Rhabdopleura decipula, 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 : 342-345

Rhabdopleura decipula n. sp.

( Figs 2B, D, F , 9B‒E , 10A‒K )

Material examined. Holotype: NIWA 162592 , Stn Z 19247, Stewart-Snares shelf, 47.8230° S, 166.7680° E, 167 m, 14 October 2016, in ethanol . Paratypes 1‒2: NIWA 161212 , same data as for holotype . Paratype 3: NIWA 158517 , Stn TRIP 2577/45, southern Snares shelf, 48.6850° S, 166.4100° E, 190 m, 16 February 2008 . Other material: NIWA 162591 (part), Stn Z 19246, Stewart-Snares shelf, 47.5910° S, 166.7170° E, 163 m, 14 October 2016 .

Etymology. Latin decipula , trap, snare, alluding to the provenance of the species northeast of The Snares Islands, Stewart-Snares shelf, New Zealand.

Diagnosis. Erect ringed tubes budded directly from surface of creeping tubes. Colony ramifying over dead shell, up to 4 cm length or diameter. Erect tubes spaced 630‒1116 μm apart, mean erect-tube diameter 136 μm, mean fusellus height 32 μm. Creeping tubes sinuous, zigzag sutures inconspicuous, on about a third to half of convexity width; zigzag angles 26‒44°. Dormant bodies common. Three to four creeping tubes can be initiated from sicula and immediate daughter tubes.

Description. Inception of ringed erect tubes is direct. Colony encrusting surface of dead shell, ramifying mostly irregularly over surface ( Fig. 9B ), with generally curved and sinuous or sections between side branches, produced at various angles at non-regular intervals; spreading up to at least 4 cm across substratum, forming conspicuous dark-brown to black continuous tracery.

Creeping tubes frequently sinuous in their course, even on very smooth shell; convex part of tube 238‒344 (293) μm (n = 18), with flattened marginal lamina along both sides, also of variable width [36‒67 (51) μm, n = 10], its edges often diffuse and not well defined. Surface fusellar sutures very thin ( Fig. 10E, F ), with point-to-point zigzag angles of predominantly 26‒44 (35)°, mode 35° (n = 75); zigzag sequence occupying c. 35‒55% of convexity width, with zigzag components mostly gently curved. Pectocaulus running along floor of creeping tubes, including under dormant bodies within tubes, 26‒33 (29) μm wide (n = 3).

Erect tubes up to 1863 μm long, spaced at distances of c. 630‒1116 μm along creeping tubes, projecting at right angles to creeping tube. Tube diameter between fusellar collars 107‒160 (136) μm (n = 35), collars generally weakly projecting circumferentially around tube [1.3‒9.2 (4.3) μm, n = 44], lightly crinkled and generally horizontal for most of tube length, more crinkled and irregular in proximal third and distalmost parts of tube. Fusellus height in standard section of tube 17‒49 (32) μm (n = 105), fuselli generally shorter in proximal third and distalmost parts of tubes; number of fusellar collars in 500 μm of erect tube 12‒20 (16) (n = 16).

Dormant bodies moderately common, mostly parallel-sided or curved, rounded at each end ( Fig. 9C, E ), each proximal to an erect tube.

Zooids seen mostly in creeping parts of tubes in preserved material, some zooids extended into proximal parts of erect tubes; zooid length 511‒689 μm (561 μm), n = 5.

Prosicula of founding individual ( Fig. 10G ) subspherical to suboval, typically slightly longer [322‒504 (442) μm, n = 12] than wide [344‒491 (403) μm, n = 12]; metasicula (proximal part of initial creeping tube) with fine oblique sutures and first zigzag angles; prosicular zooid c. 444 μm long.

