Rhabdopleura compacta Hincks 1880

Rhabdopleura compacta Hincks, 1880

(Fig. 5D‒F)

Type locality. Off Antrim, Ireland (NE or NNE of Belfast in “deep water” (c. 46‒165 m; see Stebbing 1970a, p. 211).

Key features. Inception of ringed erect tubes appears to be indirect. The mature reproductive colony is tiny and compact (Fig. 4E), with mean size <3 mm diameter (Stebbing 1970a, p. 213) and the adherent tubes closely appressed laterally, each with oblique fusellar sutures. There are up to 30 zooids per colony (average seven). Zooids are lemon-yellow with reddish-brown pigment spots on tentacles and the central part of the cephalic shield, and light green spots on the sides of cephalic shield and along one side of each tentacle. Dormant buds are enclosed in a dark casing (capsule) within the stolon.

Comment. Colony growth in this compact species has been described as monopodial by Urbanek & Dilly (2000). They described a “permanent terminal zooid” (ibid., p. 201) in which the stolon produces buds of regular feeding zooids that “form a chain … at different growth stages situated behind the terminal zooid,” with each bud separating from its neighbour by secreting a transverse partition across the stolon-bearing tube such that each chamber formed in this way becomes a basal part of an individual zooidal tube. Juvenile zooids within such chambers form, by resorption, a circular pore in the upper wall of the tube and, emerging through it, they secrete an erect free portion of the zooidal tube proper. The original drawing by Hincks (1880, pl. 72, fig. 8) and those by Stebbing (1970a, fig. 2; 1970b, fig. 1) suggest indirect lateral growth or a modified form of it. Stebbing’s (1970a, fig. 3) interpretative drawing shows no obvious “permanent terminal zooid” and what may be interpreted as blind-ending side branches are budded laterally not frontally. Stebbing’s (1970a) redescription conforms well with Hinck’s (1880) original description. Owing especially to its tiny compact size (even when in sexually reproductive mode) it is a morphologically highly distinctive species and isolated colonies are unlikely to be confused with any other species.