Rhynchozoon scimitar, 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 : 252-254

Rhynchozoon scimitar sp. nov.

( Figures 36 (e – f) and 40)

Etymology

The specific name is a noun in apposition referring to the scimitar-like rostrum of the suboral avicularian.

Material examined

Holotype: NSMT-Te 1205, dried specimen, SES site . Paratypes: NSMT-Te 1190 ( SES-47 ), upper colony (overgrowing Rhynchozoon maculosum ), bleached, on SEM stub; NSMT-Te 1207 ( SES-48 B), bleached, on SEM stub; NHMUK 2016.5 . 13.87 ( SES-48 A) (part of same colony as SES-48 B), unbleached, on SEM stub; NHMUK 2016.5.13.88-89, two dried fragments, SES site .

Measurements

AzL, 0.47 – 0.62 (0.519 ± 0.044); AzW, 0.29 – 0.45 (0.365 ± 0.047) (n = 15, 1). OrL, 0.097 – 0.132 (0.113 ± 0.009); OrW, 0.095 – 0.110 (0.104 ± 0.005) (n = 15, 2). OvL, 0.19 – 0.23 (0.213 ± 0.014); OvW, 0.22 – 0.26 (0.236 ± 0.014) (n = 15, 1). Largest colony observed 22 × 14 mm.

Description

Colony ( Figure 40 (a)) forming an encrusting sheet; unilaminar, becoming bi- or trilaminar in older parts of colony due to self-overgrowth or frontal budding; with faint yellowish tinge; exposed entooecial area on ovicell opaque white; embryos in dried specimens opaque light yellow. Zooids at colony margin ( Figure 40 (b)) irregularly hexagonal, delineated by suture line between opposing columns of areolae; with six to nine small, circular or oval areolae evident along each lateral margin. Frontal wall slightly to moderately convex, without pseudopores, surface somewhat irregular, texture smooth. With moderate calcification ( Figure 40 (e)) frontal surface becomes flatter, with zooidal boundaries indicated by areolae and suture line. In the oldest, heavily calcified zooids ( Figure 40 (f)), zooidal boundaries indistinct; frontal wall irregular, with scattered small tubercles. Primary orifice (including sinus) on average longer than broad ( Figure 40 (b, c)); anter D-shaped; proximal sinus deep, narrow, U-shaped; condyles small, rounded, directed frontodistally. Margin of anter beaded with 19 – 23 denticles (n = 8, 2). Oral spines lacking. Secondary orifice ( Figure 40 (e, f)) with distinct, asymmetrical proximal pseudosinus; sometimes a major and minor sinus occur. Around peristomial margin in older zooids are up to six short, blunt, cylindrical or conical processes ( Figure 40 (f)), arising from secondary calcification from proximal end of next-distal zooid, from one or both lateral zooids, and around proximal margin of orifice. Avicularia lacking in some zooids, but if present, only one avicularium per zooid, either lateral-suboral or frontal. Lateral suboral avicularium ( Figure 40 (b, d)) with a semicircular chamber immediately proximolateral to orifice on one side or other, arising from one or two areole. Rostrum large, length up to 2 times orifice width, raised distally at about 30° to frontal plane, directed distolaterally, strongly hooked at end; crossbar complete; mandibular part of rostrum curved, scimitar-like; rostral opening irregular due to coarse denticles around margin of opening; mandible ( Figure 40 (d)) long-triangular, weakly scimitar-shaped, sharply hooked in opposite direction to rostral hook, tip acute. Suboral avicularium not immersed in peristome, but remains conspicuous at peristomial margin. Frontal avicularia ( Figure 40 (e)) are situated along lateral zooidal margin on one side or other. Rostrum diamond shaped, parallel to frontal plane or slightly raised distally, directed laterally or distolaterally, about same length and width as rostrum of suboral avicularium but not curved laterally; crossbar complete; mandible slightly to markedly long-triangular, with a slight hook at the end. In heavily calcified zooids, the frontal avicularia can be nearly completely covered by secondary calcification ( Figure 40 (f)). Ovicell initially subimmersed ( Figure 40 (a, e)), later becoming endozooidal ( Figure 40 (f)). Ooecium wider than long; covered with secondary calcification contributed by one to three surrounding zooids and often bearing one or two small tubercles. Proximal face of ooecium with large zone of membranous (non-calcified) ectooecium of variable shape ( Figure 40 (e, f)), which in cleaned specimens becomes a window of exposed entooecium; exposed entooecium weakly sculptured (pustulated or marginally ribbed); labellum narrow, usually (but not always) complete. Zooids interconnect by uniporous septula in distolateral and transverse walls; septula lead to basal pore chambers bearing stalactite-like denticles ( Figure 40 (c)). Ancestrula not observed.

