Syllis monilaris Savigny

Álvarez-Campos, Patricia, Gil, João & Martín, Guillermo San, 2015, Unveiling the Rosetta Stone of syllids: redescription and neotype designation of Syllis monilaris Savigny in Lamarck, 1818, type species of type genus of family Syllidae Grube, 1850 (Annelida), Zootaxa 4040 (3), pp. 317-330 : 322-327

Syllis monilaris Savigny in Lamarck, 1818

Figures 1–5

Syllis monilaris Savigny in Lamarck 1818: 318. Syllis monilaris Savigny 1822: 44 –45, pl. 4, fig. 3. Syllis monilaris Audouin 1826: 71 .

Syllis monilaris Savigny 1826: 372 –373, pl. IV, fig. 3. Syllis monilaris Blainville 1828: 473 , pl. 17, fig. 2. Syllis monilaris Peters 1855: 613 .

Syllis monilaris Licher 1999: 292 .

Syllis moniliformis [sic; lapsus calami] Grube 1869: 502. Syllis moniliformis Licher 1999: 306 .

Material examined. El Tor ( Egypt, Sinai Peninsula, Gulf of Suez), C.G. Ehrenberg coll., id. A.E. Grube, 2 specimens: ZMB F1941, 1 bigger specimen, designated as neotype; ZMB 11529, 1 smaller specimen, broken in two.

Additional material. England, off Folkestone, eastern English Channel, 51.02726º N, 1.30379º E, coarse sediment with Sabellaria spinulosa crust, 33 m depth, 22 Jun 2014, Fokestone Pomerania Survey, 3 specimens (collection of APEM Ltd.). Australia, Western Australia, Houtman Abrolhos, NE entrance Goss Passage, Beacon Island, 28° 27' 54" S, 113° 46' 42" E, 25 May 1994, 33 m depth, underneath boulders embedded in coral sand, coll. Pat Hutchings, 1 specimen (AM W29492).

Redescription based on neotype (ZMB F1941). Body of large size, elongated ( Figs 2 A–C, 3), incomplete, 5.8 cm and 296 chaetigers long, almost 1 mm wide on anterior body ( Fig. 3 ), wider on mid-posterior segments (1.4 mm) ( Fig. 4 C), slender posteriorly (0.7 mm) ( Fig. 4 D). Body now very dark, opaque ( Figs. 2 A–C). Prostomium pentagonal; 4 eyes in trapezoidal arrangement. Palps robust, similar in length to prostomium. Median antenna on posterior part of prostomium, between posterior eyes, with 16 articles, slightly longer than combined length of prostomium and palps; lateral antennae shorter than median one, with 13–14 articles each. Peristomium shorter than subsequent segments ( Fig. 3 ). Dorsal tentacular cirri longer than median antenna, about twice as long as lateral ones, with about 25 articles each; ventral tentacular cirri about half length of dorsal ones, with 18 articles each. Dorsal cirri longer than body width on anterior segments, becoming similar to shorter than body width from midbody backwards, and more or less fusiform, especially from midbody backwards ( Fig. 4 A–C), with well defined cirrophores and articles; articles basally and distally smaller than medially; anteriormost dorsal cirri longer than dorsal tentacular cirri, with 28–22–29–32–20–26–21–19–27 articles (left side), 29–19–26–32–22–22–21–23– 23 articles (right side) ( Fig. 3 ); at proventricular level, all cirri becoming shorter and similar in length, with about 14–18 articles ( Fig. 4 A); 12–14 articles on midbody ( Fig. 4 B), with similar cirri alternating irregularly between long and short; on midbody-posterior segments, dorsal cirri proportionally thicker and more fusiform, with 12–17 articles ( Fig. 4 C); on posteriormost segments, dorsal cirri smaller, with 10–14 articles each ( Fig. 4 D). Parapodia conical, distally bilobed on dorsal view ( Fig. 3 ). Ventral cirri digitiform, longer than parapodial lobes on anterior parapodia, becoming shorter from midbody onwards. Chaetae mostly broken but some parapodia with complete fascicles all along the body. All chaetae heterogomph compound, with thick shafts and short, triangular, unidentate blades sometimes with short spines on margin, but blades usually smooth, similar in shape and size throughout body ( Figs 2 D–E, 5A–C). Upper blades 35 µm long, lower ones 25 µm long; usually dorsal-most falciger on each parapodium with slightly thicker shaft than remaining ones. Anterior parapodia each with 9 compound chaetae ( Fig. 5 A); occasionally some chaetae slightly bidentate; midbody parapodia with 8 compound chaetae ( Fig. 5 B); posterior parapodia with 5 compound chaetae ( Fig. 5 C). Dorsal and ventral simple chaetae not observed (specimen incomplete). Anterior parapodia with 7 aciculae ( Fig. 5 D), 4 at midbody ( Fig. 5 E), and 2 in posteriormost parapodia, one distally blunt and other slightly acuminate ( Fig. 5 F). Pharynx extending through about 13 segments; pharyngeal tooth on anterior margin of pharynx. Proventricle from chaetigers 13 to 21.

