Subfamily Autolytinae Langerhans, 1879

Diagnosis (modified from San Martín & Aguado, Reference San Martín, Aguado and Schmidt- Rhaesa2014). Body cylindrical, medium to long in size. Palps fused to prostomium, usually folded ventrally (apparently absent in Acritagasyllis and Levidorum). Three antennae, usually long, extending beyond palps (antennae absent in Levidorum Hartman, 1967; median antenna absent in Acritagasyllis). Four eyes (a genus without eyes, Acritagasyllis) and sometimes two anterior eyespots. Two pairs of tentacular (peristomial) cirri, usually long and thin (single pair in Acritagasyllis, absent in Levidorum). Two nuchal organs as nuchal epaulettes, indistinct in some genera. Pharynx slender, long, winding, with a trepan. Dorsal cirri smooth, sometimes with a distinctive cirrophore (absent in Levidorum; only present on first chaetiger in Procerastea). Ventral cirri apparently absent (clearly present but fused to parapodial lobes in Acritagasyllis). Dorsal simple chaetae bayonet-shaped. Ventral simple chaetae absent. Reproduction by epigamy (in Epigamia), schizogamous gemmiparity (in Myrianida), or by schizogamous anterior scissiparity; stolons dimorphic, with segments grouped in several regions.

Remarks. The subfamily includes 13 genera and about 91 species.

Genus Acritagasyllis Lucas, San Martín & Sikorski, Reference Lucas, San Martín and Sikorski2010

Acritagasyllis Lucas, San Martín & Sikorski, Reference Lucas, San Martín and Sikorski2010, p. 251.

Type species: Acritagasyllis longichaetosus Lucas, San Martín & Sikorski, Reference Lucas, San Martín and Sikorski2010, by monotypy.

Diagnosis (modified from Lucas et al., Reference Lucas, San Martín and Sikorski2010). Body long and slender, dorsally arched, ventrally flattened, white to yellowish, without colour pattern, opaque; anterior segments apparently fused into groups. Prostomium conical to semicircular, eyes absent. Apparently, without median antenna; lateral antennae smooth, 3.5 times longer than prostomium. Two spherical nuchal epaulettes, extending from posterior margin of prostomium to beginning of first chaetiger. Palps absent. Peristomium shorter than subsequent segments. Pair of smooth tentacular cirri, about 2/3 length of lateral antennae. Dorsal cirri smooth, somewhat longer on anterior chaetigers, shorter posteriorly. Cirrophores short, thick. Parapodia bilobed, with pre- and post-chaetal short lobes. Ventral cirri well defined, completely fused to parapodial lobes, with dark, conspicuous, granular material inside. Chaetal fascicles with numerous compound heterogomph chaetae. Shafts distinctly long and slender, with tip enlarged, and numerous small spines, one thicker and longer. Blades slender, elongated, delicate, usually curved and even spiraled, unidentate and smooth on margin. Dorsal simple chaetae thin, bayonet-shape, with filiform tip, on some posterior parapodia. Two slender, straight aciculae in each parapodium. Pygidium small, with two short anal cirri. Pharynx slender, more or less convoluted, through six segments, distally opening in a crown of 8 soft papillae; trepan small, with 9 large teeth, alternating with 2–3 small teeth. Proventricle barrel-shaped, shorter than pharynx, through three segments with about 27–28 muscle cell rows. Reproduction with schizogamy by anterior scissiparity.

Remarks. Only the type-species is known (off Norway).

Acritagasyllis longichaetosus Lucas, San Martín & Sikorski, Reference Lucas, San Martín and Sikorski2010

Acritagasyllis longichaetosus Lucas, San Martín & Sikorski, Reference Lucas, San Martín and Sikorski2010, pp. 251–257, Figures 1 & 2.

Fig. 1.

Trepan of Acritagasyllis longisetosus, surrounded by a crown of papillae. Scale: 20 µm.

Fig. 2.

Male stolon of Acritagasyllis longichaetosus, anterior end, dorsal view. Legend: a.c., central antenna; a. l., lateral antenna; p., palps; c.t.v., ventral tentacular cirri; c.t.d., dorsal tentacular cirri; c.d., dorsal cirrus; n.s., nerurochaetae; N.S., swimming chaetae. Scale: 1 mm.

Material examined. Two specimens (MNCNM 16.01/15335) and two stolons (MNCNM 16.01/15334).

Additional material. Holotype (MNCNM 16.01/11601) and paratype (MNCNM 16.01/11602).

Remarks.

This species is characterized by the absence of palps (probably reduced and fused to prostomium), eyes, and median antenna, presence of a single pair of tentacular cirri and especially by the presence of a kind of compound chaetae unique among syllids, with long, filiform blades, and shafts distally inflated and spinous, more similar to those of the family Phyllodocidae. The examined specimens agree with the original description except for the pharyngeal armature. One specimen was dissected to allow the study of the anterior end of the pharynx; it has a tiny trepan with 9 large teeth, alternating with 2–3 small teeth (Figure 1).

