Tjalfiella tristoma, Bezio & Collins, 2024

Bezio, Nicholas & Collins, Allen G., 2024, Redescription of the deep-sea benthic ctenophore genus Tjalfiella from the North Atlantic (Class Tentaculata, Order Platyctenida, Family Tjalfiellidae), Zootaxa 5486 (2), pp. 241-266 : 256-258

Species Tjalfiella aff. tristoma (2018)

Figs. 3 , 4 , 5 , 6 , 10 , 11 .

Material— Two specimens of T. aff. tristoma collected by the ROV Deep Discoverer on the NOAA Okeanos Explorer and stored in the Smithsonian National Museum of Natural History ( EX 1806_D17_02B_A01; UNSM-IZ-1490693). Additional pictures of living specimens observed during the same dive were used for morphological analyses and host availability.

Description

Body— Benthic ctenophore, “U” shaped in the tentacular axis; two large semi-gelatinous arms (h— 3.23-5.2mm) extending perpendicular from the horizontal axis of the animal, placed on opposing sides of the body; lacking lateral oral grooves, distal ends of arms flattened into a disk with an outer margin that is capable of folding inwards, contains two openings, a singular gastric accessory opening with serrated edges, and a single smaller tentacular opening; overall body surface is smooth with no warts or visible papillae, exceptions are four pairs of globular gonads on the aboral face (number of gonads vary between individuals) and two rows of brood pouches around the oral skirt.

Size— 10mm in the tentacular plane, 5mm in the stomodeal plane, and 8–11mm in height

Coloration— Translucent with orange to yellow ochre hue, intensity varies;suboral cavity deep rust or copper in color; tentacles and tentacle bulbs are a milky white.

Statocyst— Sunken into the main body via a small pocket; exact structure is unknown.

Ctene Rows— Absent in maturity but present in observable larvae. See the embryo section below.

Tentacular Apparatus— Two tentacular apparatuses (one per aboral arm) located on the innermost facing half of arm; contains a large spherical tentacle bulb (d— 2mm) with a crescentic tentacle root, a single tentacle protruding up towards the distal end of the arm (d—. 11mm), lacking tentilla; of an undefined thickness that gradually narrows to the distal end; coil counterclockwise when retracted into sheath; the shape and abundance of colloblasts were not documented.

Gastrovascular System— Large and “U” shaped with three openings: a downward-facing slit mouth on the oral face and a single accessory opening on each aboral arm; central suboral cavity directly above the slit mouth and below the statocyst, lined with numerous thin stomodeal folds attached from the cavity’s roof, folds thin out towards accessory openings; a single protrusion extends upward from the suboral cavity towards the centralized statocyst, two canals extend horizontally from protrusion into the tentacles, each horizontal canal contains four diverticula that imbed into four hermaphroditic gonads; two diverticula extend between each pair of gonadal diverticula along the outer body wall, each diverticula has numerous dichotomously branching protrusions, all protrusions lack anastomoses; development of these diverticula varies across individuals.

Reproductive System— Eight large globular gonads (d— 2.2–2.44mm) protruding from aboral face around statocyst, contain both male and female reproductive tissue separated by canal diverticula; two rows of brood pouches around oral skirt (ea. 1.36–1.62mm in d), house brooded embryos (see embryo section).

Embryo — Spherical and cydippid-like (d— 0.8mm); eight equidistant ctene rows on aboral side stretching to half body length, number of ctene plates around six but variable; statocyst present on central aboral end with large gelatinous dome; large slit on oral end that stretches halfway up the body and opens up to suboral cavity, suboral cavity voluminous and occupies half body cavity, suboral cavity converges to stomodeum, four perradial canals diverge from stomodeum, and split into two adradial canals each which merge into meridional canal roughly a quarter way from the bottom of the ctene row at an angle of 40°; tentacle bulbs halfway up body on opposing side of tentacular plane, “T” shaped, orange/rust colored, a single tentacle without tentilla each.

Ecology— Known only residing on the surface of highly ramified deep-sea octocoral of the genus Acanella , perhaps in an obligatory symbiotic relationship (it was not seen on nearby corals in other genera). The ecology of this platyctene species is otherwise unknown and requires future study.

Habitat— 1866.3m, 36°13’48.0” N 74°28’12.0” W, roughly 75 km east of North Carolina, USA

Host— Undescribed Acanella sp. but closely resembles Acanella arbuscula . Usually occupies the upper third of the host organism.