Second specimen (ZMB 11529) much smaller than neotype, broken in two pieces and also incomplete, 13 mm long and 76 chaetigers, 1 mm wide on anterior body.

Distribution. Only known from the Gulf of Suez. In agreement with the ICZN article 76.3, the type locality of the species becomes El Tor ( Egypt, Sinai Peninsula, Gulf of Suez), locality of collection of the neotype.

Habitat. Stated to be common on the coasts of the Red Sea ( Savigny 1822). Collected at El Tor under flattened grey coloured sponges, between corals (“ Sub tegmine gelatinoso-carneo-cinereo lineari lapidibus affini inter corallia ad Tor ”; Grube 1869).. “ Elle se déplace en serpentant avec beaucoup d’agilité et remuant continuellement ses cirres ” ( Savigny 1822).

Etymology. According to WoRMS ( Gil 2015) the specific epithet monilaris is formed by the Latin root monil-, meaning "string of beads", and the Latin suffix -aris, which is used to form an adjective, usually by adding it to a noun, indicating a relationship or a pertaining to, referring to the presence of moniliform antennae and cirri typical of the species.

Remarks. The specimens studied by Grube agree with the descriptions and drawings provided by Savigny and Lamarck, and were collected in the same area (Gulf of Suez), so we have no doubt that they represent the same species. Since the type material used for the original description was lost, and that with the exception of Grube (1869) nobody redescribed the species, we designate here a neotype and provide a redescription. Both specimens from the ZMB are in excellent condition, flexible, and well preserved, although most chaetae are now broken; however, specimens are very darkened, and both pharynx and proventricle are difficult to see by transparency; in order to preserve as undamaged as possible these two important specimens, we have not removed parapodia nor dissected them, so it is not possible to detail the number of muscle cell rows of the proventricle. Savigny’s description of Syllis monilaris is much more extensive than the one published in Lamarck (1818), but even so very incomplete, not detailing some important characters, especially the size and shape of the chaetae; the chaetae are described by Savigny (1822) as “ assez grosses, obtuses, jaunâtres ” (“thick enough, obtuse, yellowish”), while the genus Syllis is stated to have “ soies simples ” (“simple chaetae”); in the drawings only the bases of the shafts are shown (see Fig. 1). However, Grube (1869) describes the chaetae as “ falce plus minus brevi apice simplici ”, meaning that the chaetae did have a more or less short, simple, sickle-shaped apex. Savigny figured a dark, opaque, very long, complete specimen, with 341 segments and more than 76 mm long (from text; about 87 mm long from figure), and described to be grey-reddish coloured with some iridescence. Grube (1869) reported about 300 segments, and we counted 296 chaetigers in the incomplete neotype. Both Grube’s and Savigny’s specimens have elongated dorsal cirri on anterior segments, becoming shorter, somewhat fusiform, on midbody and posterior segments, where they are shorter than body width (see Fig. 4 ). This arrangement of dorsal cirri is similar to those of Syllis gracilis Grube, 1840 , Syllis armillaris ( O.F. Müller, 1776) and, especially, Syllis hyalina Grube, 1863 ; the general aspect of the body of Syllis monilaris is quite similar to the latter, but the compound chaetae are closer to those of S. armillaris , although clearly different. All these species have thick compound chaetae with short blades, which are fused with the shafts in the midbody parapodia only in S. gracilis . The three are probably closely related, since they also share the pentacerous reproductive stolon; however, none of the examined specimens of Syllis monilaris were developing stolons, and none of them is useful for molecular analyses. This way, we cannot assure the phylogenetic relationships of S. monilaris with other species of Syllis . Furthermore, the three species are reported worldwide and each one of them represents likely a complex of different species. Future researches will provide further details for the knowledge of S. monilaris (number of rows in the proventricle, dorsal and ventral simple capillary chaetae, type of stolon, molecular data), but the present description is sufficient and adequate for a correct identification of the species.

Three specimens from England and one from Beacon Island (Western Australia) agree perfectly with the description of Syllis monilaris , although being smaller. Whether they belong or not to the same species is not possible to assess at this moment. Licher (1999) reports this species from La Rochelle ( France) based on specimens collected by d’Orbigny (MNHN A–74, 15 specimens), and also from Sicily, as Syllis moniliformis (NHMW 621, 3 specimens). The question if the records of the species from Northern France and England, or the Mediterranean Sea represent sibling species or introduced populations remains unsolved until further detailed studies can be carried out. The Australian specimen however, comes from the same Ocean as S. monilaris , although from a very distant locality. The present redescription will make the recognition of the species easier, and additional material will probably provide an answer to the above questions.

