Euchone, Malmgren, 1866

Giangrande, Adriana & Licciano, Margherita, 2006, The genus Euchone (Polychaeta, Sabellidae) in the Mediterranean Sea, addition of two new species and discussion on some closely related taxa, Journal of Natural History 40 (21 - 22), pp. 1301-1330 : 1313-1316

Euchone (Chiade sensu Cochrane) sp.

( Figure 6 )

as Euchone southerni in Giangrande (1989).

Examined material

One specimen from Brindisi, 40 m depth.

Description

A small-sized taxon measuring 2 mm in length with eight thoracic chaetigers and 10 abdominal ones of which four form the anal depression ( Figure 6A–C ). Four pairs of flanged radioles per lobe, with few pinnules arranged alternately and longest in the midradiolar region (snowflake organization). Collar well developed with a narrow dorsal gap and slightly higher ventrally, margin entire ventrally and dorsolaterally crenulated ( Figure 6D, E ). Dorsal and ventral lips not observed. Ventral lobe of anterior peristomial ring covered by collar margin. Notopodia in chaetiger 1 with five narrowly hooded chaetae. Notopodial fascicle from chaetigers 2–8 with superior group of four elongated narrowly hooded chaetae ( Figure 6H ) and inferior group with three paleate chaetae posteriorly and three bayonet-type anteriorly. Paleate chaetae narrow with long tip ( Figure 6H ). Neuropodial uncini six to seven per torus, with teeth of similar size over the main fang, main fang short and blunt ( Figure 6I ). Abdominal neuropodial fascicles with four elongate narrowly hooded chaetae. Notopodia with seven avicular uncini, with main fang surmounted by four rows of small teeth ( Figure 6L ). Intratorus variation absent. Anal depression formed by four chaetigers with uncini more quadrangular in shape (raspshaped) ( Figure 6M ). Anal depression with very developed membranous frilly margin and a medial gap forming a pair of elongate flaps ( Figure 6F, G ).

Habitat

Coralligenous deposits.

Remarks

The paucity of Mediterranean material (only one specimen) did not allow description at the specific level, additional material is needed especially for the examination of crown internal structure.

The Mediterranean specimen represents an important record because, taking into consideration the structure of the branchial crown and the low number of chaetigers (four) forming the anal depression, it could belong to the Chiade group. Although Cochrane (2003) reports the genus having only three chaetigers in the anal depression, the species E. trilobata (Banse, 1957) has four chaetigers as well. The same statement can be inferred also for the number of radioles, since Cochrane (2003) reports the genus with only three pairs, whilst E. hancocki has four pairs ( Banse 1970) as well as the Mediterranean specimen. The Mediterranean taxon is very similar to E. trilobata also in the shape of the anal depression, but it differs in the branchial crown structure without flanges. The peculiarity of the crown and the presence of rasp-shaped uncini in the abdomen (a feature present in other small Euchone species ), led Banse (1970) to describe the species as belonging to Desdemona genus.

The main difference between the Mediterranean specimen and the other Euchone species belonging to the Chiade group is in the absence of rasp-shaped abdominal uncini.

A peculiar shape of thoracic uncini is instead present in this taxon as well as in some small Euchone species. Based on Banse’s drawing (1970), this type of uncinus is surely present in E. hancocki and E. incolor , and probably in E. trilobata , placed by Cochrane (2003) in the Chiade clade.

The recognition of this peculiar shape of the thoracic uncinus leads to some considerations: the genera Euchone and Chone , having thoracic uncini with long handles, have always been considered as having teeth of similar size over the main fang (type 1, Figure 7A ). This last feature distinguished these genera from Amphicorina , having teeth of unequal size above the main fang (type 4, Figure 7D ). However, taxa with teeth of unequal size above the main fang (type 3, Figure 7C ) can be found also in Euchone (see E. pseudolimnicola ) and Chone ( Tovar-Hernández 2005) . The difference in the thoracic uncini among Euchone , Chone , and Amphicorina must be referred not only to the dentition pattern above the main fang, but also to other features such as the shape and length of the main fang, the orientation of the handle, and development of the breast ( Figure 7E, F ).

The uncinal type found in the Mediterranean small Euchone specimen is intermediate between Chone – Euchone and Amphicorina . These uncini (type 2, Figure 7B ) have teeth of similar size over the main fang, but the main fang is shorter and blunt in comparison to that found in Chone and Euchone , and more similar to the Amphicorina type. The ratio between length of the main fang and total length of uncinal head is 0.2 in small Euchone and Amphicorina and 0.4 in Chone and Euchone . This is revealed especially by the SEM analysis ( Figure 7G, H ).

Thoracic uncini type 2 are also present in another Mediterranean taxon, which in the present paper is ascribed to the genus Chone and discussed below.

Figures

Figure 6. Euchone (Chiade) sp. (A) Entire worm, dorsal view; (B) entire worm, ventral view; (C) entire worm, lateral view; (D) collar, ventral view; (E) collar, dorsal view; (F) anal depression, dorsal view; (G) anal depression, ventral view; (H) thoracic superior notochaeta and thoracic inferior paleate chaeta; (I) thoracic uncinus; (L) abdominal uncinus from an anterior abdominal chaetiger; (M) abdominal uncinus from chaetigers of the anal depression.

Figure 7. Comparison of thoracic uncini structure in some related sabellin genera having long handle. (A) Type 1 (from Chone duneri); (B) type 2 (from Chone sp.); (C) type 3 (from Chone americana); (D) type 4 (from Amphicorina armandi); (E) scheme of handle orientation found in uncini of types 1–3; (F) scheme of handle orientation found in uncini of types 2–4; (G) SEM photograph of uncini type 1; (H) SEM photograph of uncini type 2.