Anguillosyllis Day, 1963

Neal, Lenka, Drennan, Regan, Wiklund, Helena, Stewart, Eva C. D., Rabone, Muriel, Dahlgren, Thomas G. & Glover, Adrian G., 2025, New species of Anguillosyllis Day, 1963 (Annelida, Syllidae) from polymetallic nodule exploration areas, eastern Clarion-Clipperton Zone, central Pacific Ocean, European Journal of Taxonomy 1026, pp. 30-64 : 36-40

Genus Anguillosyllis Day, 1963

Type species

Anguillosyllis capensis Day, 1963 .

Diagnosis (amended from Maciolek 2020)

Body very small, adults with limited and fixed number of chaetigers (8–11). Palps elongated, free to the base or fused partly to completely. Prostomium with three antennae, without eyes. Peristomium with one pair of tentacular cirri similar to or smaller than prostomial antennae. Parapodia uniramous, with anterior and posterior lobes developed to varying degrees; with superior (dorsal) lobe that may be contractile. Dorsal cirri may be absent on a variable number of chaetigers, with true absence on chaetiger 2 or on all chaetigers except chaetiger 1. Compound chaetae heterogomph, with falcigers and spiniger-like blades. Falcigers uni- or bidentate. Pharynx straight, eversible, with two (or three) crowns or sections, external one formed by pharyngeal sheath, distal one surrounded by several (10–12) soft papillae, tooth absent. Proventricle cylindrical, usually tapered posteriorly, muscle rows obscure; with associated glandular structure wrapped around post-ventricle. Pygidium with four cirri, two lateral, two ventromedial.

Remarks

While the distinct look of Anguillosyllis (elongated palps, low number of chaetigers) might have contributed to the lumping of similar species in the past, the use of molecular techniques has revealed a far greater diversity within this genus than previously thought ( Drennan et al. 2025). Recent taxonomic work reported on characters that could help differentiate between different species ( Aguado & San Martín 2008; Barroso et al. 2017; Maciolek 2020). Most recently, Drennan et al. (2025) revealed novel and previously overlooked characters, best observed with the use of SEM, such as a complex form of prostomium, and absence of dorsal cirri on all but chaetiger 1. Here we rely predominantly on the characters used by Maciolek (2020), supplemented with SEM observations where possible. We caution against reliance on the characters such as size and shape of antennae or dorsal cirri, as these can differ even within a single specimen (Neal pers. obs.), likely due to preservation.

Update on previously reported characters

Palpal length can range from short (defined as similar in length to prostomium), to elongate (equaling multiple lengths of prostomium, usually no more than twice the length of prostomium), while the shape of palps can be pointed, conical or finger-like.

Prostomial antennae can vary in size (relative to prostomium and lateral antennae relative to median antenna) and in shape being papilliform, digitiform, club-shaped, cirriform or ovate, although caution is needed as stated above.

Tentacular cirri can range in size and shape similar to diversity of shape displayed by the antennae. Parapodia themselves can differ in length, and in shape, size and development of the associated parapodial lobes/projections. Ventral cirri showed some variation in length, thickness and the point of insertion on parapodium (median or subdistal).

Presence/absence of dorsal cirri on chaetiger 2 was discussed by Barroso et al. (2017) and Maciolek (2020) as an important character. Although dorsal cirri on chaetiger 2 are only rarely confirmed (they may be either absent or lost), the type species of the genus, A. capensis was originally described as bearing dorsal cirri on all chaetigers ( Day 1963). However, the presence of the dorsal cirri on chaetiger 2 was not confirmed by examination of the holotype NHMUK. 1963.1.29, which is in extremely poor shape ( Fig. 2). Maciolek (2020) reported the presence of dorsal cirri on chaetiger 2 on two specimens of A. palpata ( Hartman, 1967) and one specimen of A. hampsoni Maciolek, 2020 , with basal cirrophores also observed on chaetiger 2 in A. truebloodi . No specimens examined from UKSR material in the present study or previous study ( Drennan et al. 2025) bore dorsal cirri on the second chaetiger.

True absence of dorsal cirri in other chaetigers. The true absence of dorsal cirri on chaetiger 2 has already been discussed above. Interestingly, SEM revealed true absence in all chaetigers but chaetiger 1 in Anguillosyllis dalgleishae sp. nov. This character state was already reported by Drennan et al. (2025) in Anguillosyllis cf. hessleri from the CCZ. Previously, based solely on examination using light microscopy, Maciolek (2020) suggested that in the following species the dorsal cirri are “missing” in all but chaetiger 1: Anguillosyllis acsara Maciolek, 2020 , A. hadra Maciolek, 2020 , A. taleola Maciolek, 2020 , A. elegantissima Maciolek, 2020 , A. enneapoda Maciolek, 2020 , A. hessleri , A. inornata Maciolek, 2020 and A. sepula Maciolek, 2020 .

Prostomium has been given little attention in previous descriptions, usually described in simple terms such as dome-shaped, rounded, pentagonal, longer than wide, as long as wide etc. Based on observations from light microscopy, Maciolek (2020) reported that three species A. carolina Maciolek, 2020 , A. denaria Maciolek, 2020 and A. taleola appear to have the prostomium in two parts. Our SEM observations showed that the prostomium is a complex structure composed of superimposed lobes of variable shapes, which may potentially differentiate species (see also Drennan et al. 2025).

Chaetae. Chaetae were previously considered falcigerous or spiniger-like, with variable length of blades, development of serration and number per chaetiger. Previously, only A. palpata was considered to have unique distinctly hooked chaetae ( Maciolek 2020), with slightly hooked tips reported in A. truebloodi and A. hampsoni by Maciolek (2020). Our observations confirmed the presence of bidentate chaetae in Anguillosyllis villarae sp. nov. for the first time in this genus. Additionally, chaetal blades in Anguillosyllis can vary in thickness from narrow to flat and wide. The shafts of falcigerous chaetae in several anterior chaetigers in some species have been found to be denticulated, with denticles arranged in horizontal rows.

Internal glands. Maciolek (2020) reported on the shape and size of internal glands within parapodia, but we have found these difficult to observe and quantify. A stained circle of cells post-ventricle was observed when specimens were stained with Methyl Green (Neal pers. obs.).