Abstract

Introduction

Syllids are small to medium-sized polychaetes (from 2–3 mm long and 15–30 chaetigers, up to about 140 mm and 200 chaetigers). They are extremely abundant and diverse in benthic marine shallow habitats and also inhabit deep areas; however, they are absent from fresh water and are not an important group in estuaries. They are very common on hard substrata, having an errant life among algae, biogenic structures, crevices, within porous rocks, etc. and they also inhabit marine sediments, especially coarse sand, where most species have an interstitial lifestyle. Also, numerous species are associated with other marine organisms, especially sponges and octocorals, mostly in tropical waters.

As syllids may constitute more than 50% (sometimes more than 70%) of the polychaete species that live in some substrata, they are very important in benthic studies. However, because of their small size, they are often overlooked, since most benthic ecology studies are devoted to macrofauna. Furthermore, they are difficult to identify because of their small size and the lack of taxonomic studies and monographs with keys and detailed descriptions for many areas. Syllids are very easy to recognize to family level, because they have a conspicuous modification of the gut, the proventriculus (=proventricle), which constitutes the autapomorphy of the family. The taxonomy and systematics are also complex and difficult, again because of their small size, numerous taxa (approximately 74 genera and 700 species), and the difficulty to correctly observe the characters. This paper is directed to participants of the NMBAQC Scheme and to all laboratory staff and students who need to familiarize themselves with the syllid fauna that may be found in benthic studies from British or Irish waters. Since the workshop, the keys have been modified and completed with the species identified during the workshop, many of which are not yet formally reported in the area. We have included reference numbers (in brackets after each species) to recommended descriptions cited in the references. Comparison of specimens with descriptions and figures is highly recommended. Also, it is necessary to note that fixed specimens lose their pigmentation after some time, and also that young, small specimens have appendages proportionally shorter than large, mature specimens. Also, note that the taxonomy and systematics are not yet completed and some changes and additions are probable in future years. Some genera need careful revision, and some species are only tentatively included in a particular genus, since they do not fit perfectly with the diagnosis of that genus.

Howson and Picton (1997) listed the following species as likely to be found in British water, which are herein arranged according to recent classifications (Aguado and San Martín 2009; Aguado et al. 2007, 2012; Nygren 2004; San Martín and Aguado 2014):

Subfamily Anoplosyllinae Aguado & San Martín, 2009: Streptosyllis bidentata Southern, 1914; Streptosyllis websteri Southern, 1914; Syllides benedicti Banse, 1971; Syllides longocirrata Ørsted, 1845.

Subfamily Eusyllinae Malaquin, 1893: Eusyllis assimilis Marenzeller, 1875; Eusyllis blomstrandi Malmgren, 1867; Eusyllis lamelligera Marion & Bobretzky, 1875; Nudisyllis divaricata (Keferstein, 1862); Nudisyllis pulligera (Krohn, 1852); Odontosyllis ctenostoma Claparède, 1868; Odontosyllis fulgurans (Audouin & Milne-Edwards, 1833); Odontosyllis gibba Claparède, 1863; Opisthodonta longocirrata (Saint-Joseph, 1886); Pionosyllis compacta Malmgren, 1867; Synmerosyllis lamelligera (Saint-Joseph, 1886).

Subfamily Exogoninae Langerhans, 1879: Brania pusilla (Dujardin, 1851); Erinaceusyllis erinaceus (Claparède, 1863); Salvatoria clavata (Claparède, 1863); Salvatoria limbata (Claparède, 1868); Salvatoria swedmarki (Gidholm, 1962); Exogone dispar (Webster, 1879); Exogone naidina Ørsted, 1845; Exogone verugera (Claparède, 1868); Parexogone longicirris (Webster & Benedict, 1887); Parexogone hebes (Webster & Benedict, 1884); Prosphaerosyllis tetralix (Eliason, 1920); Sphaerosyllis bulbosa Southern, 1914; Sphaerosyllis hystrix Claparède, 1863; Sphaerosyllis pirifera Claparède, 1868; Sphaerosyllis taylori Perkins, 1980.