Remarks. Rhabdopleura decipula n. sp. is very similar in most respects to R. francesca n. sp., overlapping in virtually all characters, although means may differ. The biggest character difference is in the number of fusellar collars in 500 μm of erect tube, respectively 10‒12 (11) in R. francesca and 12‒20 (16) in R. decipula . The two species are undoubtedly closely related, which is supported by the concatenated 16S+18S rRNA phylogeny ( Fig. 3A ). Nevertheless, comparison of pairwise distances and the phylogeny both concur that they are genetically distinct, when compared to distances and branch lengths in conspecific R. recondita ( Fig. 3A ; Tables S2, S3). Indeed, 16S distances between R. francesca and R. decipula stood at 0.07, which is much larger than the intraspecific distance within N. normani with an average of 0.002. Similarly, the 18S distance between the two species is 0.007, which is also several times larger than the average of 0.002 between conspecifics in R. normani .

Several examples of colony initiation were seen ( Fig. 10H‒K ), based on larvae that had settled on very smooth white shell, allowing details of budding from the prosicula to be seen. Externally, the prosicula appears featureless, and is symmetrical or nearly so, with the metasicula sometimes slightly offset, in which case two metasiculae may protrude from the prosicula ( Fig. 10K ), establishing two creeping tubes that may run together for a short distance before diverging to allow colony spread. Additionally, a curved internal longitudinal partition develops within the sicular cocoon [or dome in the terminology of Dilly (1985a) and Strano et al. (2019)], partitioning off another zooid bud that develops into a second (or third) creeping tube that immediately diverges to left and right. Thus three of four creeping tubes can be produced from the primary complex of buds. Stebbing (1970b, fig. 5) depicted a similar internal partition within the prosicula of the primary zooid in R. compacta , quickly establishing two parallel creeping tubes.

Two species were discovered to have co-occurred at the same station, one of them corresponding to R. decipula . Part of NIWA 162591 (see material examined) that was sequenced recovered at least two mitochondrial contigs of different species that could not be retrospectively separated out. Determining if two morphological entities can be recognized in the remaining registered sample labelled NIWA 162591 would compromise it, given that SEM is the easiest way to characterize the key non-genetic characters. Nevertheless, the discovery of two genetic entities at the same station highlights the potential problem posed by cryptic speciation. More sampling of Rhabdopleura on the Stewart-Snares shelf is necessary resolve this issue.

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 3. Maximum likelihood phylogeny for Rhabdopleura reconstructed on: A, concatenated 16S+18S rRNA genes; B, 13 mitochondrial protein-coding genes. Numbers adjacent to nodes represent bootstrap support (≥ 50) /posterior probability (≥0.8) values.

FIGURE 9. Rhabdopleura francesca n. sp.: A, section of creeping tube of holotype with a broken erect tube; note the small erect-tube diameter relative to the width of the creeping tube. Rhabdopleura decipula n. sp.: B, holotype NIWA 162592 (left) and paratype 1 (NIWA 161212) colonies on broken shells; C, reflected-light image of part of a paratype 2 colony (NIWA 161213) seen in transparency on smooth shell, showing a loop of one creeping-tube branch curving back on itself, with a narrow crossconnection (cc) of the pectocaulus; erect tubes and zooids (z) and a dormant body (db) also seen. D, SEM image of same colony at lower magnification, with branch loop in C at top right. E, another part of same colony seen in transparency, showing a zooid inside a creeping tube to the left (distal) of a dark dormant body. Scale bars: A, 500 μm; B, 10 mm; C, 1 mm; D, 2 mm; E, 400 μm.

FIGURE 10. Rhabdopleura decipula n. sp., A, C, D, holotype 162592; B, E, paratype 3 NIWA 162591; F, paratype 4 NIWA 158517; G‒K, paratype 2 NIWA 161213. A, broken-tipped erect tube. B, distal end of young erect tube. C, D, subdistal and middle parts of same fully formed erect tube. E, F, parts of creeping tubes with some point-to-point suture zigzags highlighted. G, a prosicula (ps) and metasicula (ms); H‒K, reflected-light (H, I) and SEM (J, K) images of new colonies, with numbers indicating the inferred budding sequence (subsequent development seems to vary) of creeping tubes; the metasicula produces one or two metasiculae while one side of the prosicula is internally partitioned off to produce both a backwards-directed tube and an opposing creeping tube. Black stolons occur in H and a zooid in I. Scale bars: A, E‒H, 200 μm; B, 100 μm; C, D, K, 50 μm; I, J, 400 μm.