Megapora ringens ( Busk, 1856 )

Martino, Emanuela Di & Taylor, Paul D, 2012, Pyrisinellidae, a new family of anascan cheilostome bryozoans, Zootaxa 3534, pp. 1-20 : 16-18

Megapora ringens ( Busk, 1856)

( Figs 39−44 ; Table 6)

Lepralia ringens Busk, 1856: 308 .

Megapora ringens: Hincks, 1880: 172 , pl. 22, fig. 1; Prenant & Bobin, 1966: 273; Ryland & Hayward, 1977: 107, fig. 47; Hayward & Ryland, 1998: 196 −199, fig. 58B −59.

Material examined. NHML 1911.10.1.629, Recent (specimen figured by Hincks, 1880, pl. 22, fig. 1), Shetland (Barlee), 80−170 fathoms. NHML 1911.10.1.631, Recent, Porcupine Bight, several encrusting three small pebbles.

Description. Colony encrusting, multiserial, unilaminar, developing small rounded patches. Pore chambers present, distal pore chamber slightly larger than distolateral pore chambers; pore windows oval (mean L = 37 µm, mean W = 10 µm), surrounded by a mural ring, that of the distal pore chamber facing frontally. Ancestrula oval and smaller than astogenetically mature autozooids (L = 223 µm, mean W = 166 µm); gymnocyst absent or hidden by later zooids, cryptocyst densely granular, occupying half of the frontal area, opesia trifoliate with two small oral spines (D = 12 µm) placed distally ( Fig. 44 ). Autozooids in zone of early astogeny transitional in size and morphology between ancestrula and later autozooids ( Fig. 43 ). Autozooids small, rounded hexagonal or oval, longer than broad (mean L/W = 1.27), separated by deep furrows. Gymnocyst narrow, slightly broader proximally. Cryptocyst extensive, coarsely and densely granular, depressed centrally, submarginally raised to form a rounded ridge which together with distal rim of the opesia forms a pear-shaped outline on the frontal wall and around the opesia. Opesia strongly trifoliate, the proximal edge gently convex with rounded indentations at the proximolateral corners; a pair of rounded thick lateral denticles directed downwardly divide the larger, distal semicircular part of the opesia from the smaller but broader proximal part ( Fig. 41 ). Oral spines numbering six, arranged in an arch around the distal edge of the opesia, the most proximal pair almost level with the denticles; distalmost pair of oral spines spaced more widely in ovicellate than non-ovicellate zooids. Complete ovicell not observed but that figured by Hayward & Ryland (1998, p. 197, fig. 58B) is hyperstomial, prominent, rounded and elongated, with a narrow, median triangular ectooecial window with thickened edges; a small rounded triangular kenozooid with a cryptocystal frontal area and a tiny central lacuna is present immediately distally of each ovicell ( Fig. 41 ). Closure plates developed in some autozooids as distal extensions of the cryptocyst, with a small tuberculum located medially level with the proximal edge of the opesia, which is completely occluded ( Fig. 40 ). Kenozooids of irregular shape frequently present along lateral margins of lobate colonies, slightly smaller (L = 258−291 µm, W = 195−218 µm) than autozooids; gymnocyst narrow but wider than in autozooids; cryptocyst coarsely and densely granular, continuous over the entire frontal area except for a small, round, lacuna (mean D = 25 µm) near the centre ( Fig. 42 ). Avicularia absent.

Remarks. None of the material examined of this species had an intact ovicell; in all instances the roof of the ovicell was broken, revealing a floor consisting of an exterior skeletal wall with planar spherulitic microstructure delineating two lobes on either side of a median depression ( Fig. 41 ). The presence of a small kenozooid distal of each ovicell, which is unique among the species described in this paper, recalls various other cheilostomes (e.g. Bishop & Househam 1987; Ostrovsky et al. 2008), and may signify evolutionary reduction of the zooid distal of the maternal zooid from an autozooid to a kenozooid.

Hayward & Ryland (1993) introduced the new genus Apiophragma for M. hyalina Waters, 1904 , referring to the shape of the mural rim from the Greek apios, a pear and phragma, a wall. Apiophragma resembles Megapora in its depressed cryptocyst, thick mural rim and distal arc of spines but differs in having only a weakly trifoliate opesia which is much smaller and narrowly bell-shaped, and the presence of opesiules. The inclusion of Apiophragma in the new family Pyrisinellidae should be considered after restudy of the type material of A. hyalina .

Distribution. Recent of boreal and arctic areas, widely distributed in the northeast Atlantic, extending southwards, in deep waters, as far as the Shetland Isles and Bergen, Norway ( Hayward & Ryland 1998).

N (colonies, zooids) Mean SD Range Zooid length 3, 30 414 39 367–489 Zooid width 3, 30 333 32 294–384 Orifice length 3, 15 100 13 82–116 Orifice width 3, 15 93 3 90–95 Ovicell length 2, 3 180 3 177–182 Ovicell width 2, 3 219 11 206–228

N, Number of colonies and number of zooids measured; SD, standard deviation.

Figures

FIGURES 39 − 44. Megapora ringens (Busk, 1856), NHML 1911.10. 1.631, Recent, Porcupine Bight. 39. view of several autozooids and kenozooids, scale bar = 200 µm; 40. group of autozooids showing a closure plate and lateral, distolateral and distal pore windows, scale bar = 200 µm; 41. detail of the strongly trifoliate orifice with six oral spine bases and inner view of a broken ovicell with associated distal kenozooid, scale bar = 100 µm; 42. zooid and kenozooid at the colony growing edge, scale bar = 100 µm; 43. early astogeny, scale bar = 200 µm; 44. close-up of ancestrula, scale bar = 20 µm.