Stomhypselosaria robusta, Hayward & Winston, 2011

Hayward, Peter J. & Winston, Judith E., 2011, Bryozoa collected by the United States Antarctic Research Program: new taxa and new records, Journal of Natural History 45 (37 - 38), pp. 2259-2338 : 2276-2279

Stomhypselosaria robusta sp. nov.

( Figure 9 )

Material

Holotype. NMNH 1154054 : Islas Orcadas cruise 876, station 120, 59 ◦ 53.1’ S, 32 ◦ 19.5’ W, 523–671 m, 25 February 1976; part of colony, 25 mm high, with spread of 20 mm.

Paratypes. VMNH 0136668.00: same data as holotype; numerous fragments, to 7 mm high, including many juvenile colonies .

Description

Colony erect, rigid, branching dichotomously in more than one plane, attached by bundles of chitinous rhizoids. Branches cylindrical, stout, thickly calcified in later ontogeny, tapering distally and sometimes anastomosed; holotype with maximum proximal diameter 2 mm. Colony founded by a single, proximally tapered ancestrula, budding first two autozooids; autozooids then ordered in spiralled whorls of three for the next few (up to 10) astogenetic generations; thereafter, in whorls of four or five through later astogeny. Dichotomy occurs irregularly, with variable intervals of 2.5–18 mm; internodes straight or curved. Autozooids elongate, hexagonal in early ontogeny, each separated by clear sutures; opesia situated in distal third of autozooid, longer than wide, with a thin raised rim, proximal border convex and thickened, projecting frontally as a distinct lip. Cryptocyst calcification thick, coarsely and irregularly nodular, distal and disto-lateral parts raised and projecting, forming a cowl around the opesia, with sinuous crenulated rim, extending proximal to opesia on each side and converging at proximal end of autozooid. A low, median longitudinal ridge extends from immediately proximal to the opesia to the convergence of the lateral cryptocyst ridges. Calcification thickens considerably in later ontogeny; autozooids become more broadly hexagonal, although the sutures between remain distinct, and the lateral cryptocyst ridges are obscured by the thickening and deepening of the lateral and distal hood above the opesia. The cryptocyst becomes distinctly convex laterally and proximally, dipping medio-distally towards the median longitudinal ridge; the opesia is deeply recessed and the curved proximal lip is especially prominent, its intersection with the median longitudinal ridge forming a distinct anvil shape. Ovicell indicated by a cowl-like structure overhanging the opesia. Vicarious avicularia occur very rarely; as large as autozooids, recognized by enlarged opesia, medially situated, as wide as long; distal rim raised to form a broad, semi-elliptical rostrum, the proximal rim deeply notched medially between conspicuous lateral condyles.

Measurements

For all measurements, n = 20, mean ± SD: autozooid length 0.95 ± 0.06 mm; autozooid width 0.73 ± 0.10 mm; opesia length 0.13 ± 0.01 mm; opesia width 0.11 ± 0.01 mm.

Etymology

Latin, robustus: strong, with reference to the colony morphology in later ontogeny.

Remarks

Avicularia were not described in the type species of the genus, Stomhypselosaria condylata Canu and Bassler, 1927 , or in the next Recent species to be attributed to it, S. dupliforma Canu and Bassler, 1929 , and Gordon’s (1984) diagnosis stated that they were absent. However, the Antarctic/subantarctic S. watersi Hayward and Thorpe, 1987 , which in all other morphological characteristics conforms to the generic diagnosis, does have vicarious avicularia. These are situated singly in the axil of each branch dichotomy and are not very conspicuous. The avicularia of S. robusta sp. nov. are so sparsely developed, and inconspicuous, that just one was discovered in the process of SEM ( Figure 9D ). It is possible that avicularia also occur in the first two species attributed to the genus, but have been overlooked simply as a consequence of a paucity of material.

The number of juvenile colonies in the sample could not be easily counted, most were attached to the larger colony fragments and were very much smaller than them. About 20 juveniles were attached to the holotype colony, the smallest consisting of just the ancestrula and buds of one or two autozooids. Each juvenile colony arose from a cluster of about five tubular rhizoids, which were spread out, and branching, across the surface of the larger colony. The smallest juveniles arose from a single rhizoid tangled among those of the larger colony, perhaps suggesting that they had budded from the large colony, and that perhaps the whole may represent a clonal assemblage.

Figures

Figure 9. Stomhypselosaria robusta sp. nov., Paratype. (A) A branch dichotomy; (B) part of an internode in early ontogeny; (C) part of an internode in late ontogeny; (D) a vicarious avicularium. Scale bars: 1.0 mm (A), 0.1 mm (B), 0.5 mm (C, D).