Micropora rimulata Canu and Bassler, 1929

Arakawa, Shinji, 2016, Taxonomy of Some Microporids (Bryozoa: Cheilostomata) from the Pacific Coast of Japan, Species Diversity 21 (1), pp. 9-30 : 16-18

Micropora rimulata Canu and Bassler, 1929 ( Fig. 5 )

Micropora rimulata Canu and Bassler, 1929: 137 , pl. 14, fig. 4; Dick et al. 2006: 2211, fig. 5D.

Micropora coriacea (not of Esper in Johnston, 1847): Arakawa 1999: 56 (in part, misidentified).

Material examined. NMNS PA 16838 (on mollusc shell), Station 1734, Hakurei-Maru cruise GH80-2; NMNS PA 16839 (on rhodolith), Ogasawara, Station 37, Hakuho- Maru cruise KH80-3; NMNS PA 16840 (on mollusc shell), Awa-Kominato, Chiba Prefecture, Japan; NMNS PA 16841 (on mollusc shell), Aburatsubo , Kanagawa Prefecture, Japan; SGBC-0405 (on mollusc shell), Merahama, Chiba Prefecture, Japan . See Table 1 for coordinates and depths of cruise samples.

Measurements (in milimetres). NMNS PA 16838, 16839, 16840, and 16841. Autozooids (n =122, 4): ZL, 0.37–0.82 (0.570±0.092); ZW, 0.28–0.66 (0.427±0.064); OrL, 0.05–0.09 (0.067±0.008); OrW, 0.08–0.18 (0.133±0.015). Ovicellate zooids (n =13, 1): ZL, 0.51–0.79 (0.621±0.076); ZW, 0.34–0.53 (0.441±0.062); OrL, 0.08– 0.12 (0.096±0.013); OrW, 0.13–0.18 (0.158±0.014). Ovicells (n =13, 1): OvL, 0.18–0.31 (0.252±0.042); OvW, 0.25– 0.33 (0.285±0.030). Avicularia (n =15, 4): AvL, 0.10–0.16 (0.125±0.016); AvW, 0.05–0.09 (0.069±0.013).

Description. Colony encrusting, forming unilaminar sheet; maximum observed size in largest dimension 7.5 mm (NMNS PA 16838). Zooid subhexagonal to elliptical, but irregular in outline. Frontal shield flattened or slightly convex, with granular surface, surrounded by mural rim; frontal pores somewhat unevenly distributed ( Fig. 5A ). Proximal gymnocyst generally lacking. Latero-oral knobs not well developed. Opesiule elliptical or circular, rarely with exposed inner layer. Orifice semicircular, wider than long, dimorphic, larger in ovicellate zooids than in non-ovicellate zooids. Oral spines lacking. Ovicell initially distinct, with finely granulated surface like frontal shield of zooid; proximal margin thickened, chevron-shaped, smooth, with tubercle at apex ( Fig. 5B ), becoming smooth by secondary calcification ( Fig. 5A ); central umbo sometimes present ( Fig. 5C ). Avicularium distal to orifice of non-ovicellate zooids, directed proximolaterally, mandible portion subtriangular, pivot bar complete ( Fig. 5D ).

Distribution. This species has been reported from the Philippines, Hawaii, and Japan. My material was collected on the seashore and the continental shelf (155 m in depth) around the Boso Peninsula, and the continental slope of Ogasawara at a depth of 222– 300 m. Kataoka (1961: 234, pl. XXXVII, fig. 12) reported this species from the Pleistocene Ryukyu Limestone, but his identification seems doubtful, because the orifice width is larger than 0.20 mm and the ovicells and the avicularium are small for the large orifice.

Remarks. This species fits the diagnosis of Micropora rimulata from the Philippines, especially in relatively salient frontal pores; dimorphism of the orifice, which is large in ovicellate zooids; a proximal band fringing the ovicell; and the proximolaterally directed avicularium. Zooids in my specimens are somewhat smaller in average length and narrower in average width than in the Philippine material of Canu and Bassler (1929), and the orifice is somewhat smaller in average length and width than in the Hawaiian material of Dick et al. (2006).

Ovicells are present in only one colony (NMNS PA 16838). They are finely granulated like the frontal wall of zooids and have a chevron-shaped thickening of the proximal margin ( Fig. 5B ), similar to specimens from the Philippines and Hawaii. However, the surface texture varies depending on the degree of secondary calcification, and some ovicells are obscured by secondary calcification. Sometimes no tubercle is evident ( Fig. 5A ), or a blunt umbo develops ( Fig. 5C ).

There is distinct variation in the avicularia in my material. In specimens from the continental shelf of the Boso Peninsula and the coast of Awa-Kominato, the avicularium has a nearly symmetrical rostrum ( Fig. 5A, D, G ), whereas the rostrum is asymmetrical in the specimens from Ogasawara and Aburatsubo ( Fig. 5E, F ). The former are more similar to Philippine material, the latter more similar to Hawaiian material. This character varied even within colonies, however, and the angle between the rostrum and the proximal border of orifice (or the straight line joining the distal margins of paired opesiules in the cryptocyst) ranged from 12° to 44° in NMNS PA 16838.

Figures

Fig. 5. Micropora rimulata Canu and Bassler, 1929, A–D, NMNS PA 16838; E, NMNS PA 16839; F, NMNS PA 16841; G, NMNS PA 16840: A, autozooids and one ovicellate zooid; B, ovicell; C, umbonate ovicell; D, avicularium with complete pivot bar; E–G, avicularium. Scale bars: 200µm (A); 100µm (B–G).