Plumatella raoi, Wood, 2022

Wood, Timothy S., 2022, Phylactolaemate bryozoans at the Zoological Survey of India and a taxonomic key to Indian Phylactolaemata, Zootaxa 5200 (2), pp. 401-435 : 421-422

Plumatella raoi n. sp.

( Fig. 17 )

Type material. No. P 1387 collected 25 December 1958 at “Jor Pokhri, 1800 feet, Sitong, Darjeeling District, East Himalayas Shilong, Assam,” now Shillong , capital of Meghalaya State in northeast India, by “ ZSI ”, originally labelled “ Plumatella emarginata .”

Diagnosis. Sessoblast with cellular annulus (uninflated).

Etymology. The specific epithet honors the late K.S. Rao (1938–2012), an active freshwater biologist based for many years at Vikram University in Maharashtra, whose numerous publications on the freshwater bryozoans of India (1961–1985) made significant contributions towards the understanding of this group.

Description. Colony spreading and diffuse with many free branches; ectocyst thin, colorless and transparent. Floatoblasts short and broad, only about 350 μm in length with a length/width ratio <1.5 ( Table 1 ); dorsal valve flat or even slightly concave; dorsal fenestra small with the width often greater than the length, and the length less than half floatoblast length; floatoblast ventral valve highly convex with the fenestra only slightly smaller than the underlying capsule; fenestrae of both valves bearing a dense arrangement of tubercles with hypertubercles, many surrounded by 5 or 6 pore-like depressions. Sessoblast annulus cellular, with the cells open externally ( Fig. 17a, b, d ); frontal valve crowded with tubercles; basal valve having sharply reduced adhesive material by which plumatellid sessoblasts are normally attached to the substratum.

Distribution. The species is known only from its type locality.

Remarks. It is generally understood that among plumatellid bryozoans the sessoblast annulus is homologous to the gas-filled annulus of floatoblasts ( Mukai 1982). The sessoblast annulus typically occurs as a blade-like ribbon of chitin extending around the periphery of the frontal valve. The annulus can be variable in width and ornamentation, but the underlying structure appears always to be the same. However, this species is unique by the cellular nature of the sessoblast annulus, even including communicating pores, which normally occur only in floatoblasts ( Rubini et al. 2011). Lacourt (1968, pl. 12i) illustrated a sessoblast of P. vorstmani (now Rumarcanella vorstmani ) in which a small portion of the annulus showed cellular structure, but this appears to have been an isolated developmental anomaly. In the ZSI specimen every sessoblast carries a cellular annulus.

The small floatoblast size is another distinguishing feature of P. raoi n. sp. The length and width are similar to floatoblasts of Rumarcanella himalayana , but in P. raoi n. sp. the dorsal fenestra is much smaller and the dorsal valve is flat or even slightly concave, not convex. Hypertubercles appear mostly on the ventral valve and are not expressed on every tubercle.

The specimen consists of small bits tree bark and a few loose strings of zooids with about ten sessoblasts and numerous floatoblasts.

Figures

FIGURE 17. Plumatella raoi n. sp. (a) Two sessoblasts showing unusual cellular annulus and heavily tuberculated frontal valve, scale bar = 100 μm; (b) Closer view of sessoblast annulus, scale bar = 10 μm; (c) SEM images of floatoblast valves with dorsal valve on the left, scale bar = 100 µm; (d) Enlargement of a portion of previous image showing hypertubercles and porelike structures; (e) Floatoblast valves with dorsal valve on the left, scale bar = 100 μm; (f) sessoblast valves with frontal valve on the left, scale bar = 100 μm.