Rhynchozoon latiavicularium Dick & Ngai & Doan 2020, n. sp.

Rhynchozoon latiavicularium n. sp.

(Fig. 17)

urn:lsid:zoobank.org:act: D543ACD0-6CEA-46F5-8624-F9FD808E0A37

Etymology. The specific name combines the Latin word lata (wide) with avicularium, referring to the relatively wide frontal avicularia.

Material examined. Holotype, VNMN-0264 (CT-14); paratypes, VNMN-0265 (CT-4), VNMN-0266 (CT-25), VNMN-0267 (CT-34), all on SEM stubs.

Measurements. AzL, 0.45–0.63 (0.536 ± 0.062); AzW, 0.30–0.54 (0.399 ± 0.056); OrL, 0.10–0.14 (0.121 ± 0.010); OrW, 0.10–0.12 (0.106 ± 0.006) (n = 15, 2). OvL, 0.14–0.21 (0.173 ± 0.023); OvW, 0.19–0.23 (0.208 ± 0.013) (n = 15, 1). Frontal AvRL, 0.14–0.18 (0.157 ± 0.014); frontal AvRW, 0.066 –0.102 (0.080 ± 0.008) (n = 30, 2).

Diagnosis. Suboral sinus U-shaped, moderately shallow, narrow, flanked by straight orificial margin and broad, semicircular condyles. Oral denticles indistinct. Oral spines lacking. Zooids with suboral avicularium and single frontal avicularium. Suboral avicularium relatively broad; uncinus scarcely evident. Frontal avicularia relatively short, broad; weakly diamond shaped or semicircular proximally; scarcely decurved, with triangular mandibular portion lacking setiform end; paired crossbar elements V-shaped. Ooecium with transversely oval endooecial panel in peristome; labellum present but not markedly broad. Cylindrical projections around secondary orifice in older parts of colony.

Description. Colony encrusting, sheet-like, initially unilaminar, developing bilaminar zones due to frontal budding, but not nodular.

Zooids irregularly hexagonal but boundaries distinct only at colony margin. Frontal wall at margin smooth, convex, imperforate, with 7–10 small, circular areolae along each lateral margin; with thickening frontal calcification, areolar openings become larger and fewer, five or six along each lateral margin. Primary orifice with anter D-shaped or nearly circular; median sinus variable, ranging from narrowly U-shaped to more broadly rounded-V-shaped, flanked on each side by small shelf, with rounded condyle filling angle between shelf and lateral orifical margin; condyles not directed distally. Oral spines lacking. With increased calcification, scattered low, rounded tubercles appearing on frontal wall, though surface texture remaining smooth; from two to seven short, blunt cylindrical processes forming around secondary orifice as outgrowths from the frontal walls of surrounding zooids; and deep, asymmetrical secondary orifical sinus forming to one side of midline or other.

Zooids with suboral avicularium, usually 20–25% longer than orifice width; poster broadly semicircular, with smooth cryptocyst. Rostrum directed laterally, with rostral plane also tilted slightly toward distal end of zooid; mandibular portion asymmetrical, forming right triangle, or symmetrical, forming short-equilateral triangle; opesial margin of mandibular portion irregular with one or two coarse, nodular denticles on each side. Hinge element on each side of rostrum narrowly V-shaped, apex pointing medially, with or without complete crossbar extending between opposing elements. Suboral avicularium occasionally larger, twice as long as orifice width, with rostral plane convex in frontal direction. Most zooids have single frontal avicularium located in proximal half of frontal shield, directed distolaterally, laterally, proximolaterally, or proximally; frontal avicularia similar in size and shape to suboral avicularia; ratio of average rostrum length to rostrum width = 1.9; proximal end of rostrum usually acute (rostrum diamond-shaped), but sometimes rounded as in suboral avicularia; mandibular portion usually symmetrical and long-triangular, sometimes asymmetrical and comprising right triangle. Opesial margin of mandibular portion irregular, with up to six coarse, rounded denticles on each side. Rostrum slightly decurved at end, indicating mandible is probably hooked distally.

Ooecium immersed with age; non-calcified ectooecium leaving circular or oval zone of exposed endooecium vertically oriented in sunken peristome, with broad labellum along proximal ooecial margin; secondary calcification atop ooecium typically giving rise to one or two coarse tubercles.

Remarks. Rhynchozoon latiavicularium n. sp. is quite similar to R. setiavicularium n. sp. above, but differs in the following characters (character state of R. setiavicularium n. sp. in parentheses). Rhynchozoon latiavicularium n. sp. has larger zooids, average ZL × ZW = 0.54 × 0.40 mm (0.46 × 0.32 mm); the orifical condyles are not discrete, filling angle between lateral and proximal margins of orifice (discrete, distally directed knobs); the frontal avicularian rostrum is wider relative to length, with an average L/W ratio ≈ 2.0, with a triangular mandible not distinctly nar- row distally (L/W ratio ≈ 3.0, tapering and narrow distally); the frontal avicularian rostrum is not markedly decurved (rostrum often markedly decurved); the poster in the frontal avicularia varies from acute and triangular to rounded and semicircular (sharply acute and triangular); the avicularian hinge element comprising each side of the crossbar is sharply V-shaped (hinge elements uniform in width, not V-shaped); the endooecial panel on the proximal side of the ooecium is smaller, and transversely oval or semicircular in outline (proportionally larger, semicircular).

Rhynchozoon setiavicularium n. sp. and R. latiavicularium n. sp. are probably closely related, as the differences between them are slight. Dick & Mawatari (2005) documented a parallel case for two similar Rhynchozoon species co-occurring at a locality in southeastern Alaska. In that study, a mitochondrial DNA (mtDNA) analysis detected two mitochondrial clades, and a morphological analysis showed slight but consistent qualitative and quantitative, clade-specific differences. One species had already been described and the other was eventually described as new (Dick et al. 2005).

Distribution. Co To Island is the only known locality.