Craseoschema thyasiricola Ravara & Aguado, 2019

Ravara, Ascensão, Aguado, M. Teresa, Rodrigues, Clara F., Génio, Luciana & Cunha, Marina R., 2019, Description of a new genus and species of Chrysopetalidae (Annelida: Polychaeta) from the NE Atlantic, with some further records of related species, European Journal of Taxonomy 539, pp. 1-21 : 13-16

Craseoschema thyasiricola Ravara & Aguado sp. nov.

urn:lsid:zoobank.org:act:146CA9DD-11B3-46F0-81A8-2E6B085458C0

Figs 3–5

Diagnosis

Chaetous neuropodia and dorsal and ventral cirri present on the first segment. Notopodial lobes poorly developed. Notochaetae stick-like, with denticulated tips. Neurochaetae compound, with serrated blades and denticulated tips.

Etymology

The species name is derived from the host bivalve genus Thyasira , with the Latin suffix -cola denoting ‘inhabiting’.

Material examined

Holotype

PORTUGAL • 1 af, 1 pf, several mf; Gulf of Cadiz , Carlos Ribeiro mud volcano; 35°47.238′ N, 08°25.272′ W; 2200 m; specimen found inside mantle cavity of Thyasira vulcolutre (Bivalvia, Thyasiridae ); St TTR16_AT615; ANEA NHMUK 2019.7339 • 2 mf; same locality; 1 slide and 1 SEM stub; DBUA 0002275.

Description

Only one fragmented specimen was found, inside the mantle cavity of a bivalve of the species Thyasira vulcolutre Rodrigues & Oliver, 2008 . The anterior fragment has three chaetigers, the posterior nine chaetigers; largest median fragment with seventeen chaetigers, 5.82 mm long. Body elongate, tapering at anterior and posterior ends, arched dorsally and flattened ventrally, rectangular in crosssection ( Fig. 5A ). Colour in ethanol white. Prostomium very short, broadly rounded anteriorly, fused to first chaetiger ( Figs 3A , 4A ). Eyes absent. Paired antennae cirriform, with basal swelling, inserted dorsolaterally ( Figs 3A , 4A ). Palps as low mound structures, inserted ventrally just anterior to mouth ( Fig. 3B ). Pharynx strongly muscularised; jaws not examined (specimen not dissected in order to keep its integrity as holotype). Parapodia much shorter on first segment than on following ones ( Fig. 3A–B ). Dorsal and ventral cirri digitiform, present from first chaetiger. Notopodia clearly present from segment 4, rounded, with straight notoacicula and dorsal cirri inserted distally; neuropodia much longer and conical, neuroacicula with bent tips ( Fig. 4D ), ventral cirri inserted close to base ( Figs 3 C–D, 4B, 5B).

Notochaetae simple, stick-like, laddered basally, with a few rows of small spines subdistally, and ending in blunt and slightly curved tips with several denticles (only visible in SEM; Figs 3E , 5 C–D); absent in first three segments, up to 15 present in median and posterior segments; a single notochaeta in last segments. Neurochaetae compound falcigers, with long, laddered shafts and short, finely serrated blades ( Figs 3 F–G, 4E, 5E–H). Neurochaetae longer in dorsalmost position, gradually shortening ventrally, and present in greater numbers than notochaetae in all segments. Blades of dorsalmost neurochaetae with two rows of longitudinal spines and numerous denticles distally, with a worn appearance under compound microscope ( Fig. 5 E–F); blades of ventralmost neurochaetae with longer spines arranged in two longitudinal rows that are wider apart distally, resembling a spaghetti-spoon ( Fig. 5 G–H). Pygidium button-like, with terminal anus and no appendages ( Fig. 4C ).

Remarks

Carlos Ribeiro is the deepest and most active mud volcano among the five mud volcanoes from where chrysopetalid specimens are recorded in this study. Two species of thyasirid bivalves, Thyasira vulcolutre (3 specimens) and T. obsoleta (Verrill & Bush, 1898) (1 specimen), were collected in the crater of this mud volcano ( Rodrigues et al. 2008), but only one large specimen of T. vulcolutre was infested. This thyasirid species was also reported to occur in higher abundance on Captain Arutyunov MV (37 spms) and Sagres MV (32 spms). These mud volcanoes were also reported as active cold seeps in the deepwater field, but none of the small-sized specimens collected from them (at depths of 1321 m and 1562 m, respectively) were infested. In the Gulf of Cadiz, Thyasiridae are represented by nine species collected from 10 different mud volcanoes, but only two are recognized as chemosymbiotic and both occur in MVs of the Deep Field: Thyasira vulcolutre (Captain Arutyunov, Sagres and Carlos Ribeiro MVs) and Spinaxinus sentosus Oliver & Holmes, 2006 (Captain Arutyunov MV) ( Rodrigues et al. 2013). Another two species are mixotrophic and occur across a wider bathymetric range: Axinulus croulinensis (Jeffreys, 1847) (Mercator, Meknès, Captain Arutyunov, Bonjardim and Porto MVs; 350–3900 m) and Thyasira granulosa (Monterosato, 1874) (Mercator, Meknès, Chechaouen, Darwin and Captain Arutyunov MVs; 350–1321 m) ( Rodrigues et al. 2013 and unpublished data). Except for the large specimen of T. vulcolutre from Carlos Ribeiro MV, all the other thyasirid specimens were not infested.

Ecology and distribution

NE Atlantic (Gulf of Cadiz), on Carlos Ribeiro MV, at a depth of 2200 m. Found in the mantle cavity of the chemosynthesis-based bivalve Thyasira vulcolutre Rodrigues & Oliver, 2008 .

Figures

Fig. 3. Craseoschema thyasiricola gen. et sp. nov. Line drawings. A. Anterior end, dorsal view. B. Prostomium, ventral view. C. Third parapodium, posterior view. D. Median parapodium, posterior view. E. Notochaeta from median parapodium. F. Ventralmost neurochaeta from the second parapodium. G. Dorsalmost neurochaeta from median parapodium.

Fig. 4. Craseoschema thyasiricola gen. et sp. nov. Stereo (A–C) and compound (D–E) microscope images. A. Prostomium, dorsal view. B. Median region, dorsal view. C. Pygidium, ventral view. D. Neuroacicula of a median parapodium. E. Neurochaetae of a median parapodium.

Fig. 5. Craseoschema thyasiricola gen. et sp. nov. SEM images. A. Fragment from median region, posterior view. B. Median parapodium, posterior view. C. Notochaetae of median parapodium. D. Notochaetal tips. E–F. Dorsalmost neurochaetae. G–H. Ventralmost neurochaetae.