Rhynchozoon lunifrons Dick & Grischenko 2016, sp. nov.

Rhynchozoon lunifrons sp. nov.

(Figures 37 and 38)

The specific name is a noun in apposition combining the Latin luna (moon) and frons (forehead, brow), referring to the large zone of exposed entooecium on the ovicell.

Holotype: NSMT-Te 1195, colony on worn coral fragment, SES site. Paratypes: NSMT-Te 1196 (SES- 44), bleached, on SEM stub; NSMT-Te 1197 (SES- 8), bleached, on SEM stub; NSMT-Te 1198 (SES- 46), precocious form, bleached, on SEM stub; NSMT-Te 1199, dried colony, SES site; NHMUK 2016.5.13.78-80, three dried fragments, SES site.

Common form. AzL, 0.40 – 0.75 (0.540 ± 0.077); AzW, 0.28 – 0.44 (0.339 ± 0.044) (n = 30, 2). OrL, 0.112 – 0.138 (0.125 ± 0.008); OrW, 0.116 – 0.144 (0.128 ± 0.008) (n = 26, 2). OvL, 0.160 – 0.213 (0.185 ± 0.018); OvW, 0.197 – 0.249 (0.216 ± 0.015) (n = 11, 2). Largest colony observed 15 × 13 mm.

Precocious form. AzL, 0.36 – 0.51 (0.432 ± 0.044); AzW, 0.25 – 0.37 (0.307 ± 0.034) (n = 15, 1). OrL, 0.101 – 0.122 (0.110 ± 0.007); OrW, 0.102 – 0.131 (0.117 ± 0.009) (n = 10, 1). OvL,

0.151 – 0.200 (0.174 ± 0.020); OvW, 0.185 – 0.223 (0.200 ± 0.016) (n = 5, 1). One colony observed, 3.1 × 4.1 mm.

Common form (Figure 37). Colony (Figure 37 (a)) forming an encrusting sheet, mostly unilaminar, with occasional small areas of frontally budded zooids comprising a second layer; dried specimens white, with orange embryos. Marginal zooids (Figure 37 (b – d)) distinct, delineated by a groove; oval, irregularly hexagonal or barrel-shaped in outline. Frontal wall of marginal zooids highly convex, inflated, smooth (Figure 37 (b)) or rugose (Figure 37 (c)), without pseudopores; four to six small, circular areolae along each lateral margin. With increasing calcification (Figure 37 (b)), zooidal boundaries become less distinct and areolar openings larger and more irregular, frontal surface develops coarse tubercles, and three to five tapering cylindrical processes develop around proximal peristomial margin. Typically there are three peristomial processes (Figure 37 (b)), one in midline and two others paired laterally. Primary orifice (Figure 37 (d)) variable within colony; on average about as broad as long; suboral sinus broadly arcuate between rounded, proximomedially sloping condyles, variable in depth. Orifice beaded with 12 – 15 denticles around periphery distal to condyles (Figure 37 (c, d)). Most but not all zooids have a suboral avicularium; chamber evident only in marginal zooids (Figure 37 (b – d)), slightly larger than area of developing peristome, arising from one or two marginal areolae; abutting peristomial rim on one side or other of midline. Rostrum raised at high angle to frontal plane, pointing frontolaterally; with age, slightly hooked distally; crossbar complete. Mandible long-triangular, lanceolate, decurved towards end, not hooked. With increasing calcification, suboral avicularian rostrum becomes immersed in peristome and not visible in frontal view (Figure 37 (e, f)). Frontal avicularia variably present; some colonies lack them, but in other colonies, heavily calcified zooids frequently bear a single frontal avicularium along proximal or lateral margin (Figure 37 (e, f)); rostrum long-triangular, about same size as that of suboral avicularia or somewhat smaller, raised from frontal plane, directed perpendicular to zooidal margin; crossbar complete. Oral spines lacking. Ovicell initially subimmersed (Figure 37 (e)) but with age becoming endozooidal (Figure 37 (f)). Entire proximal face of ooecium comprises a large zone of membranous (non-calcified) ectooecium that, in cleaned specimens, is seen as exposed entooecium; labellum usually lacking; if present, quite narrow. In colony NSMT-Te 1197, some maternal zooids have initiated the formation of two ooecia (Figure 37 (f)), one endozooidal in the usual distal position, the other hyperstomial, lying on colony surface at lateral edge of secondary orifice (Figure 37 (f), arrowheads). Vertical walls (Figure 37 (d)) low; pore chambers small, at base of wall, each bearing a uniporous septulum; two or three septula in transverse wall, and four or five in each distolateral wall. Ancestrula not observed.

Precocious form (Figure 38). Small, irregularly discoid colony (Figure 38 (a)); precocious, with fully developed ovicells starting in second daughter-zooid generation from ancestrula. Morphology as described above, except zooids smaller; young marginal zooids often bear one or two ephemeral distal oral spines (Figure 38 (b)); frontal avicularia lacking; suboral avicularia rare, with only one zooid bearing a small, broken chamber (Figure 38 (c), arrowhead). Mature, ovicelled zooids (Figure 38 (d)) as in the common form above.

Young zooids in the one or two generations inside the marginal row in Rhynchozoon lunifrons resemble zooids in R. tristelidion Tilbrook, 2006, in having a uniformly dimpled frontal surface and three distally directed conical processes arising from the peristomial margin; the size of the chamber of the suboral avicularium and the shape and direction of the rostrum are also similar between the two species. Tilbrook (2006) described R. tristelidion from the Solomon Islands (presently the only known locality for that species) on the basis of four colonies, all of which were young, lacking secondary frontal calcification and key diagnostic characters such as frontal avicularia (if they typically occur) and ovicells. Rhynchozoon tristelidion may be similar in growth form to R. zealandicum Gordon, 2009, another species with zooids having three peristomial processes. In R. zealandicum, colonies produce an extensive, unilaminar basal layer in which zooids lack secondary calcification, ovicells or frontal avicularia; mature zooids showing these characters develop in a zone of frontal budding in the centre of the colony (Gordon 2009, p. 49, fig. 22). In R. lunifrons, zooids inside the growing edge rapidly accumulate secondary calcification; the frontal wall becomes more heavily tuberculate, and the peristomial processes become thicker, rougher, and more digitiform than conical. The orifice in our specimens is about as long (including the sinus) as broad (broader than long in R. tristelidion), and the orificial sinus varies in depth, from broad and shallow as in R. tristelidion to narrower and deeper. While the different pattern of astogenetic change suggests that R. lunifrons is a different species from R. tristelidion, the possibility remains that the former represents mature colonies of the latter; resolving this question will require examination of mature colonies of R. tristelidion from the Solomon Islands.

We found one colony of what we term the ‘ precocious form ’ of this species, at the same locality as the common form. Zooid and ovicell morphology are quite similar; the precocious form differs from the common form in having smaller values for all characters measured, and in producing ephemeral oral spines on marginal zooids. Ostrovsky (1998) has described the development of ovicells in small colonies consisting of a few zooids among cribrimorph species that typically produce larger colonies, and so the precocious form might simply represent an early astogenetic stage of the common form, with marginal zooids in larger colonies ceasing to produce oral spines, and older zooids in the colony interior beginning to produce frontal avicularia. More material is necessary to resolve this question.

We found 11 colonies at the SES site, the only known locality.