Microporina okadai Silén, 1942

(Figs 9, 13C)

Microporina okadai Silén, 1942: 68, figs 79–82; Gontar 1993a: 201 (only listed); Grischenko and Mawatari 2006: 193 (only listed); Hirose 2010: 46, pl. 79A–D.

Cyclostomella articulata (not of Fabricius, 1821): Okada 1921: 29, text-fig. 6.

Material examined. NMNS PA 18154 (two internodes, A, B and additional fragments, C), and 18160 (inner surface of frontal shield), and SGBC-0412 (two internodes, A, B), Station 1709, Hakurei-Maru cruise GH80-2; NMNS PA 18155, Station 1711, Hakurei-Maru cruise GH80-2. See Table 1 for coordinates and depths of cruise samples.

Measurements (in milimetres). NMNS PA 18154, 18155, and SGBC-0412. Autozooids (n =23, 6): ZL, 0.67– 1.28 (0.957±0.146); ZW, 0.23–0.45 (0.326±0.052); OrL, 0.07–0.14 (0.108±0.015); OrW, 0.12–0.22 (0.176±0.024). Avicularia (n =14, 6): AvL, 0.08–0.12 (0.102±0.013); AvW, 0.08–0.12 (0.103±0.011).

Description. Colony erect, consisting of slender internodes; only one unbroken internode (NMNS PA 18155) observed, 2.9 mm long by 0.74 mm in diameter; fragmented internodes ranging from 0.54 mm to 0.74 mm in diameter. Internodes nearly rhomboidal in section, composed of 5 to 6 columns of zooids (Fig. 9A, C, E); proximal and distal ends of internodes occupied by small kenozooids. Zooids elongate; nearly rectangular, or hexagonal, and tapering proximally. Frontal shield cryptocystal, flat, but somewhat inflated in proximal part of internodes, coarsely granulated, perforated by many pores with inner spicules, and surrounded by finely granulate mural rim reaching sides of orifice distally (Fig. 9B, D). Opesiules elliptical, longer than wide, completely occluded by somewhat uneven calcification. Orifice elliptical or semielliptical, proximal border straight or broadly concave. Oral spines lacking. Ovicells absent. Avicularium distal to orifice, small, directed proximally, circular or oval in outline.

Distribution. The known distribution is Sagami Sea, north off Hachijo-jima, and the continental shelf east of the Boso Peninsula. Gontar (1993a) listed this species from the Kuril Islands. Specimens from Sagami Sea came from depths of roughly 285 m (156 fms: Okada 1921), 450–600 m (Silén 1942), and 124–126 m (Hirose 2010). The specimen from Hachijo-jima came from depth of 160–190 m (Grischenko and Mawatari 2006; Hirose 2010). My material was collected at depths of 144 m (Station 1709) and 125 m (Station 1711).

Remarks. This species is characterized by slender internodes and small avicularia. The frontal shield is rather thin. All avicularia in my specimens were poorly preserved, but according to Okada (1921) and Silén (1942), the mandible is triangular and points proximally. Okada (1921) illustrated a narrow but complete pivot bar.

Grischenko (2013) recorded this species from the continental slope of the Japan Sea, but the identity of his specimen with M. okadai needs to be confirmed because he described the colony as “unjointed” and did not illustrate a triangular avicularian mandible.