Rhabdopleura emancipata, Gordon & Randolph Quek & Huang, 2024

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 : 347-350

Rhabdopleura emancipata n. sp.

( Figs 2C , 11A‒E , 12A‒E , 13A‒H )

Material examined. Holotype: NIWA 13590 , NIWA Stn E861, Wanganella Bank , near West Norfolk Ridge, 32.4167° S, 167.5833° E, 318 m, unattached, 18 March 1968, in ethanol . Paratype 1: NIWA 162566 , same station data as holotype, dried colony fragments . Paratype 2: NIWA 161211 , glycerol slide mount of small fragment of holotype .

Etymology. Latin emancipatus, set free, alluding to the unattached sole colony.

Diagnosis. Mature colony unattached, three-dimensional, erect and tangled, c. 4.5 cm diameter, erect ringed tubes budded directly from surface of principal tubes (stems) at any angle; stems lacking zigzag sutures. Pectocaulus varying in position along stems. Erect tubes spaced 309‒521 μm apart, mean erect-tube diameter 178 μm, mean fusellus height 31 μm. Darkly sclerotized tubes occurring sporadically on stems, slightly tapered proximally and distally.

Description. Sole colony not attached to any substratum ( Fig. 11A, B ), relatively large, forming ~ 4.5 cm diameter ‘ball’ of branching tubes in small jar of ethanol, flattening to 6 cm in petri dish for microscopic examination. No obvious colony attachment point, but many tubes trending in a similar direction, otherwise tangled.

Inception of ringed erect tubes corresponds to a direct mode, except that, inasmuch as principal stems, equivalent to creeping tubes in attached forms, are unattached to a substratum, erect ringed tubes can emerge in any direction. Principal tubes (stems) circular in cross section ( Figs 2C , 11C‒E , 12C‒E ) [diameter 241‒389 (312) μm, n = 30], with fairly regularly spaced annular fuselli encircling the tubes [ Figs 11C , 12D ; distance between annular fuselli 56‒96 (78) μm, n = 30]; tubes c. 4‒12 μm wider at annular fusellar sutures than between annular sutures. Branching of principal tubes initially almost at right angles, between 80° and 90°. No tube fusions or anastomoses. Principal tubes lacking flanking basal layer owing to being unattached; zigzag fusellar sutures lacking.

Erect ringed tubes of fully formed mature feeding zooids ( Fig. 11C, D ) c. 2 mm long, delicate, generally emerging at angles of 70‒80°. Spacing between erect-tube originations relatively close [309‒521 (394) μm, n = 17]. Diameter of erect tubes between fusellar collars narrower [133‒230 (178) μm, n = 50] than principal tubes, with well-developed fusellar collars over much of their length ( Fig. 12A, B ), a majority of collars curled and sinuous in the proximal half to two-thirds of long tubes, as are most of the collars in short tubes. Fusellus height 21‒46 (31) μm (n = 50), with 15‒21 (18) collars [n = 25] contained within 500-μm lengths of tube in middle two-thirds of tube length. Fusellar collars weakly projecting 2‒17 (7) μm circumferentially around tube.

Dark, sclerotized tubes 2.2‒3.0 mm long ( Fig. 12E ) occurring sporadically throughout colony, branching off principal tubes to which they are connected internally by a black-stolon branch. As seen by light microscopy, sclerotized tubes darkest proximally, almost black, becoming a little paler distalwards, with the terminal part of the tube transparent. Sclerotized tubes 153‒307 (250) μm diameter (n = 26), tending to be a little narrower proximally and distally, with well-developed fusellar collars in distal half to third of tubes (fusellus height 30‒82 (52) μm; n = 50). Darkened zooids partly visible in transmitted light (bright-field) within sclerotized part of tube; terminal aperture open.

Pectocaulus (black stolon) of principal tubes dark brown in transmitted light, 22‒42 (34) μm diameter [n = 10]. Stolon can run along one side of principal tube for some distance on side opposite erect-tube openings if these open in a series along one side, but can also trace a sinuous course within principal tube where erect tubes open all around ( Fig. 12C ). Stolon bifurcates at angles of c. 35°, sending a branch into another principal tube or sclerotized tube. Transverse septa cross width of principal tubes at angles of c. 30‒35° to compartmentalize zooids. A bowl-shaped or conical diaphragm complex (cdc; Figs 11C , 12C ) accommodates proximal end of each zooidal gymnocaulus. Zooid buds and immature zooids can occur in lineal series within principal tubes prior to lengthening and stretching into developing erect tubes once erect-tube openings established.

Remarks. Rhabdopleura emancipata n. sp. is globally unique among extant rhabdopleurids, distinguished by its loose, tangled, free-living habit. It almost certainly would have originated from an attached base, having been captured by a Menzies Trawl at 318 m depth, but the extent of that attachment is presently unknown. A New Zealand ctenostomatous bryozoan may provide an analogue. Parachnoidea rowdeni Gordon, 2013 begins life as a linearly ramifying encrusting growth on a vent-faunal mussel species that occurs on the Kermadec Ridge. Once established, it produces erect branches that eventually form a much-larger tangle above the mussel. This tangle looks remarkably like that of R. emancipata at the macro level, with free branches of zooids, now somewhat cylindrical, with erect peristomes. The zooids have typical bryozoan anatomy, however. It is not known what induces this morphology in the ctenostome; Gordon (2013) hypothesized that erect growth into free space may have been an adaptation to limited substratum space after initial colonization of the mussel shell, with enhanced access to particulate food, as well as, in this particular setting, compensating for the rate of coating of zooids by metal-sulfide precipitates— growth into three-dimensional space potentially exploits micro-variations in precipitation. There is no evidence of volcanism at the locality where R. emancipata occurred, however.

