Rhynchozoon ryukyuense, Dick & Grischenko, 2016

Dick, Matthew H. & Grischenko, Andrei V., 2016, Rocky-intertidal cheilostome bryozoans from the vicinity of the Sesoko Biological Station, west-central Okinawa, Japan, Journal of Natural History 51, pp. 141-266 : 249-251

Rhynchozoon ryukyuense sp. nov.

( Figure 39 )

Etymology

The specific name is a Latinised adjective from the Japanese Ryûkyû, the name of the island group that includes Okinawa.

Material examined

Holotype: NSMT-Te 1200, dried colony, SES site . Paratypes: NSMT-Te 1201 ( SES-16 ), bleached, on SEM stub; NSMT-Te 1202 ( SES-49 ), bleached, on SEM stub; NSMT-Te 1203 (REEF-8), bleached, on SEM stub; NSMT-TE 1204 , three dried specimens, SES site; NHMUK 2016.5 . 13.81 ( SES-45 ), bleached, on SEM stub; NHMUK 2016.5.13.82-86, five dried fragments, SES site .

Measurements

AzL, 0.38 – 0.55 (0.475 ± 0.038); AzW, 0.26 – 0.42 (0.336 ± 0.039) (n = 30, 2). OrL, 0.093 – 0.108 (0.101 ± 0.004); OrW, 0.097 – 0.114 (0.108 ± 0.005) (n = 9, 4). OvL, 0.17 – 0.22 (0.191 ± 0.016); OvW, 0.21 – 0.24 (0.220 ± 0.010) (n = 7, 3). Largest colony observed 6.5 × 5.0 mm.

Description

Colony ( Figure 39 (a)) small, roughly circular, forming a mostly unilaminar, encrusting sheet; greyish off- white. Marginal zooids ( Figure 39 (b – d)) irregularly hexagonal or barrel-shaped in outline, delineated by a shallow groove and suture line with flanking areolae ( Figure 39 (d)); zooidal boundaries rapidly becoming indistinct with age, delineated primarily by areolae. Frontal wall moderately convex, lacking pseudopores, initially smooth or minutely pitted, with a few scattered tubercles ( Figure 39 (b – d)); young zooids at colony margin with six to 10 small areolae evident along each lateral margin. Frontal wall of heavily calcified zooids ( Figure 39 (e, f)) mildly rugose, with or without scattered tubercles; peristomial margin with up to six blunt, tapering processes; secondary orifice cormidial, with processes formed on proximal wall of next-distal zooid, from calcification from lateral zooids, and proximal to orifice. Primary orifice (including sinus) slightly broader than long ( Figure 39 (d)); proximal sinus deep, broad, rounded between blunt condyles; each condyle separated by a groove from edge of sinus. Margin of primary orifice beaded, usually with 18 – 22 (n = 13, 3) denticles, although some zooids in all colonies have fewer (13 – 17, n = 3). In mature zooids, primary orifice deeply immersed in peristome. Oral spines lacking. Most zooids have a suboral avicularium ( Figure 39 (b – d)), with a large, circular or oval chamber evident in marginal zooids on one side of midline or other; chamber often bearing a tubercle. Rostrum large, raised at a moderate angle to frontal surface and strongly tilted laterally towards distal end of zooid; with a conspicuous, rounded process on inner proximal margin. Rostrum hooked distally; crossbar complete; mandible long-triangular. Young zooids have a deep sinus in secondary orifice between proximal end of rostrum and opposing lateral wall, which often bears a conical process. With age, suboral avicularium becomes deeply immersed in peristome, often not evident in frontal view except for proximal rostral process projecting into peristome. Scattered zooids in some colonies have a hypertrophied suboral avicularium ( Figure 39 (e, f)), with a greatly enlarged rostrum lying outside the peristome, parallel to frontal plane or raised distally, with a scoop-like distal end; broad distal margin hooked and often coarsely serrate; proximal rostral process conspicuous; crossbar complete. Most zooids in mature colonies have a single frontal avicularium ( Figure 39 (e, f)); rostrum diamond-shaped, often raised distally, sometimes slightly hooked, crossbar complete; mandible equilateral or somewhat longer than broad, pointing in any direction. Ovicell ( Figure 39 (f)) endozooidal. Ooecium broader than long, covered by frontal calcification from one or more surrounding zooids, surface irregular, often bearing one or more tubercles. Proximal face of ooecium with transversely oval zone of membranous (non-calcified) ectooecium that, in cleaned specimens, becomes a window exposing the entooecium; labellum narrow, usually complete. Zooids interconnect by uniporous septula, each leading to basal pore chamber ( Figure 39 (c, d)); usually two or three septula in transverse wall and three or four in each distolateral wall. Most pore chambers bear sharp, stalactitelike denticles projecting from roof of chamber, up to six per chamber ( Figure 39 (d)). Ancestrula not observed.

Remarks

With diamond-shaped frontal avicularia and a well-developed lateral-suboral avicularium having a prominent proximal process and a hooked rostrum, Rhynchozoon ryukyuense greatly resembles Rhynchozoon splendens Hayward, 1988 , which has been reported from Mauritius ( Hayward 1988), Vanuatu ( Tilbrook et al. 2001), the Great Barrier Reef ( Ryland and Hayward 1992) and Hawaii ( Dick et al. 2006), and (as R. rostratum ) from Indonesia ( Winston and Heimberg 1986) and the Philippines (Scholz 1991). Measurements of zooidal characters are not markedly different among these populations and R. ryukyuense . The two species differ primarily in the occurrence of hypertrophied suboral avicularia, which have not been reported in any population of R. splendens . Although the sharp, stalactite-like denticles on the roof of pore chambers, as seen in R. ryukyuense , have not been mentioned in previous reports of R. splendens , this character may simply have been overlooked, because Scholz (1991, p. 399, pl. 18, fig. 5); illustrated porechamber denticles in Philippine material that Tilbrook et al. (2001) later identified as R. splendens . Indeed, pore-chamber denticles may be widespread in Rhynchozoon , as R. scimitar (see the next species description) also has them. Finally, several previous reports have indicated that zooids in R. splendens can bear one or two frontal avicularia, whereas we did not observe more than one per zooid in R. ryukyuense . It is noteworthy that among three mature R. ryukyuense colonies with ovicells, hypertrophied suboral avicularia were abundant in one, rare (one only) in another and lacking in the third, which we would probably have identified as R. splendens had it been the only specimen examined.

Occurrence

We found a total of 18 colonies, at the SES and REEF sites, the only known localities.

Figures

Figure 39. Rhynchozoon ryukyuense sp. nov., NSMT-Te 1202 (a–c), NHMUK 2016.5.13.81 (d, e), NSMT- Te 1201 (f), all paratype specimens: (a) colony view; (b) marginal autozooids, showing aspect of suboral avicularian chamber; (c) marginal autozooids, showing shape and orientation of avicularian rostrum; (d) marginal autozooids, showing orifice shape; note denticles associated with basal pore chambers; (e) autozooids, with diamond-shaped frontal avicularia and hypertrophied suboral avicularia; (f) ovicelled autozooids. All panels are scanning electron microscopic images of bleached specimens. Scale bars: a = 1.0 mm; b–f = 300 µm.