Syllis monilaris is the type species of the type genus of the family Syllidae , but it was poorly known, as a detailed description was lacking. This lack of knowledge caused numerous synonymies (see above list) and taxonomic problems. The most remarkable attempt to simplify the situation was done by Langerhans (1879), who split the genus into four subgenera: Haplosyllis , only with simple chaetae; Typosyllis , only with compound falcigerous chaetae; Syllis , with both compound falcigers and some secondarily simple, thick chaetae; and Ehlersia , with spinigerous and falcigerous compound chaetae; this division was followed by Fauvel (1923) and subsequent numerous different systems of classification, depending on the authors, who considered them as subgenera or raised them to genera. San Martín (1984, 1992, 2003) offered a detailed discussion and revision of this problem, and with the exception of Haplosyllis , considered as a genus per se, proposed the combination of the remaining three genera under the genus Syllis , at least until more species were better known and more data available: “[...] the division of Syllis [...] both as subgenera or genera, is an artificial division and it has been followed for practical reasons more than scientific ones [...]” ( San Martín 1992).

Furthermore, San Martín (1992) stated that “ All these considerations do not mean that Syllis must be considered as a homogeneous genus. There are many other characters, which have not been described for most of the species of this genus, but could be useful for segregating Syllis into groups of species or subgenera. These characters are mainly three: the shape of posterior aciculae, shape of the solitary dorsal setae, and the kind of reproductive stolon.”

Some recent authors (e.g. Licher 1999, Tovar-Hernández et al. 2002, Aguado et al. 2015) have maintained the existence of two genera: Syllis , with some simple chaetae in midbody resulting from the fusion of the blades with the shafts, and Typosyllis , with the rest of the species, including those with thick simple chaetae resulting from the loss of blades, and the ones with pseudospinigers (previously considered as spinigers). This contradicts the opinion expressed by San Martín (1984, 1992, 2003), in that “ the presence of thick simple setae, which would define the subgenus Syllis , cannot be used as a generic or subgeneric character, but only as a specific character ” ( San Martín 1992). The fusion of blades and shafts is progressive during the ontogeny, and younger specimens of Syllis gracilis , for instance, do not have fused chaetae, and they become more markedly fused as the specimen grows; the presence of thick simple chaetae is thus a secondary character. This species would be a special case in the Animal Kingdom, changing of genus during its ontogeny, being Typosyllis during the early stages, and Syllis when adult. Furthermore, there are some other similar species with only partially fused chaetae, which would not fit clearly in the two genera. There are two more considerations supporting Typosyllis as a non-valid genus, and just one more among many other synonymies of Syllis :

1. The type species of the genus Syllis , Syllis monilaris , does not have fused chaetae, contrary to what is stated in the diagnosis of the genus by Savigny (1822) (see above redescription of neotype).

2. There are several older genera synonymized with Syllis that would have priority over Typosyllis (see list of synonymies above).

Two of the authors (P.A.-C. and G.S.M.) are currently revising the available types of each one of these synonymies, and preparing redescriptions of some older species, in the framework of a global revision of the systematics of the genus, supported by molecular data whenever available (Álvarez-Campos et al. in press). Considering the possible future division of Syllis into several genera as a result of the ongoing studies, the name Typosyllis could only be applied for its type species ( Syllis krohnii Ehlers, 1864 ) and closely related species, and only in the case there would be no available previous name to designate the group, according to the ICZN article 23.3.

Figures

FIGURE 2. Syllis monilaris. Photographs of neotype (ZMB F 1941). A, complete specimen, with original ZMB’s label. B, anterior end, dorsal view. C, midbody, dorsal view. D, compound chaetae, anterior parapodia. E, compound chaetae, midbody.

FIGURE 3. Syllis monilaris. Neotype (ZMB F 1941). Anterior end, dorsal view. Scale: 0.4 mm.

FIGURE 4. Syllis monilaris. Neotype (ZMB F 1941). A, dorsal cirri, proventricular level. B, dorsal cirri, midbody. C, midposterior segments, dorsal view. D, posterior segments, dorsal view. Scale: 0.4 mm.

FIGURE 5. Syllis monilaris. Neotype (ZMB F 1941). Compound chaetae: A, anterior. B, midbody. C, posterior. Aciculae: D, anterior. E, midbody. F, posterior. Scale: 50 µm.