Rhynchozoon setiavicularium, Dick & Ngai & Doan, 2020

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 : 235-237

Rhynchozoon setiavicularium n. sp.

( Fig. 16 )

urn:lsid:zoobank.org:act:E9957EDC-7E61-4D15-ADD3-5FA8627BF14A

Etymology. The specific name is a combination of the Latin seta (bristle) and avicularium, referring to the inferred setiform end of the frontal avicularia.

Material examined. Holotype, VNMN-0263 (CT-40), on SEM stub.

Measurements. AzL, 0.40–0.54 (0.458 ± 0.044); AzW, 0.30–0.35 (0.323 ± 0.018); (n = 10, 1). OrL, 0.11–0.12 (0.116 ± 0.005); OrW, 0.11–0.13 (0.113 ± 0.006) (n = 9, 1). OvL, 0.16–0.21 (0.175 ±0.018); OvW, 0.18–0.21 (0.197 ± 0.012) (n = 7, 1). Frontal AvRL, 0.19–0.24 (0.221 ± 0.018); Frontal AvRW, 0.067 –0.089 (0.075 ± 0.007) (n = 10, 1).

Diagnosis. Suboral sinus U-shaped, moderately shallow, narrow. 20–23 oral denticles. Oral spines lacking. Zooids with suboral avicularium and single frontal avicularium. Suboral avicularium with mandibular part of rostrum right-angled, and with proximal uncinus. Rostrum of frontal avicularia diamond shaped, decurved, with narrow, distal channel for setiform mandible; avicularian crossbar simple. Ooecium with large endooecial panel vertical in peristome, flat, with marginal pustulation; labellum wide. Scattered surface nodules in older parts of colony.

Description. Colony encrusting, sheet-like, unilaminar, possibly becoming bi- or trilaminar with age through frontal budding, but not nodular.

Zooids irregularly hexagonal or barrel-shaped; discrete only near margin, delineated by line of calcification between adjacent rows of marginal areolae. Frontal wall initially convex, smooth, imperforate, with six or seven small, irregular areolae along each lateral margin. Orifice with anter broader than long; overall (including sinus) about as long as broad. Median sinus distinct, moderately wide and deep, U- or rounded-V-shaped, flanked by prominent, rounded condyles, bounded by small secondary condyles. Periphery of orifice distal to condyles beaded with 20–23 rounded denticles. Oral spines lacking. With age and increasing secondary calcification, zooidal boundaries obscured; frontal surface smooth, but irregular, with scattered coarse tubercles; secondary orifice with distinct, wide pseudosinus on one side or other, with up to five or six tuberculate prominences around proximal and lateral margins.

Most zooids have suboral avicularium, offset toward one side or other of midline; rostrum length equal to or somewhat greater than orifice width; rostral plane tilted toward distal end of zooid, almost perpendicular to frontal plane; rostrum with complete crossbar; anter semicircular, with smooth cryptocyst, and bearing slight uncinus; mandibular part asymmetrical but not curved, forming right triangle with narrow base along crossbar, side toward orifice at right angle to crossbar, other side longer; cryptocystal margin of rostral part irregular with coarse, rounded denticles. Most zooids having single frontal avicularium roughly in center of frontal shield, directed laterally or proximally. Rostrum up to twice as long as orifice width; ratio of average rostrum length to rostrum width = 2.9. Rostral poster triangular, with smooth crytocyst. Mandibular part long-triangular, tapering and quite narrow distally, forming groove for setiform mandible; often decurved; opesial opening long-triangular, opesial margin irregular, with several coarse, rounded denticles on each side.

Ooecia seen forming rapidly near colony margin, appearing in second row of fully formed zooids in from margin; completely immersed with age, visible only inside sunken peristome; window of non-calcified ectooecium exposing large, semicircular zone of endooecium on proximal face of ooecium; endooecial zone vertically oriented in peristome, smooth, flat, but with small pustules around margin; with unusually broad labellum along proximal margin of ooecium, beneath exposed endooecial area.

Remarks. This species resembles Rhynchozoon scimitar Dick & Grischenko, 2017 in having a deep, narrow oral sinus; in having a single elongate, diamond-shaped frontal avicularium per zooid; in lacking oral spines; and in having the ooecium with a large endooecial panel of similar texture. However, in R. setiavicularium n. sp., the oral sinus is shallower, broader, and varies from rounded-V-shaped to U-shaped; the suboral avicularium is shorter, and the mandibular portion, while asymmetrical, is not curved but forms a right triangle. In R. setiavicularium n. sp., both a suboral and a frontal avicularium can occur on the same zooid, whereas in R. scimitar , zooids have one or the other. The frontal avicularium, while similarly elongate and diamond-shaped in the two species, is narrower distally and forms a distinct channel for a setiform end of the mandible in R. setiavicularium n. sp.; furthermore, the frontal avicularian rostrum is often decurved in R. setiavicularium n. sp.; in R. scimitar , occasional frontal rostra are curved in the frontal direction, but never decurved. While the ooecia are similar in the two species, they are smaller in R. setiavicularium n. sp.; the endooecial panel seems proportionally larger; and the labellum is much wider.

Rhynchozoon haha Hayward, 1988 and R. splendens Hayward, 1988 have a similarly diamond-shaped frontal avicularium. The former differs from R. setiavicularium n. sp. in having a broad, shallow proximal orifical sinus; the latter differs in having smaller, less-elongate frontal avicularia, the ooecium with a much smaller ectooecial panel and narrow labellum, and the suboral avicularian rostrum with a distinct uncinus, or projection, on the proximal end.

Distribution. Co To Island is the only known locality.

Figures

FIGURE 16. Rhynchozoon setiavicularium n. sp., holotype, VNMN-0263. A. Young autozooids at colony margin. B. Older portion of colony, showing suboral avicularia with slight uncinus (arrowhead), and frontal avicularia with narrow rostrum and simple crossbar (arrow). C. Orifice. D. Immersed ooecia, with wide labellum (arrowhead) and flat, vertical exposed endooecial panel in peristome. E. Autozooids in frontodistal view, showing right-angled suboral avicularian mandibular rostra. F. Colony surface, showing decurved frontal avicularia and scattered nodules. Scale bars: A, 500 µm; B, F, 250 µm; C, 100 µm; D, E, 150 µm.