Plesiocleidochasma arcuatum Ramalho & Moraes, 2021

Ramalho, Laís V., Moraes, Fernando C., Salgado, Leonardo T., Bastos, Alex C. & Moura, Rodrigo L., 2021, Bryozoa from the reefs off the Amazon River mouth: checklist, thirteen new species, and notes on their ecology and distribution, Zootaxa 4950 (1), pp. 1-45 : 20-22

Plesiocleidochasma arcuatum Ramalho & Moraes n. sp.

( Fig. 10 )

Material examined. Holotype: MNRJBRY-1435: Brazil, Amapá state (Sta #3, 03°35.4267’N – 049°07.6028’W), 90 m, on rhodolith, 26 September 2014, collected by Fernando Moraes & Rodrigo Moura (NHo Cruzeiro do Sul ).

Etymology. From the Latin (arcuatus, -a, -um) meaning arched, referring to the shape of the avicularium.

Diagnosis. Plesiocleidochasma having orifice with a transverse elliptical or sometimes broader V-shaped sinus, strong and blunt condyles and 3–5 spines. Single adventitious avicularium placed on a tumid cystid; rostrum triangular with tip curving downwards. Ovicell bearing a small central umbo.

Description. Colony encrusting, multiserial, unilaminar ( Fig. 10A ). Autozooids polygonal, longer than wide [L 306–355–402 (SD 30, N 19); W 200–248– 319 µm (SD 25, N 20)], slightly inclined distally; frontal shield convex, smooth, with up to four small and rounded marginal areolar pores ( Fig. 10A–C ). Orifice cleithridiate [L 87–96–106 (SD 6, N 9); W 68–79– 87 µm (SD 8, N 10)] with horseshoe-shaped anter and transversely elliptical or sometimes broader V-shaped sinus; condyles strong and blunt, somewhat curved and oriented proximo-medially; 3–5 distal oral spines (two in ovicellate zooids) that disappear later in ontogeny ( Fig. 10B–D ). Orifice sometimes slightly obscured by a pair of umbones, developing on each side of the orifice, plus a third smaller umbo proximo-centrally placed below the orifice ( Fig. 10B, C ). Single adventitious avicularium [L 70–77– 84 µm (SD 10, N 2)], absent in some zooids, laterally oriented, placed on a tumid cystid at about zooidal half length, marginal, budded from an areolar pore ( Fig. 10D ); rostrum triangular, tip curving downwards (arched); palate with a triangular foramen and narrow crossbar, without columella ( Fig. 10D ). Ovicell globular [L 139–155–172 (SD 14, N 4);W 157–213– 253 µm (SD 35, N 5)], usually depressed; ento- and ectooecium calcified, frontal labellum distinct, incised obliquo-laterally on both sides, trapezoidal in outline ( Fig. 10E ); sometimes secondary calcification leaves visible only two circular pores at each side with a very thin horizontal slit between the pores ( Fig. 10C ); ooecial surface smooth, bearing a small central umbo ( Fig. 10A, C, E ).

Remarks. Two Plesiocleidochasma species have been recorded from Brazil: P. foliosum Winston, 2005 , from Rio de Janeiro state, and P. acuminata Ramalho et al., 2018 , from the Abrolhos Bank, Bahia state. The former species has orifice with smaller and rounded to arrow-shaped sinus, leaf-shaped frontal avicularia with four uncalcified windows on the palate, while the latter species has granular frontal shield, larger and pointed avicularia, and palate with four pores (three pores aligned transversely plus a fourth one above the median transverse pore line).

Plesiocleidochasma perspicum ( Hayward & Cook, 1983) from South Africa, is the most similar to the new species. They share the short oral umbones, the presence of a single avicularium on a tumid cystid and ovicell with distinct labellum and small median umbo. However, P. perspicum differs in having slender avicularium rostrum, a stout crossbar with a thickened and quadrate columella, plus an orifice with a distinctly V-shaped sinus. Plesiocleidochasma immersum ( Soule et al., 1991) from Thailand, also resembles the new species in having acute avicularium on a bulbous cystid and crossbar without columella, but it differs in having rounded and arcuated sinus, three oral spines, and an ovicell with horizontal slit placed frontally and above the ooecial aperture.

Figures

FIGURE 10. Plesiocleidochasma arcuatum Ramalho & Moraes n. sp. (Holotype MNRJBRY-1435).A. General view of a colony on an irregular substrate. B. Zooids showing orifice with spines and umbones (lateral to orifice and suboral). C. Some zooids showing oral distal spines, lateral umbones and one ovicell (asterisk). D. Detail of zooids showing the triangular avicularia. E. Part of the colony showing an ovicellate zooid (asterisk). Scale bars: A, 500 µm; B, 250 µm; C, E, 200 µm; D, 100 µm.