Euchoneira knoxi, Licciano, Margherita, Giangrande, Adriana & Gambi, Maria Cristina, 2009

Licciano, Margherita, Giangrande, Adriana & Gambi, Maria Cristina, 2009, A new genus of Sabellidae (Annelida, Polychaeta) from Antarctica, with discussion of relationships among plesiomorphic genera within Sabellinae, Zootaxa 2226, pp. 28-42 : 33-38

publication ID	https://doi.org/ 10.5281/zenodo.190215
DOI	https://doi.org/10.5281/zenodo.6213865
persistent identifier	https://treatment.plazi.org/id/8F2B193E-D710-A664-FF60-FCBF0956FCE5
treatment provided by	Plazi
scientific name	Euchoneira knoxi
status	sp. nov.

Treatment

Euchoneira knoxi View in CoL sp. nov.

Figures 3 View FIGURE 3 , 4 View FIGURE 4

Material examined. Type material: Holotype, on muddy-sand sediment mixed with gravel and small pebbles at 77 m depth off Livingston Island (62° 43.117 S; 60° 42.900 W), Gambi M.C. legit, MNCN 16.01/10650. Paratypes, 186 specimens from the same locality. PCZL 231: 10 specimens, SZN POLY 12: 172 specimens, AM W35683: 4 specimens.

Description. Holotype complete with 8 thoracic and 30 abdominal chaetigers ( Fig. 3 View FIGURE 3 ) of which 9 form an anal depression ( Fig. 4 View FIGURE 4 C, D). Body length 3 cm; maximum width 0.3 cm ( Fig. 3 View FIGURE 3 A, B). Branchial crown length 0.7 cm, branchial lobes each with 16 fully developed radioles with palmate membrane developed for ¾ of length. Flanges present, radioles with filiform tip ( Fig. 3 View FIGURE 3 C). One or two dorsal “pinnular appendages” on the dorsal most radioles basally fused together and separated from remaining pinnules. Basalmost pinnular appendage highly developed and thicker ( Fig. 3 View FIGURE 3 E, F). Dorsal lips rounded, extending from inner, dorsal margin of branchial lobes and terminating just dorsal to mouth, without radiolar appendages ( Fig. 3 View FIGURE 3 E). Ventral lips rounded, with up to 16 ventral radiolar appendages about same length of branchial crown ( Fig. 3 View FIGURE 3 E). Posterior peristomial ring collar high, slightly decreasing in length dorsally, forming two deep and thick pouches terminating at the end of the second chaetiger and leaving visible the junction between the crown and the thorax ( Figs 3 View FIGURE 3 A, 4A, B). Mid dorsal collar attached to the peristomium in its uppermost portion. Anterior peristomial ring with two small C-shaped depressions situated just above pouches dorsally ( Fig. 4 View FIGURE 4 A).

Glandular ridge on chaetiger two. Green methyl staining pattern visible only on ventral shields both in the thorax and abdomen, extending laterally only on collar ( Fig. 3 View FIGURE 3 D); ventral shields visible also without colouration ( Fig. 3 View FIGURE 3 B). Notopodial fascicle from first chaetigers bearing 35 elongate narrowly hooded chaetae arranged as C-shape; chaetigers 2–8 with superior group of 10 elongate narrowly hooded chaetae ( Fig. 4 View FIGURE 4 E) and an inferior group with 25 broadly hooded elongated chaetae posteriorly ( Fig. 4 View FIGURE 4 F) and 10 bayonet type chaetae anteriorly. About 40 neuropodial uncini per torus in chaetigers 2–8, with very long handle and small teeth of similar size over main fang ( Fig. 4 View FIGURE 4 G). Abdominal neuropodial fascicles with modified, elongate chaetae ( Fig. 4 View FIGURE 4 H). Notopodia with 45 avicular uncini, with main fang surmounted by rows of small teeth, breast well developed and rounded, handles present ( Fig. 4 View FIGURE 4 I). Variation within a torus absent. Uncini of anal depression slightly smaller ( Fig. 4 View FIGURE 4 L).

