Antropora minor ( Hincks, 1880 )

Dick, Matthew H., Ngai, Nguyen Danh & Doan, Hung Dinh, 2020, Taxonomy and diversity of coelobite bryozoans from drift coral cobbles on Co To Island, northern Vietnam, Zootaxa 4747 (2), pp. 201-252 : 210-211

Antropora minor ( Hincks, 1880)

( Figs 4 C–F; 20B)

“? Membranipora trifolium, S. Wood , var. minor ” Hincks, 1880, p. 87, pl. 11, fig. 6.

Antropora minor: Tilbrook 1998, p. 34 , fig. 2A–F, and synonyms therein; Tilbrook et al. 2001, p. 41, fig. 3A; Tilbrook 2006, p. 30, pl. 4B; Dick et al. 2006, p. 2209, fig. 5A, B; Gordon et al. 2007, p. 48, fig. 1G; Taylor & Tan, 2015, p. 13, fig. 7A.

Material examined. VNMN-0217 (CT-11), VNMN-0218 (CT-20), VNMN-0219 (CT-37), VNMN-0220 (CT-47), VNMN-0221 (CT-48), VNMN-0276, (CT-52), on SEM stubs; VNMN-0222, 11 colony fragments.

Measurements. AzL, 0.28–0.40 (0.350 ± 0.034); AzW, 0.20–0.26 (0.230 ± 0.015); OpL, 0.15–0.22 (0.185 ± 0.018); OpW, 0.12–0.15 (0.133 ± 0.012) (n = 15, 1).

Description. Colony encrusting, sheet-like; initially unilaminar but becoming bi- or trilaminar as additional layers are produced, initiated by budding in frontal direction.

Zooids small, distinct, delineated by sharp groove, distal third raised, forming sharp mural rim; gymnocyst negligible, sometimes evident at proximal end of zooid; cryptocyst extensive, coarsely granulate, widest proximally, narrowing laterally, continuing around inside of raised distal rim. Opesia occupying half or more of frontal length; irregular, pear-shaped, or oval; small, smooth gymnocystal papillae occasionally present proximally or proximolaterally. Spines lacking.

Small interzooidal avicularia common in angular positions formed by zooidal intersections; on raised cystid, mandible rounded, no crossbar. Vicarious avicularia rare; only one found among all specimens examined; similar in size (0.35 × 0.21 mm) to autozooids, with raised, smooth-walled, circular mandibular part of rostrum separated from smaller, proximal, cryptocystal part by sharp, medially directed condyles.

Reproducing zooids with vestigial ooecium manifested as crescent-shaped zone of smooth, inflated calcification at proximal end of next-distal zooid; when ooecium fully developed, cap entirely covering sharp distal rim of reproducing zooid. Embryos brooded endozooidally.

Ancestrula similar to but smaller than subsequent zooids; appears to bud first daughter zooid(s) distolaterally on one or both sides, with subsequent spiral budding pattern from first daughter zooid on one side.

Remarks. Diagnostic characters for this species include the irregular occurrence of “kenozooidal papillae” at the proximal end of some zooids, and large vicarious avicularia interspersed with autozooids ( Tilbrook 1998). Some zooids in our specimens have proximal papillae, but these do not appear to be kenozooidal, but rather are raised swellings in the exposed proximal gymnocyst. Some of the supposed kenozooidal papillae illustrated by Tilbrook (1998) likewise appear to be gymnocystal swellings rather than interzooids (e.g. Tilbrook 1998, fig. 2C), whereas others are ambiguous (e.g. Tilbrook 1998, fig. 2E). We observed only one vicarious avicularium, with most colonies lacking any; Tilbrook (1998) noted that vicarious avicularia may be absent from large areas of some colonies of A. minor .

Distribution. Originally described from Bahia, Brazil ( Hincks 1880 b), this nominal species putatively has a circumtropical distribution in shallow shelf areas ( Tilbrook 1998, 2006; Tilbrook et al. 2001). Given the improbability of connectivity between Atlantic and Pacific populations in the era preceding widespread transoceanic shipping, this species has either been widely dispersed anthropogenically over the past few centuries, or comprises a complex of morphologically similar but genetically divergent geographical clades. The closest previous records to northern Vietnam are from the Philippines, Sabah ( Malaysia), and China ( Tilbrook 1998, 2006).

Figures

FIGURE 4. A, B. Cranosina coronata (Hincks, 1881), VNMN-0215. A. Part of colony. B. autozooids and avicularia. C–F. Antropora minor (Hincks, 1880). C. Young colony with ancestrula (asterisk), partly overgrown by the foraminiferan Planorbulina larvata (bottom) and forming stand-off with it (upper left), VNMN-0218. D. Autozooids and avicularia, VNMN-0217. E. Autozooids and avicularia, with left-center autozooid having vestigial ooecium, and several autozooids having smooth, raised prominence (gymnocystal papilla) at proximal end, VNMN-0217. F. Non-ooeciate autozooid (left-center), ooeciate autozooid (lower right), and vicarious avicularium (upper right), VNMN-0276. Scale bars: A, C, 1 mm; B, 500 µm; D, 300 µm; E, 150 µm; F, 200 µm.