Rhabdopleura compacta Hincks, 1880

Gordon, Dennis P., Randolph Quek, Z. B. & Huang, Danwei, 2024, Four new species and a ribosomal phylogeny of Rhabdopleura (Hemichordata: Graptolithina) from New Zealand, with a review and key to all described extant taxa, Zootaxa 5424 (3), pp. 323-357 : 334

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.

Figures

FIGURE 4. Original illustrations of described species of Rhabdopleura characterized by indirect inception of ringed erect tubes from creeping tubes via an intervening adnate portion that comprises a blind-ending branch. A, Rhabdopleura normani (from Allman 1869, pl. 8, fig. 1). Note that most of the colony is adherent; only the ringed tubes (rt) are erect, the proximal part of each side branch (ppsb) is adherent and the oblique sutures are not shown. B, schematic interpretative depiction of R. normani (after Andres 1977). C, D, Rhabdopleura grimaldii (from Jullien 1890, p. 181); note that ringed erect tubes (rt) are mostly lacking, with only the adherent proximal parts of side branches (ppsb) present. D is a cropped part of C to show the proximal parts of two side branches, with fusellar sutures and Jullien’s putative ‘very fine tubes’ (vft)—actually the side walls of the tapering pectocaulus, the proximal end of which is transparent, hence the apparent gap between it and the pectocaulus of the creeping tube. E, F, Rhabdopleura striata (from Schepotieff 1909, pl. 7, figs 2, 7); E shows longitudinally striated erect parts of side branches; oblique sutures on prostrate parts omitted in E but shown in F, with characteristic web of fusellar sutures (wfs) in branch angles from main creeping tube. G, Rhabdopleura manubialis (from Jullien & Calvet, 1903, pl. 1, fig. 2) with dormant bodies (db).

FIGURE 5. Original illustrations of three described species of Rhabdopleura characterized by direct-frontal inception of ringed erect tubes (A‒C). Erect-tube inception in R. compacta (D‒F) appears to be a form of indirect erect-tube inception. Note that the direction of zooid budding is the same in A to C. A, Rhabdopleura mirabilis (from Sars 1872, pl. 1, fig. 5). B, Rhabdopleura annulata (from Norman 1921, fig. 4). C, Rhabdopleura recondita (after Beli et al. 2018, fig. 2C). D‒F, Rhabdopleura compacta (respectively after Hincks 1880, pl. 72, fig. 8; Stebbing 1970b, fig. 1; Stebbing 1970a, fig. 3).