Remarks

Rhynchozoon scimitar is distinguished by the deep, narrow orificial sinus; zooids having either a moderately large, scimitar-like lateral suboral avicularium that is not submerged in the orifice, or a similarly sized diamond-shaped frontal avicularium, but not both; the distinctly irregular margin of the opening of the avicularian rostrum; the large, weakly sculptured entooecial area on the ovicell that is typically opaque white; and the lack of spines. Few other Rhynchozoon species have a similarly deep, narrow orificial sinus. The orifice shape in R. scimitar is much like that in R. solitarium Tilbrook, 2006 , described from the Solomon Islands; orifice size is identical in the latter, and the ovicell has a similarly large, pustulated area of exposed entooecium. Rhynchozoon solitarium differs in having hypertrophied suboral avicularia with a spatulate rostrum, and frontal avicularia uncommonly present, with the rostrum broadly rounded proximally rather than diamond shaped. Rhynchozoon ardeolum Ryland and Hayward, 1992 , described from the Great Barrier Reef, Australia, also has a narrow orificial sinus, but this is shallower than in R. scimitar ; R. ardeolum also has larger condyles and a hypertrophied lateral-suboral avicularium that is very broad proximally, with a narrow mandible.

One specimen (a fragment measuring 8 × 3 mm) entirely lacked avicularia and ovicells but was identifiable from the orifice shape. By light microscopy we observed between zooids what appeared to be a raised line of calcification, which upon bleaching and SEM examination ( Figure 36 (e, f)) proved to be a thin layer of gymnocystal calcification secreted around a filamentous epibiont extending throughout the colony in the interzooidal grooves or between zooids (arrowheads, Figure 36 (e, f)), and occasionally emerging to the surface through circular openings (arrows, Figure 36 (e, f)). This epibiont may be same that affected Rhynchozoon maculosum (see Remarks for that species, and Figure 36 (a – d)). As with R. maculosum , the epibiont appears to have inhibited avicularium production in R. scimitar . This apparent inhibition raises the question whether the epibiont somehow protects the bryozoan, thus obviating the need of the latter to produce avicularia; alternatively, the epibiont might inhibit avicularium production to avoid damage to itself.

Occurrence

We found five colonies at the SES site, the only known locality.

Figures

Figure 36. Interzooidal epibiont in two Rhynchozoon species. (a–d) Rhynchozoon maculosum, NSMT- Te 1187 (paratype): (a, b) calcified tubes (arrowheads) secreted by bryozoan around epibiont, with occasional openings (arrows); (c) enlargement of calcified tube (asterisk), with broken area showing tubular structure of epibiont (arrowhead) in cross section;(d) epibiont emerging from between vertical walls of two newly forming zooids (asterisk); annular structure of emergent epibiont (arrowhead) suggests it may be a hydroid; note that epibiont appears to have suppressed avicularium formation.(e, f) Rhynchozoon scimitar, NSMT-Te 1206 (paratype): calcified tubes (arrowheads) secreted by bryozoan around epibiont, with epibiont opening indicated by arrow; note that epibiont appears to have suppressed avicularium formation. Panels are scanning electron microscopic images of bleached (a, b, e, f) or dried but non-bleached (c, d) material. Scale bars: a, b, d–f = 300 µm; c = 50 µm.

Figure 40. Rhynchozoon scimitar sp. nov., NSMT-Te 1207 (paratype) (a, b, d–f), NSMT-Te 1206 (paratype) (c): (a) colony view; (b) marginal autozooids, showing orifice shape and aspect of suboral avicularian chamber; (c) orifice; note denticles associated with basal pore chambers; (d) avicularian mandibles; (e) ovicelled autozooids, with diamond-shaped frontal avicularia and scimitar-like suboral avicularium (top centre); (f) old part of colony, with secondary calcification covering frontal avicularia. Panels are scanning electron microscopic images of bleached specimens, except for (d), which shows the specimen before bleaching. Scale bars: a = 1.0 mm; b, e, f = 300 µm; c, d = 100 µm.