Subfamily Syllinae Grube, 1850: Eurysyllis tuberculata Ehlers, 1864; Haplosyllis spongicola (Grube, 1855); Syllis amica Quatrefages, 1866; Syllis armillaris (O.F. Müller, 1771); Syllis cornuta Rathke, 1843; Syllis gracilis Grube, 1840; Syllis garciai (Campoy, 1981); Syllis hyalina Grube, 1863; Syllis krohnii Ehlers, 1864; Syllis prolifera Krohn, 1852; Syllis variegata Grube, 1860; Syllis vittata Grube, 1840; Trypanosyllis coeliaca Claparède, 1868; Trypanosyllis zebra (Grube, 1860).

Subfamily Autolytinae Langerhans, 1879: Epigamia alexandri (Malmgren, 1867); Myrianida brachycephala (Marenzeller, 1874); Myrianida edwarsi (Saint-Joseph, 1886); Myrianida inermis (Saint-Joseph, 1886); Myrianida langerhansi (Gidholm, 1967); Myrianida pinnigera (Montagu, 1808); Myrianida prolifera (O.F. Müller, 1788); Myrianida quinquedecimdentata (Langerhans, 1884); Myrianida rubropunctata (Grube, 1860); Proceraea aurantiaca Claparède, 1868; Proceraea cornuta (Agassiz, 1862); Proceraea picta Ehlers, 1864; Proceraea prismatica (O.F. Müller, 1776); Procerastea halleziana Malaquin, 1893; Procerastea nematodes Langerhans, 1884.

Incertae sedis:Amblyosyllis formosa (Claparède, 1863); Dioplosyllis cirrosa Gidholm, 1962; Palposyllis prosostoma Hartmann-Schröder, 1977; Paraehlersia ferrugina (Langerhans, 1881); Streptodonta pterochaeta (Southern, 1914).

Another 18 syllid taxa were also reported, but they are synonyms of other species, invalid, or doubtful species, or even not recognized as Syllidae.

This number of species is quite low for such an area and it is certain that many other species live in British waters. In the keys below, we have included all previously reported species (excluding invalid or doubtful ones) plus those that have been reported from nearby areas of the NE Atlantic and that could be also present in the study area. Some of these were noted at the NMBAQC workshop or since that time but are not yet formally recorded. It is important to remember the possibility that other species, not in the keys presented here, may yet be found in the area and reference should be made to additional literature for any specimens that do not fit descriptions. Books with keys for syllids of nearby areas include those by Fauvel (1923) (France), Hartmann-Schröder (1996) (Germany), and San Martín (2003) (Iberian Peninsula). A previous NMBAQC workshop (2006) included work on syllids led by Peter Garwood but the resulting key was not published or circulated via the website. Recently, Dietrich et al. (in press) revised the Autolytinae from the area (North Sea and NE Atlantic). Their results are followed here; we strongly recommend use of these keys as a complement to ours for that subfamily.

Main morphological characters

Body

Cylindrical in section (Fig. 1A, B, E, F), but may be flattened, ribbon-like (Fig. ​(Fig.1C).1C). The surface is smooth (Fig. 1A–C, F), but may also bear papillae on the dorsal (Fig. ​(Fig.1E)1E) and ventral surface, and on the parapodia. Some bear rugosities, tubercles, rows of cilia, etc.

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Figure 1.

Anterior end of: A Syllis amica (SF. Syllinae), body cylindrical, smooth surface, two pairs of tentacular cirri, antennae, tentacular and dorsal cirri moniliform, nuchal organs as ciliated pits, palps basally fused B Nudisyllis pulligera (SF. Eusyllinae), body cylindrical, smooth surface, two pairs of tentacular cirri, antennae, tentacular and dorsal cirri smooth, nuchal organs as ciliated pits, palps free C Trypanosyllis coeliaca (SF. Syllinae), body flattened, smooth surface, two pairs of tentacular cirri, antennae, tentacular and dorsal cirri moniliform, nuchal organs as ciliated pits, palps free (see figure D) D Same species, prostomium in ventral view E Sphaerosyllis pirifera (SF. Exogoninae), body cylindrical, papillated surface, single pair of tentacular cirri, antennae, tentacular and dorsal cirri smooth and short, nuchal organs as ciliated pits, palps totally fused F Myrianida convoluta (SF. Autolytinae), body cylindrical, smooth surface, two pairs of tentacular cirri, antennae, tentacular and dorsal cirri smooth, nuchal lappets, palps totally fused, pharynx coiled.

