Robbea Gerlach 1956

Scharhauser, Florian, Saavedra, Daniel E. M., Pröts, Philipp, Ot, Jörg A., Geier, Benedikt, Gruber-Vodicka, Harald R., Polikarpov, Maxim, Bourenkov, Gleb & Leisch, Nikolaus, 2025, Revision of the genus Robbea (Stilbonematinae: Desmodoridae), worldwide abundant marine nematodes with chromophoric Fe-Br inclusions and the description of a new stilbonematine genus, Zoological Journal of the Linnean Society 203 (1), pp. 1-28 : 4-5

The genus Robbea Gerlach 1956

Diagnosis Stilbonematinae

Desmodoridae , with short, unarmed pharynx; minute buccal cavity; pharynx three partite, corpus may be distinctly muscular and swollen, thin isthmus containing nuclei, bulbus round, muscular or mainly glandular; cuticle finely transversely striated or coarsely annulated; amphidial fovea spiral, loop shaped or reduced to a porus; cuticle covered by ectosymbiotic sulphur-oxidizing chemoautotrophic gamma proteobacteria.

Emended diagnosis of the genus Robbea

Stilbonematinae ; body long (L = 2.8–8.3 mm), slender (a> 110), cuticle thin, transverse striation extremely fine, cephalic capsule with block layer, creating a honey-comb structure in surface view; first circle of cephalic sensillae as six finger-like papillae retracted into mouth opening, second circle as six conical papillae (<1 µm long) at margin of buccal membrane in lateral and submedian positions; four long cephalic setae in sublateral position, directed anteriorly, close to the anterior margin of the amphidial fovea; one circle of eight subcephalic setae with four in submedian position (either at about the level of the cephalic setae or at the level of the centre of the amphidial fovea) and four sublateral near the posterior margin of the amphidial fovea. Somatic setae minute or absent, except for subventral rows of papilliform setae on the male tail, instead eight rows of cuticular pores corresponding to the GSOs; GSOs in eight rows, two sublateral rows on each side and two submedian rows dorsally and ventrally, respectively. GSOs dark-purple coloured in live animals only. Amphidial fovea flat with shallow spiral groove, ventrally wound. Pharynx with distinctly swollen muscular corpus, distinctly set off from the long and thin isthmus, small mainly glandular terminal bulb. Spicules curved, cephalate, capitulum appearing heart-shaped under the light microscope, gubernaculum with paired dorsocaudally directed plate-shaped apophysis with reinforced margin; tail cylindro-conical. Vulva with conspicuous sclerotization.

Dense monolayer of ectosymbiotic coccobacilli attached to the cuticle, starting several pharynx-lengths from anterior end to near tip of tail. Body diameter decreases at onset of bacterial coat, accommodating the thickness of the symbiont layer. Bacteria arranged with their longer axis parallel to the nematode’s cuticle, dividing transversally (see Figs 2 for an overview of host and symbiont characteristics).

Remarks: Whether the dark coloration of the GSOs can be used as a genus character is unclear. The lack of coloration in all species that had not been observed live is most probably due to the fixation and processing of the nematodes prior to observation, which leads to a fading of the colour. This underlines the importance of observation of live material in the study of meiofauna.

List of species

Robbea caelestis Gerlach 1956 (type species)

R. gallica Vitiello 1973

R. porosum ( Hopper and Cefalu 1973) Tchesunov 2013

Syn: Catanema porosum Hopper and Cefalu 1973

R. macintyrei ( Platt and Zhang 1982) nov. comb.

Syn: Catanema macintyrei Platt and Zang 1982

R. smo ( Platt and Zhang 1982) nov. comb.

Syn: Catanema smo Platt and Zang 1982

R. judithae sp. nov. this publication

R. weberae sp. nov. this publication

R. lotti sp. nov. this publication

R. seahi sp. nov. this publication

R. bernarditae sp. nov. this publication

The following specimens are tentatively assigned to the genus Robbea :

R. sp. Tchesunov 2013

R. sp. ‘Bahamas’, R. sp. ‘Bermuda’, R. sp. ‘Red Sea’, R. sp. ‘Great Barrier Reef’ (this publication)

R. sp. ‘ Cuba’ [ R. porosum in Armenteros et al. (2014b)]

Figures

Figure 2. Characteristics of the genus Robbea. A, total view of a live R. seahi sp. nov.. The transparent anterior body part is symbiont free, the opaque appearance of the rest of the body is due to the symbiont coat. The rows of dark coloured GSOs extend throughout the body. LM B-M Morphological details of R. judithae sp. nov.. B, anterior end, optical section. LM of preserved specimen.C, anterior end, surface view.LM of live specimen D, volume rendering of a CLSM stack showing setation of anterior end. E, mouth opening surrounded by circle of six papilliform outer labial sensillae, SEM. F, inner labial sensillae in form of six finger-like papillae retracted into mouth opening, SEM. G, midbody, lateral view, optical section. Double row of lateral GSOs, in median rows the connection to the outside via pores is visible (arrowheads). LM of live specimen.H, SEM of midbody region showing rows of cuticular pores and annulation where the coat of symbiotic bacteria is disrupted. I, CLS micrograph of pharyngeal region showing muscular corpus, thin isthmus and small, glandular bulbus. J, spiculum of holotype, LM. K, spicula and gubernaculum, ventral and oblique dorsal views.µ-CT images. L, optical section showing edge of bacterial coat with reduction of bacterial coat. The reduction of body diameter from the symbiont-free anterior end (right) to the posterior body region (left), LM. M, SEM micrograph of same region.