Rhynchozoon lunifrons Dick & Grischenko, 2017

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 : 233-235

Rhynchozoon lunifrons Dick & Grischenko, 2017

( Fig. 15 )

Rhynchozoon lunifrons Dick & Grischenko, 2017, p. 245 , figs 37, 38.

Material examined. VNMN-0262 (CT-1), on SEM stub.

Measurements. AzL, 0.40–0.62 (0.524 ± 0.073); AzW, 0.28–0.34 (0.309 ± 0.021); OrL, 0.10–0.11 (0.105 ± 0.005); OrW, 0.10–0.12 (0.106 ± 0.004); OvL, 0.16–0.19 (0.177 ± 0.010); OvW, 0.15–0.21 (0.189 ± 0.017); frontal AvRL, 0.076 –0.107 (0.092 ± 0.011); frontal AvRW, 0.031 –0.055 (0.050 ± 0.006) (n = 15, 1).

Description. Colony encrusting, sheet-like, unilaminar at margin, multilaminar in older parts of colony due to frontal budding, but not becoming nodular.

Zooids in marginal zone distinct; spindle-shaped or oval, delineated by groove; in older parts of colony, zooidal boundaries obscured; zooidal orientation haphazard in zones of frontal budding. Frontal shield in young zooids markedly convex, with three to five small areolae along each lateral margin; imperforate, texture smooth but surface slightly irregular. Orifice in marginal zooids parallel to frontal plane or tilted down distally, flanked by lateral flanges and sometimes low proximal projection. Primary orifice about as broad as long; with distinct, shallow, rather narrow, U-shaped median sinus; sinus flanked by sharp lateral projections that merge with fine diagonal ridges leading proximolaterally; low, rounded condyle between sinus and orificial rim on each side. Periphery of orifice distal to condyles beaded with 14–17 blunt denticles. Primary orifice sunken in low, D-shaped peristome; sharp rim around orifice distal to condyles. Oral spines lacking.

Zooids in large parts of colony lack suboral or other avicularia. When present, suboral avicularium with rostrum longer than width of orifice, offset to one side or other, directed frontolaterally and also tilted toward distal end of zooid; crossbar complete; rostrum semicircular proximal to crossbar, long-triangular distal to crossbar, blunt or acute, slightly hooked at end, rostral plane straight or convex. Frontal avicularia developing primarily in secondary layers of colony; same size or somewhat smaller than suboral avicularia but similar in shape; usually along lateral or proximal margin, directed toward margin; usually one per zooid; chamber often raised, rostrum tilted frontally at angle of up to 45 degrees from frontal plane.

Ooecium hyperstomial or sub-immersed, broader than long in frontal view, with extensive circular or semicircular window of non-calcified ectooecium, manifested as smooth endooecial layer when soft tissue removed; with or without narrow labellum along proximal margin of ooecium. Secondary orifice in older, haphazardly arranged zooids surrounded by up to four cylindrical projections that may arise from any of several surrounding zooids contributing to secondary orifice.

Remarks. Features that identify this specimen as R. lunifrons include zooid and ooecium sizes similar to those in the original description ( Dick & Grischenko 2017); the characteristic ooecium with a large, moonlike zone of smooth, exposed endooecium; the nearly circular orifice having a distinct, rather shallow U-shaped sinus; lack of oral spines; lateral and median processes flanking the orifice in both young and old zooids; relatively few, small areolae in marginal zooids; a large suboral avicularium oriented at a high angle to the frontal plane; frontal avicularia similar in shape to the suboral avicularia; and the development of cylindrical processes around the secondary orifice in older zooids, with these processes arising from two or more surrounding zooids. There are also differences with the specimens from Okinawa, the type locality and the only previously known locality for this species. In the Okinawa population, the marginal zone with well-delineated zooids having a relatively smooth frontal wall is much narrower, with zooids rapidly become coarsely pustulose, whereas in the Co To specimen, even the oldest zooids in the colony remain comparatively smooth.At Okinawa, the proximal orificial sinus is wider, without the sharp flanking projections leading to diagonal ridges, and the surrounding shallow peristome is not conspicuously D-shaped. Suboral avicularia are uncommon in the Co To specimen, and the highly raised avicularian chambers developing proximolateral to the orifice in marginal zooids were not seen. We conclude that the similarities outweigh the differences and that the two populations are conspecific, although examination of more material from Co To could alter this conclusion. Some of the differences observed between the two populations may be ecophenotypic; the specimens from Okinawa came from the rocky-intertidal habitat on a shore exposed to strong wave action, whereas the Co To specimen probably lived subtidally in a cavity and was not exposed to strong waves.

Distribution. Previously known only from west-central Okinawa ( Dick & Grischenko 2017).

Figures

FIGURE 15. Rhynchozoon lunifrons Dick & Grischenko, 2017, VNMN-0262. A. Colony. B. Autozooids near colony margin. C. Orifice shape. D. Autozooids near colony margin, showing suboral avicularia. E. Colony, showing numerous small frontal avicularia. F. Autozooids with ooecia. Scale bars: A, 1 mm; B, E, 500 µm; C, 100 µm; D, F, 250 µm.