Prostomium

Semicircular to pentagonal or oval and has four eyes and, sometimes, also a pair of ocular spots, three antennae, which may be smooth (Fig. 1B, E, F) or articulated (also known as moniliform) (Fig. 1A, C), short or long, and one pair of palps, triangular in shape, rounded or oval, that may be fully separated from each other (Fig. ​(Fig.1D),1D), basally fused or fused along their entire length (Fig. ​(Fig.1E1E).

Tentacular (= peristomial) cirri

Usually two pairs (Fig. 1A–D, F), but in some genera only one pair (Fig. ​(Fig.1E),1E), or absent, which may be smooth (Fig. 1B, E, F) or articulated (moniliform) (Fig. 1A, C, D), short or long.

Nuchal organs

Most commonly as ciliated pits (the most common) but also as nuchal lappets (nuchal epaulettes) (Fig. ​(Fig.1F1F).

Parapodia

Uniramous (except on some segments, during reproduction), with dorsal cirri, parapodial lobes, ventral cirri, chaetae, and aciculae (Fig. 2A–D).

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Figure 2.

Lateral view of parapodia of: A Syllis amica, dorsal cirrus moniliform and long B Nudisyllis pulligera, dorsal cirrus smooth and long C Parapionosyllis brevicirra (SF. Exogoninae), dorsal cirrus smooth and short D Epigamia labordai (SF. Autolytinae), dorsal cirrus smooth, short, without ventral cirrus E anterior end, dorsal view of Syllides fulvus (SF. Anoplosyllinae), without any pharyngeal armature F trepan, without middorsal tooth of Myrianida convoluta (SF. Autolytinae) G everted pharynx of Eusyllis assimilis (SF. Eusyllinae), showing an incomplete trepan and middorsal tooth H anterior end of Odontosyllis fulgurans (SF. Eusyllinae), with an incomplete trepan, teeth directed to posterior part of body.

Dorsal cirri

May be long or short, alternating between long and short, smooth (Figs 1B, E, F, 2B–D) or moniliform (Figs 1A, C, ​,2A).2A). Typically filiform, but may be of different shapes.

Ventral cirri

Present, except in the subfamily Autolytinae, in which they appear to be absent (Fig. ​(Fig.2D)2D) but are in fact fused to parapodial lobes.

Pharynx

Usually straight, but coiled in some genera, sometimes very slender and complex (Fig. ​(Fig.1F1F).

Pharyngeal armature

Absent in the subfamily Anoplosyllinae (Fig. ​(Fig.2E),2E), but most often as a single pharyngeal tooth, or as a crown of denticles on the pharyngeal opening, i.e. the trepan, with (Fig. ​(Fig.2G)2G) or without a pharyngeal tooth (Fig. ​(Fig.2F).2F). The trepan may be complete or incomplete, and the denticles may be directed to the anterior or posterior parts of the body (Fig. ​(Fig.2H2H).

Proventriculus (= Proventricle)

Rectangular, squared or barrel-shaped. Size (number of segments) and number of muscle cell rows vary between species.

Chaetae

Typically, compound heterogomph, with capillary dorsal and ventral simple chaetae on posterior parapodia but many modifications may occur. Some may be elongated and similar to the spinigers of nereidids, known as spiniger-like (Fig. ​(Fig.3D),3D), or pseudospingers. Falcigers usually bidentate, with both teeth similar (Fig. ​(Fig.3B),3B), the proximal teeth either smaller than the distal (Fig. ​(Fig.3F)3F) or larger (Fig. ​(Fig.3C).3C). Blades may also be unidentate (Fig. ​(Fig.3A).3A). The blades may be smooth, or have a row of marginal spines, which may be long (Fig. ​(Fig.3E)3E) or short (Fig. 3C, F).

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Figure 3.

