Hyalinella africana

(Fig. 2 a – c)

Description. In his original description, Wiebach (1964) accurately described the spindle-shaped colonies growing around plant stems. The zooid body wall is clear, colorless, and soft (Fig. 2 a, b). Notable is what Wiebach described as the “ common colonial ectocyst (forming) a hard-gelatinous mass, which is filled with fine detritus and epizoic unicellular organisms and over which the orifice of the individuals rises at intervals like a hump or fingertip. ” In fact, in some of the preserved material the entire colonial mass has started to peel away intact from the plant stem. Wiebach described and illustrated (with photos) two types of floatoblasts: pycnoblasts and leptoblasts (Wiebach 1964, figs. 23, 24). Unfortunately, among all three specimens I was able to find only a single floatoblast, in Specimen No. 267, not yet fully developed, corresponding roughly to Wiebach’s “ pycnoblast ” (Fig. 2 c). The two valves are about equally convex. The overall length = 311 μm, width = 195 μm; the dorsal fenestra length = 181 μm, width = 144 μm; ventral fenestra length = 19 μm, width = 162 μm.

Remarks. Wiebach worked under the assumption that many plumatellid species had two types of floatoblasts: robust pycnoblasts and more delicate leptoblasts. In the case of H. africana any such distinction is slight and can be attributed to normal variation. With the single remaining floatoblast, I separated the valves for detailed examination (Fig. 2 c) and isolated them in a small vial, now included in the larger vial with Specimen No. 267. Although Wiebach described (but did not illustrate) sessoblasts in this species none could be found among any of the MRAC specimens. Assuming his written diagnosis is correct, the presence of sessoblasts would preclude accommodating this species in the genus Hyalinella. According to the current understanding, no sessoblasts are produced in this genus (Wood et al. 2006). The thick gelatinous base suggests a possible affinity with Gelatinella toanensis (Hozawa & Toriumi, 1940), but the validity of that genus remains questionable. At this time, I am reluctant to move the species to the already unwieldy genus Plumatella. Recognizing these difficulties, the species name, Hyalinella africana, should remain unchanged until more specimens become available.

Distribution. Known only from Rwanda and south to Lake Tanganyia.

Material examined. Specimen Nos. 204 and 205 from Lukoma, Rwanda (both labeled “ Typus ”), No. 267 from Lake Mugesera, Rwanda, 33 km SE Kigali; and No. 439 from Lake Tanganyika. All specimens are well preserved with polypides extended (Fig. 2 a, b). No. 205 is probably in the best condition, consisting of three fairly large colonies.