Remarks. Characteristic of this species is the shape of the collar with the presence of very thick pouches extending to chaetiger two. Among the prostomial structures the dorsal filaments, here referred as “pinnular appendages”, appears unique with a mid rib present. However, the appendage closest to the dorsal lip is not fused to it indicating that it is not a dorsal radiolar appendage. The abdominal uncini are very similar to those present in the genus Jasmineira , but different from those occurring in Claviramus . As in many species of Euchone , the uncini are slightly different in the pre- and depression regions of the abdomen. Lastly, it is possible that the structure observed as a circular depression on the peristomium are actually the structures detected in Fabrisabella , and in some Euchone and Jasmineira species, where they have been referred to as coils or vascular loops. Detailed histological analysis could clarify this feature.

The collected population (187 specimens) showed a body length (crown excluded) ranging from 20 to 40 mm, while width of the 2nd chaetiger ranged between 2.0 to 3.8 mm. The species is gonochoric, and population structure showed for both males (97 specimens) and females (86 specimens) a modal class at 3.4 mm width. Females prevailed in smaller size classes (from 2 to 3 mm width), while males were more abundant in slightly larger size classes (from 3 to 3.8 mm width) ( Fig. 5 View FIGURE 5 A). Egg diameter ranged from 100 to 225 µm in the abdomen and from 100 to 250 µm in the thorax. Larger eggs were, however, more abundant in the thorax, although, the modal class was 175 µm in both the regions ( Fig. 5 View FIGURE 5 B). Mature spermatozoa appear basically similar to those of E. pallida , with a long cylindrical nucleus capped by a conical, “nip-like” acrosome and a number of 3–4 mitochondria around the axoneme ( Fig. 2 View FIGURE 2 C, D).

All specimens were collected inside their tubes which were formed by a thin layer of mucus and fine, muddy sediment, heavily encrusted with coarse gravel.

Type locality: Livingston Island

Etymology: The species is dedicated in memory of Prof. George A. Knox (University of Canterbury, New Zealand), a leading authority of the Antarctic research at sea and a pioneer of polychaete studies in the Southern Ocean.

Cladistic analysis. The matrix was computed on the basis of type descriptions. Characters and states utilized in the analysis are listed in the Appendix. Features of the new described genus were included in a matrix (Table 1) computed on the basis of the data set previously utilized by Tovar-Hernández (2008). However, for the present analysis some changes have been made with the exclusion of some features which have been particular useful in defining taxa within the genus Chone , especially concerning the peristomium and collar features, and some additional characters regarding the shape of radiolar tips, and the handles of anterior abdominal uncini were added. For each genus only two or three species of the previous analysis, including the type species, were maintained, with the monospecific species Fabrisabella vasculosa , two species of the genus Claviramus and Euchone pallida being added.

The lip-associated structures are still the object of a great deal of confusion, as there is little useful histological studies on which to infer homologies ( Orrhage, 1980; Fitzhugh, 2003; Bick and Randal, 2005; Tovar-Hernández and Sosa-Rodríguez, 2006). As already pointed out, dorsal radiolar appendage assessment has produced a lot of misinterpretation both in Euchone and Chone . For instance Cochrane (2003), in her cladistic analysis considered Euchone as lacking this structure; in contrast Bick and Randal (2005) reported based on an histological analyses the presence of skeleton in the radiolar appendage of E. analis . However, Fitzhugh (2003) stated that in these plesiomorphic genera the radiolar appendages never contain skeleton support as in Euchone , but without branchial skeleton extension. The same conclusion was reached by Giangrande and Licciano (2006). Tovar-Hernández and Sosa-Rodríguez (2006) re-describing the type material of C. infundibuliformis stated the absence of mid-rib or dorsal radiolar appendages in the dorsal lips for the genus Chone and the absence of radiolar skeleton support. Tovar-Hernández (2008) considered Chone sensu stricto with rounded dorsal lips lacking any branchial skeleton extension, and similarly Dialychone and Paradyalichone with elongated dorsal lips lacking any branchial skeleton extension, and Euchone as having elongate lips with branchial skeleton extension. Following Tovar-Hernández (2008), in our analysis three states were considered for dorsal lips: broadly rounded, elongate without skeleton extension and elongate with skeleton. Radiolar appendages in Euchone are always less developed, and the dorsal lips appears more rounded and not as elongate as in some species of Dialychone .