No information about reproduction was included in the original description. We have found two detached reproductive stolons, one male and another female. The male stolon (‘Polybostrichus’, Figure 2) has 23 segments and 4 mm in length, posterior part broken, incomplete. The ‘prostomium’ is bilobed, with two pairs of eyes, one pair in dorsal position slightly larger than the ventral pair, two forked palps, one pair of lateral antennae and one median antenna, all small and globose, but the left one is barely distinguishable. Two pairs of ‘tentacular cirri’, the dorsal pair distinctly longer than the ventral pair. The six anteriormost segments are not modified, but the parapodia have long, swimming chaetae from the seventh segment. Neurochaetae are like those of parental individuals, with shafts distally spinose, and blades long, thin, slender and delicate, smooth on margin. The dorsal cirri are slender and short, most of them lost, only cirrophores remaining. The ventral cirri are completely fused to the parapodial lobes but, in the adults, the cirri are perfectly distinguishable.

The female stolon (Figure 3), has 36 segments, 7 mm in length, ending in a pygidium with two anal cirri. The ‘prostomium’ is bilobed, with two very small palps at the base, hardly visible in dorsal view, two pairs of eyes, one dorsal and another ventral, one pair of lateral antennae and a median antenna, all long and backwards directed. One pair of dorsal ‘tentacular’ cirri. Ventral tentacular cirri slightly smaller in comparison with those of the Polybostrichus. Dorsal cirri of most segments only the cirrophores remaining. The neurochaetae are the characteristic of the species; natatory chaetae absent. Specimen filled with ova inside the coelom, ova about 8 µm in diameter.

Fig. 3.

Female stolon of Acritagasyllis longichaetosus, anterior end, dorsal view. Legend: a.c., central antenna; a. l., lateral antenna; c.t.v., ventral tentacular cirri; c.t.d., dorsal tentacular cirrus, c.d., dorsal cirrus; n.s., nerurochaetae; ov., ova. Scale: 1 mm.

Among the characters originally described, the absence of a trepan and the presence of a middorsal pharyngeal tooth were remarkable features. Observations of these characters were due to mistakes in the examination. In fact, this species has a minute trepan with 9 large alternating with 2–3 smaller teeth, as shown in Figure 1.

Furthermore, the reproduction of this species was unknown up to now. Also, its systematic position was uncertain, presenting some similarities with the subfamily Autolytinae, such as the sinuous pharynx, the shape of the nuchal organs and the bayonet-shaped dorsal simple chaetae. However, the ventral cirri are well defined, although distinctly fused to parapodial lobes.

In this study we have obtained two specimens and two stolons, with the typical sexual dimorphism of the stolons of the subfamily Autolytinae (Nygren, Reference Nygren2004; Aguado & San Martín, Reference Aguado and San Martín2009).

These stolons, however do not exhibit the same characteristics of the appendages of atokous specimens, because they have 3 antennae, which is not the case of the atokous, since the median antenna is absent in all specimens so far examined; also stolons present two pairs of ‘tentacular cirri’, while the atokous individuals seem to have lost one of these pairs.

In order to search about the systematic position of this genus, we have performed a maximum parsimony analysis using binary morphological characters (Table 1). It was not possible to use the specimens for analysis of genetic information because the samples were fixed in formalin.

Table 1.

Morphological characters and character states used in the phylogenetic analysis.

Based on Aguado & San Martín (Reference Aguado and San Martín2009), we have compiled a table with some of the characters that were used by these authors taking into account species representing the five subfamilies of the Syllidae (Appendix 1).

Phylogenetic analysis

The consensus tree is shown in Figure 4. Four of the five subfamilies (Autolytinae, Exogoninae, Syllinae and Anoplosyllinae) have been found clearly monophyletic in accordance with Nygren (Reference Nygren1999), Nygren & Sundberg (Reference Nygren and Sundberg2003), Aguado et al. (Reference Aguado, Nygren and Sidall2007) and Aguado & San Martín (Reference Aguado and San Martín2009).

Fig. 4.

Consensus cladogram showing the systematic position of Acritagasyllis within the Syllidae; numbers at nodes refer to bootstrap support values.

Acritagasyllis, with its single species represented, Acritagasyllis longichaetosus, is clearly located within the subfamily Autolytinae. The main character that relates Acritagasyllis to this subfamily is linked to their reproductive biology: the presence of stolons with sexual dimorphism.

Nygren & Sundberg (Reference Nygren and Sundberg2003), defined the subfamily Autolytinae as clearly separated into three main groups: one with species of Myrianida that reproduce by epigamy, a second group comprising some Myrianida with reproduction by schizogamy by scissiparity, and a third group with Proceraea Ehlers, 1864, Procerastea Langerhans, 1884, and Virchowia Langerhans, 1879, with anterior scissiparity.

With the inclusion of Acritagasyllis in the subfamily Autolytinae, we had to slightly modify the diagnosis of the genus and the subfamily Autolytinae.