Plesiocleidochasma porcellanum ( Busk, 1860 ) Busk, 1860

Berning, Björn, 2012, Taxonomic notes on some Cheilostomata (Bryozoa) from Madeira, Zootaxa 3236, pp. 36-54 : 48-50

Plesiocleidochasma porcellanum ( Busk, 1860) n. comb.

( Figs 23–28 , Table 5)

Lepralia porcellana Busk, 1860: 283 , pl. 31, fig. 3; Norman 1909: 305, pl. 40, figs 1, 2.

Lepralia cleidostoma Smitt, 1873 (part?): Waters 1899: 10, pl. 3, fig. 16;?not pl. 3, fig. 17.

Cleidochasma porcellanum (part): Cook 1964: 11, text-fig. 4D; not pl. 1, fig. 4; pl. 2, figs 1, 2; text-fig. 4A–C, E.

Material examined. Lectotype: NHMUK 1899.7.1.1726, chosen by Cook (1964, p. 11), Madeira, on bivalve shell (no further information provided as to the exact location or depth).

Description. Colony encrusting, unilaminar, multiserial. Zooids hexagonal to quadrangular, broadest at about mid-distance, separated by shallow grooves. Vertical walls with 1–2 large communication pores per neighbouring zooid, few distal pore chambers present. Frontal shield only very slightly convex to flat, with orifice proximolaterally encircled by a broad bulge that may later be levelled owing to extensive secondary calcification; surface irregularly covered by granular nodules that are surrounded by smooth calcification, perforated only by a single large round and rimmed areolar pore near each lateral corner. Primary orifice cleithridiate, distinctly longer than wide, with the poster comprising four-fifths of a full circle and the anter about half of a transverse ellipse, the lower distolateral orifice margin with a smooth broad shelf on which the operculum rests; condyles strong, short, blunt, somewhat curved, directed proximomedially, demarcating proximal fourth of total orifice length; distal autozooidal margin initially with 3, rarely 4, oral spines (2 remaining in ovicellate zooids) that are lost and covered by secondary calcification during ontogeny.

Ooecium formed by distal autozooid, hyperstomial, globular but somewhat depressed, broader than long, endo- and ectooecium both calcified, ectooecium delicately striated but quickly covered and disguised by thick secondary calcification of distal zooid early in its formation, a short but broad proximal area of ectooecium containing a small median window of variable shape remaining exposed; proximal ooecial margin concave, paralleling distal orificial rim, lateral incisions absent or only very faintly developed, aperture depressed but broad and opening above primary orifice, not closed by operculum.

Avicularia adventitious, monomorphic, usually single or occasionally absent, situated proximolateral to orifice at about mid-point, directed laterally; rostrum elongate-triangular, distal tip slightly downcurved, distal uncalcified area semicircular to triangular, distolaterally demarcated by calcified shelves with a median groove, proximal area D-shaped; crossbar complete, with distal hemispherical ligula.

Ancestrula not observed.

ZL ZW OL OanW OpoW OvL OvW AL AW mean 399 348 149 71 113 185 234 87 50 SD 27 43 7 6 5 16 17 4 4 min. 356 282 135 62 104 161 200 78 45 max. 441 451 160 80 121 208 253 92 58 n 20 20 20 20 20 9 9 20 20

Remarks. Plesiocleidochasma porcellanum has been recorded from almost all tropical and subtropical regions in the world (see Cook 1964, and references therein). As already pointed out by Hayward & Cook (1983: 75), and in the light of recent taxonomic revisions of other cheilostome bryozoan species with an allegedly circumglobal distribution (e.g. Tilbrook 1999; Harmelin 2006; Harmelin et al. 2011), it actually represents a species complex. Although, like many other Plesiocleidochasma species, P. porcellanum is a shallow-water form (see below) and therefore likely to be rafted on natural and/or anthropogenic substrata, comparison with specimens from other regions reveal a number of differences that are today considered as species specific (see the figures in, for example, Cook 1964: fig. 4; Winston 1984: fig. 56). Accordingly, all records of P. porcellanum from regions other than the Madeiran archipelago should be regarded with doubt and need to be revised.

There may also be other closely related species around Madeira. Waters (1899) reported considerable variation in orificial morphology in the species he referred to as Lepralia cleidostoma Smitt, 1873 [see Winston (2005: 87, figs 244–246) for an account of this species], which Norman (1909) included in the synonymy of Lepralia porcellana . Although there is a slightly greater variation in anter width (see Table 5), orifice size and shape are generally fairly stable in the type of P. porcellanum . Thus, while Waters' (1899) figure 16 certainly depicts an operculum of P. porcellanum , his figure 17 may show a distinct species.

Whereas no depth was indicated by Busk (1860) for the shell encrusted by P. porcellanum, Norman (1909: 305) reported its occurrence on stones in the intertidal.

Figures

FIGURES 23 – 28. Plesiocleidochasma porcellanum (Busk, 1860) n. comb., lectotype, NHMUK 1899.7.1.1726. 23, surface of mature colony near the growth margin; note the presence of three spines in early ontogenetic zooids. 24, primary orifice. 25, autozooids and maternal zooids at some distance to the growth margin, with the ooecia covered and immersed by secondary calcification. 26, close-up of ovicell; note the small, median fenestra in the proximal ooecial margin and the absence of lateral incisions. 27, early stages of ooecium formation, with most of the exposed ectooecium quickly covered by secondary calcification of the distal zooid (arrows indicate the advancing fronts). 28, close-up of avicularium. Scale bars: Fig. 23 = 200 µm; Fig. 24 = 40 µm; Fig. 25, 27 = 100 µm; Fig. 26 = 50 µm; Fig. 28 = 10 µm.