The “dorsal pinnular appendages” also need to be better defined. Although histological analysis available in Orrhage (1980) on the genus Chone , shows structures resembling pinnules, the structures present in Chone , Paradialychone , Euchone and Euchoneira , which we defined as pinnular appendages, appear different from the pinnular appendages found in other Sabellinae , where they are fused to dorsal lips. In our material of E. pallida and Euchoneira the basalmost pinnula seems to be completely separated from the dorsal lip. Moreover in Euchoneira this structure seems to show a radiolar support (mid-rib). This is in accordance with Tovar- Hernández and Sosa-Rodríguez (2006) observations, who refer to the presence in C. infundibuliformis of several pinnular appendages united by a palmate membrane, but with the external pinnula of each dorsalmost radiole separated from the dorsal lips and containing a central blood vessel surrounded by the coelom, associate with the pinnular skeleton and hyaline cartilage. Further histological analysis conducted on more species will clarify the homologies among plesiomorphic and apomorphic genera as well as the terminology.

The analysis produced only 2 trees (tree length: 71 steps). The following descriptive indices were obtained CI= 0.66, RI= 0.76, and RCI= 0.50, where CI is the consistency index, RI the retention index, and RCI the rescaled consistency index.

The consensus tree ( Fig. 6) confirmed A. armandi as the most plesiomorphic taxon separated from all the others. The new genus Euchoneira is located close to F. vasculosa , in an intermediate position between Jasmineira - Claviramus and the clade containing Chone ( Paradialychone ( Dialychone ( Euchone ))) species.

The shape of dorsal lips with radiolar appendages (2) seems to be homoplasious in Euchone and Claviramus , while Euchoneira shares the complete absence of this structure and rounded dorsal lips with Chone sensu stricto. The presence of dorsal pinnular appendages (3) defines Chone , Paradialychone , Euchone and Euchoneira species, but these structures are absent in Dialychone , which also lacks the ventral radiolar appendages (4). The presence of anal depression with lateral wings (24) in the new genus is homoplasious with Euchone , as well as well differentiated ventral shields (15) and biannulate segments (16). By contrast the breast of the anterior abdominal uncini (20) is a synapomorphy defining the clade Jasmineira ( Claviramus ( Fabrisabella ( Euchoneira ))). Among them Claviramus is defined by expanded foliaceous tip of radioles and by the absence of handles in the anterior abdominal uncini (7 e 21), Jasmineira by the presence of an abscission zone in the branchial crown (9), and Euchoneira by the mid-dorsal collar attached to the peristomium (11), which represents the apomorphy for the new genus. It must be stressed that the whole collar morphology of E. knox i with deep pouches appears very peculiar.

It is interesting to note that Euchone pallida differs from the other examined species of the genus in the presence of modified abdominal uncini in the posterior end (19) which was used by Tovar-Hernández (2008) in distinguishing Dialychone and Paradialichone from Chone sensu stricto, it is therefore suggested that this feature, which was not considered by Cochrane (2003), could be useful also in Euchone phylogeny.

In conclusion, according to the present analysis the new genus appears morphologically closely related to F. vasculosa with an intermediate position between the clade containing Jasmineira and that containing Euchone . The spermatozoa morphology of E. knoxi , and especially the shape and development of the acrosome, closely resembles more that of E. pallida than that of Jasmineira sp. ( Rouse, 1999), although in Euchoneira the nucleus is slender, the tip of the acrosome more pronounced, and the sub-acrosomal space slightly larger than that of E. pallida .