Metamicropora, Arakawa, 2016

Arakawa, Shinji, 2016, Taxonomy of Some Microporids (Bryozoa: Cheilostomata) from the Pacific Coast of Japan, Species Diversity 21 (1), pp. 9-30 : 23

Genus Metamicropora gen. nov

Diagnosis. Colony encrusting, unilaminar, multiserial. Zooids elongate, with lateral wall raised as mural rim. Frontal shield cryptocystal, granular, minutely perforate, with pair of large opesiules near orifice and small additional opesiules along lateral and proximal margins. Opesiules occluded, with somewhat large marginal hole closed by cribriform covering with radial or reticulate slits and pores. Orifice semicircular, without oral spines. Ovicell absent. Avicularium present, distal to the orifice, with complete pivot bar. Uniporous or pauciporous septula present; basal pore chambers absent.

Type species. Verminaria areolae Sakakura, 1935 .

Etymology. The generic name comes from addition of the Greek meta to the name Micropora , referring to its zooidal characteristics beyond the diagnosis of Micropora in spite of the gross similarity.

Remarks. Verminaria areolae was subsequently placed in the genus Microporina by Sakakura (1936) but it differs from Microporina in the presence of radially arranged or cribriform openings on the thin layer occluding the opesiules.

Colony form and zooidal characteristics of Verminaria areolae are more similar to those in some species of Micropora from the Pacific and adjoining seas than Microporina . For example, Micropora finisterrae Moyano, 1994 resembles in having multiple opesiules and paired tubercles near the orifice of proximal zooid ( Moyano 1994b), and Micropora angusta MacGillivray, 1887 in having elongate zooids and an obliquely directed avicularium distal to the orifice ( d’Hondt 1986; Ryland and Hayward 1992). Despite these similarities, V. areolae —the absence of ovicells, an avicularium distal to the orifice, opesiules occluded by calcification, and septula in the vertical walls—are common to the genus Microporina , as Sakakura (1936) discussed.

In this study, I found the difference in the occlusion of opesiules between Microporina and Verminaria areolae .

Levinsen (1909: 162) proposed the genus Microporina on the basis of the calcification filling the opesiules, noting that the opesiule were “sometimes” filled up. However, this occlusion is fundamentally not secondary calcification, but an exposed inner layer of the frontal shield, as shown above in the description of the two Microporina species. Such occluded opesiules are also observed in V. areolae , but they have radial or reticulate slits and pores ( Fig. 10C, D ). In the case of zooids which the cryptocystal frontal shield has ceased growing (NMNS PA 16848), the circular boundaries of opesiules are distinct, and their position corresponds to the hole with radial or cribrate openings in V. areolae ( Fig. 11D ).

The cribrate openings are also observed in the frontal pores of V. areolae ( Fig. 10C , 13A ). The two Microporina species described above also have cribrate frontal pores, but they are evidently not marginal ( Fig. 13B, C ); therefore, the hole occluded by cribrate calcification in the multiple opesiules of V. areolae can be expressed with marginal pores. The presence of marginal pores and the formation of opesiules reflecting their position are important difference between V. areolae and Microporina . This distinction has led to my conclusion that V. areolae cannot be considered as a species of Microporina that forms sheet-like colonies.

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. 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.