Remarks— Compared to T. tristoma (see description above), most morphological features have no significant difference between measured T. tristoma and T. aff. tristoma (2018) ( Table 4 ). However, both T. tristoma and T. aff. tristoma (2018) possess slight variations in coloration and host preference. Mortensen most notably described T. tristoma as white to pale-yellowish, while T. aff. tristoma (2018) is consistently observed as bright orange with a red suboral cavity. Additionally, even in the presence of other suitable host organisms, T. tristoma was documented only attaching to Umbellula lindahli , while T. aff. tristoma (2018) was observed only attaching to Acanella sp. ( Fig. 3 ). Additional sightings of T. aff. tristoma (2018) along EX1806 showed an affinity for Acanella sp. with no additional sighting on neighboring corals ( Fig. 4 ). Similar situations are described between individual species of Coeloplana , which often have no significant anatomical differences between species. Instead, external features like coloration, papillae arrangement, and host preference allow identification. Genetically, T. aff. tristoma (2018) differs from T. aff. tristoma (2022), but not by great amounts. The often-employed mitochondrial cox1 gene shows 3.08% divergence ( Table 3 ), comparable to the lower end of interspecific differences among platyctenes sampled by Alamaru et al. (2017), and far exceeding any intraspecific variation among their samples.

Figures

FIGURE 3. Still images of living specimens of TJalfiella aff. tristroma (2018); A. In situ observation of four individuals on host octocoral Acanella sp. Scale bar—2cm, B. Still image of a collected individual in a retracted state, showing the body shape and reproductive system, C. Closeup of the aboral side, showing the opening of the arm with a simple tentacle, D. Closeup of arms, showing the coiling of tentacles and the relaxed state of the animal. Scale bar B to D—0.25cm.

FIGURE 4. Additional observation by NOAA of platyctenes of similar morphology to TJalfiella aff. tristoma (2018) on dive EX1806_D17. Arrows point to individual platyctenes in the branches of Acanella sp. Scale bar—2cm.

FIGURE 5. Still images of living specimens of TJalfiella aff. tristroma (2022) with respect to host coral; A. In situ observation of the host coral Adinisis sp. Scale bar—2cm, B. Close-up still image of the collected host with several platyctenes in a retracted state. Scale bar—0.75cm, C. Tentacular view of two individual platyctenes separated from host coral.Arrows point to individual platyctenes on the collected Adinisis sp. Scale bar—0.5cm.

FIGURE 6. MicroCT 3D reconstruction of three TJalfiella samples:A. Tentacular view of T. tristoma, B. Tentacular view of T. aff. tristoma (2018), C. Tentacular view of T. aff. tristoma (2022), D. Stomodeal view of T. tristoma, E. Stomodeal view of T. aff. tristoma (2018), F. Stomodeal view of T. aff. tristoma (2022), G. Colorized internal anatomy of T. tristoma with low opacity body overlay, H. Colorized internal anatomy of T. aff. tristoma (2018) with low opacity body overlay, I. Colorized internal anatomy of T. aff. tristoma (2022) with low opacity body overlay, J. Closeup of aboral arms face with centralized opening and tentacle, K. Isolated view of intact larval T. aff. tristoma (2018) within the brood chamber, tentacular view, I. Closeup of developing brood chambers along diverticula protrusions. Blue—gastrovascular cavity (e.g., suboral cavity), Purple—Gonads, Magenta—brood pouches, Pink—tentacular apparatus, Green—canals, Teal—generalized larval anatomy. Scale bar: A–F— 3mm, G–I—1.6mm, J–L—0.3mm.

FIGURE 10. Illustrated Plate of TJalfiella aff. tristoma (2018) collected by NOAA: A. representation of specimens in their natural position on the stalk of Acanella sp., B. mature T. aff. tristoma (2018), tentacular view, C. Cross section of a gonad, D. Stomodeal view of mature T. aff. tristoma (2018), E. cutaway illustration featuring internalized anatomy, tentacular view. F. Closeup drawing showing the development of brood pouch on diverticula protrusions, G. view of the flattened face of aboral arm, H. aboral schematic of the arrangement of diverticula and associating canals; Tj—TJalfiella aff. tristoma (2018), H—Host coral (Acanella sp.), a.a.—aboral arm, a.f.—aboral face, b.—brooding pouch, d.b.—diverticula that transverse along the body, d.g.—gonadal diverticula, d.p.—diverticula protrusions, d.t.—tentacular diverticula, g.—gonads, m.—mouth, o.s.—oral skirt, s.c.—suboral cavity, s.—statocyst, s.f.—stomodeal folds, s.o.—secondary opening (mouth), t.—tentacle, t.b.—tentacle bulb, t.s.—tentacle sheath, t.o.—tentacle opening.

FIGURE 11. Illustrated Plate of larval TJalfiella aff. tristoma (2018) collected by NOAA: A. tentacular plane, B. aboral view, C. oral view, a.c.—anal canal, c.p.—ctene plate, c.r.—ctene row, m.—mouth, p.—pharynx, s.—statocyst, t.—tentacle, t.b.—tentacle bulb.