecture, Japan; SGBC-0410 (on mollusc shell, not coated with metal), Jizodo Formation , Pleistocene, Nagara Dam, Chiba Prefecture, Japan . See Table 1 for coordinates and depths of cruise samples.

Measurements (in milimetres). NMNS PA 16845, 16846, 16847, and 16848; SGBC-0002, -0105, -0370 and -0398. Autozooids (n =515, 10): ZL, 0.49–1.49 (0.780±0.124); ZW, 0.25–0.79 (0.396±0.0073; OrL, 0.06– 0.12 (0.082±0.010); OrW, 0.11–0.22 (0.162±0.020). Avicularia (n =184, 10): AvL, 0.07–0.18 (0.124±0.018); AvW, 0.05–0.12 (0.075±0.012).

Description. Colony encrusting, unilaminar, oπen becoming multilaminar; maximum size in largest dimension estimated to be more than 20 mm (SGBC-0407). Zooids irregularly hexagonal or rectangular. Frontal shield cryptocystal, flattened, finely granulated, with scattered frontal pores, surrounded by raised rim ( Fig. 10A ); frontal pores occluded, with three or more slit-like openings each ( Fig. 10C ). Six to nine pairs of circular opesiules present, closed by thin internal layer with radially arranged slits, or cribriform; distalmost pair generally larger than the others ( Fig. 10A, C, D ). Orifice semicircular, without oral spines. Operculum smooth ( Fig. 10B ). Paired tubercles lateral to orifice, derived from proximal gymnocyst of next-distal zooids ( Fig. 11 A–D). Avicularium, when present, situated distal to orifice, interzooidal, directed distolaterally, with complete pivot bar; rostrum acute to frontal plane, mandibular opening semicircular to rounded triangular ( Fig. 11A, B ). Ancestrula similar to later zooids; rhomboidal or hexagonal, with one pair of large, irregularly shaped opesiules and a few additional smaller ones; ancestrula budding three daughter zooids, one distally and two distolaterally ( Fig. 12A, B ). Zooids interconnected by six or more basal uniporous septula in each lateral wall ( Fig. 12C ) and several basal septula (some of them pauciporous) in transverse wall ( Fig. 12D ). Ovicells absent.

Distribution. Sagami and Tokyo Bays ( Sakakura 1935; Silén 1942; Nishizawa 1997; Hirose 2010) and the continental shelf east of the Boso Peninsula (station 1748, Hakurei- Maru cruise GH80-2). The recorded depth range is from 65 m (station 1748, this study) to more than 100 m ( Silén 1942).

Fossils of this species have been collected from Miocene to Pliocene deposits in Hokkaido, Miyagi, and Ishikawa Prefectures ( Hayami 1970, 1976; Nishizawa and Sakagami 1986), and from Pleistocene deposits in Chiba Prefecture ( Sakakura 1935; Arakawa 1995; Nishizawa 1997). It is one of the predominant species among cheilostome bryozoans in the Pleistocene Jizodo Formation ( Arakawa 1995).

Remarks. Silén (1942) advanced Sakakura’s (1936) discussion and concluded that Verminaria areolae is synonymous with Microporina elongata ( Hincks, 1880) from southern Africa. However, Silén’s conclusion is incorrect, as it was based on Waters’ (1889) misinterpretation that Micropora variperforata is synonymous with M. elongata . Those two species are distinct, as Brown (1952: 128) discussed in detail. The opesiules of M. elongata are occluded with calcification lacking any small openings ( Hincks 1880: pl. XVI, fig. 4).

On the other hand, Silén (1942) correctly described that the tubercles lateral to the orifice in this species are derived from the next-distal zooids. Similar structures are evident in Verminaria oblonga (Busk, 1859) , Calpensia rebeshovensis Zágoršek, 2010 and Micropora finisterrae , although the tubercles in these species are situated distal to the orifice of the proximal zooid ( Bishop 1987; Zágoršek 2010; Moyano 1994b). The occurrence of such tubercles thus seems to be polyphyletic.

Figures

Fig. 10. Metamicropora areolae (Sakakura, 1935), NMNS PA 16846: A, autozooids, B, enlargement of operculum; C, enlargement of additional opesiules and frontal pores; D, opesiules occluded by inner layers with cribrate marginal pores. Scale bars: 200µm (A, D); 50µm (B); 20µm (C).

Fig. 11. Metamicropora areolae (Sakakura, 1935), A–C, NMNS PA 16847; D, NMNS PA 16848: A, B, autozooids and avicularia; C, marginal part of colony, showing adoral tubercles derived from proximal gymnocyst; D, cryptocystal frontal shield aπer cessation of its growth. Scale bars: 200µm (A, B); 100µm (C, D).

Fig. 12. Metamicropora areolae (Sakakura, 1935), A, B, NMNS PA 16845; C, NMNS PA 18159A; D, NMNS PA 16846: A, B, ancestrula; C, lateral wall; D, transverse (distal) wall. Scale bars: 200µm (A, B); 100µm (C); 50µm (D).

Fig. 13. Inner surface of frontal shield, A, Metamicropora areolae (Sakakura), NMNS PA 18158; B, Microporina japonica Canu and Bassler, NMNS PA 18156; C, Microporina okadai Silén, NMNS PA 18160. Scale bars: 50µm.