Could R. emancipata be a free-living form of R. annulata , the type locality of which is c. 460 km distant? Just as free zooids of the ctenostome P. rowdeni are more elongate-cylindrical and stoloniform, could growth into free space induce creeping rhabdopleurid tubes to become cylindrical, thereby modifying the behavior of zooids and suppressing the formation of a zigzag suture in favor of annular sutures? There is considerable overlap in zooid-tube (‘erect’-tube) diameter in R. emancipata and R. annulata [respectively 133‒230 (178) μm vs 167‒200 (180) μm] with practically identical means, but the number of fusellar collars in 500 μm of erect-tube length is non-overlapping (respectively 15‒21 and 11‒13). There is also no equivalent in R. annulata of the blind-ending dark sclerotized tubes in R. emancipata .

The colony of R. emancipata has numerous principal tubes (stems). The distal end of each has a lineal series of zooids developing consecutively distalwards ( Fig. 11E ). Each zooid in turn becomes cut off by a septum and appears to initiate an erect ringed tube via a secondarily formed opening (resorption foramen).

Figures

FIGURE 2. Interpretative images illustrating morphological terms and measured characters. A, part of creeping tube (ct) of an unidentified Rhabdopleura (NIWA 158518) from Kermadec Ridge, with flat lateral margin (flm), faint oblique fusellar sutures and two ringed erect tubes (et) produced directly from the frontal surface of a creeping tube. B, distal end of an erect tube of Rhabdopleura francesca n. sp. (NIWA 158157), showing a vertical series of ring-like fuselli (f), the rim of each comprising a curled fusellar collar (fc). Tube diameter (td) is measured between fusellar collars; fusellus height (fh) is the distance between the bases of a pair of fusellar collars. C, transmitted-light image of part of a principal (‘creeping’) tube of R. emancipata n. sp. (NIWA 161211) showing a young zooid with contracted arm tentacles (at), stomach (st), rectum (r) and cephalic shield (cs), part of which is concealed by the reddish-brown stolon (black stolon or pectocaulus). D, aperture of an erect tube of R. francesca n. sp.; note the smooth-surfaced interior devoid of vertical fusellar fibrils. E, part of a creeping tube of an unidentified Rhabdopleura (NIWA 90267) from Cavalli Seamount with frontal fusellar sutures (fs, two arrowed). The white lines show the zigzag (zz) portion of the sutures (note the proximalwards shift to the left), with zigzags rendered as straight lines between points of intersection, regardless of suture curvature, to obtain measured angles (one example shown). F, close-up of part of R. francesca n. sp. (NIWA 158157) showing chains of dark ‘dormant buds’ (db) lying upon short stretches of the black stolon (bs); walls of enveloping creeping tubes almost invisible in image.

FIGURE 11. Rhabdopleura emancipata n. sp., A, B, holotype NIWA 13590; C, E, paratype 161211; D, paratype NIWA162566: A, B, close-up and whole colony (inset); principal tubes are identifiable in A by the dark-brown stolon that passes through them; abundant ringed tubes diverge almost at right angles from principal tubes. C, transmitted-light view of a principal tube (pt) from which two ringed tubes (rt) diverge and, between them, a branch of the principal tube with a bifurcation of the black stolon (bs); other abbreviations: cdc, cone of diaphragm complex; cs, contractile stalk (gymnocaulus) of zooid; fc, fusellar collar; s, septum. D, SEM of a principal tube (pt) (stem), with its aperture at left, and five ringed tubes in various stages of development. E, transmitted-light view of a principal tube (pt) (stem) and its black stolon (bs) in which are three young zooids (z), the middle one of which shows, from below, the opening of a potential ringed tube (ort). Scale bars: A, 3 mm; B, 3 cm; C‒E, 0.5 mm.

FIGURE 12. Rhabdopleura emancipata n. sp., A, B, D, E, paratype NIWA 161566; C, paratype 161211: A, B, distal part of a ringed tube, the bracketed section enlarged in B. C, transmitted-light view of a principal tube (stem) with openings of four ringed tubes all around; tentacles of one young zooid partly emergent; note how black stolon changes position within principal tube; cdc = cone of diaphragm complex. D, close-up of exterior surface of a principal tube showing parallel fusellar annuli marked by thin sutures. E, sclerotized tube, with terminal aperture at left, branching off a principal stolon. Scale bars: A, 200 μm; B, 100 μm; C, 400 μm; D, 100 μm; E, 1 mm.

FIGURE 13. Epibionts of Rhabdopleura emancipata n. sp.: A, at top, an ancestrula of Chaperia sp. (Bryozoa) and, at bottom, a juvenile of the homotrematid foraminiferan Sporadotrema cylindricum. B, Chaperiopsis n. sp. (Bryozoa). C, Celleporina n. sp. (Bryozoa). D, annelid tube (at) draped along a principal tube. E, Galeopsis brevissimus (Bryozoa). F, units of a chain-like unidentified epibiont on a principal tube. G, Strongylopora gracilis (Bryozoa). H, Microtylostylifer n. sp. (Porifera) at left and Tubulipora sp. (Bryozoa) at right. R = Rhabdopleura host. Scale bars: A, D‒G, 500 μm; B, C, H, 1 mm.