Compound chaetae of A Sphaerosyllis pirifera (falciger, unidentate, almost smooth on margin) B Trypanosyllis coeliaca (falciger, bidentate with both teeth similar, moderate spines on margin) C Eusyllis assimilis (falciger, bidentate, proximal tooth longer than distal one, short spines) D Syllis garciai (spiniger-like, long spines on margin); E Syllis garciai (falciger, bidentate, both teeth similar, long spines) F Syllis krohnii (falciger, bidentate, proximal tooth shorter than distal one, short spines on margin) G Syllis amica (thick simple chaeta by blade loss and shaft enlargement) H Syllis gracilis (thick simple chaeta by blade and shaft fusion). Aciculae of: I Trypanosyllis coeliaca (straight, pointed) J Eusyllis assimilis (distally bent at an angle) K Syllis gracilis (acuminate) L Syllis prolifera (distally rounded).

Sometimes, there may be thick simple chaetae due to the loss of blades and enlargement of shafts (Fig. ​(Fig.3G)3G) or by fusion of blade and shaft (Fig. ​(Fig.3H).3H). The capillary dorsal and ventral simple chaetae are usually very slender, bifid or entire, with or without subdistal spines. Typically, these capillary simple chaetae are present only on posterior parapodia.

Aciculae

Numerous different kinds of tips may be present: straight and pointed (Fig. ​(Fig.3I),3I), acuminate (Fig. ​(Fig.3K),3K), bent at an angle (Fig. ​(Fig.3J),3J), distally rounded (Fig. ​(Fig.3L),3L), and other variations.

Reproduction

There are two main reproductive strategies in syllids: Epigamy and Schizogamy.

Epigamy in syllids is quite similar to that of other polychaetes but long, slender notochaetae appear for swimming (natatory chaetae) (Fig. ​(Fig.4A)4A) in some parapodia (from the mid-body backwards). There are two kinds of epigamy: without brooding or with brooding eggs. Brooding eggs may be dorsal (attached by capillary notochaetae) or ventral (attached to nephridial openings). In the latter, juveniles grow attached to mother’s body.

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Figure 4.

A epigamic male of Exogone naidina. Anterior end of stolons B acephalous (male, still attached to parental), Haplosyllis spongicola C acerous (male), Trypanosyllis zebra D dicerous (male), Syllis prolifera E dicerous (female, still attached to parental), Syllis prolifera F tetracerous (male), Syllis pulvinata G tetracerous (female), Syllis pulvinata H pentacerous (male), Syllis hyalina I pentacerous (female), Syllis hyalina. All, dorsal view, except H ventral view.

Schizogamy by means of sexual stolons. Stolons are detached individuals budded off from the adult, without gut, composed of few segments, filled with gametes: a short life, purely for reproduction. There are two kinds of schizogamy: scissiparity (formation of a single stolon) and gemmiparity (formation of a chain of stolons) (Fig. 5A, B).

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Figure 5.

A, B Chain of stolons (Myrianida spp.) C Polybostrichus D Sacconereis, with brooding ventral sac E Sacconereis.

Stolons of the Syllinae have no sexual dimorphism, but are easily distinguished because males store spermatozoa and females store oocytes; there are different kinds of stolons: acephalous (without ‘head’) (Fig. ​(Fig.4B),4B), acerous (=‘Tetraglene’) (a ‘head’ without appendages, and with two pairs of eyes) (Fig. ​(Fig.4C),4C), dicerous (= ‘Chaetosyllis’) (a bilobed ‘head’ with two pairs of eyes and two antennae) (Fig. 4D, E), tetracerous (a ‘head’ with two palps and two antennae) (Fig. 4F, G), pentacerous (=‘Ioida’) (a ‘head’ with two pairs of eyes, three antennae, and two palps) (Fig. 4H, I).

Stolons of the Autolytinae have marked sexual dimorphism. Male stolons (‘Polybostrichus’) have a ‘head’ with two pairs of eyes, two bifid, elongated palps and three antennae, the median one long and spiral (Fig. ​(Fig.5C).5C). Female stolons (‘Sacconereis’) have a ‘head’ with two pairs of eyes, two short, simple palps, and three antennae (Fig. 5D, E). Both also have two pairs of ‘tentacular cirri’.

Viviparity has also been reported in some species.

Key to subfamilies and ‘incertae sedis’ genera

Acknowledgments

Notes

References