INTRODUCTION
The Scalibregmatidae (Annelida: Polychaeta: Scolecida) are sedentary polychaete worms usually present in marine subtidal sediments and in some species the presence of epitokes swarming has been reported in the plankton (Clark, Reference Clark1954). They are active burrowers and subsurface deposit feeders and never form tubes (Schüller & Ebbe Reference Schüller and Ebbe2007; Snowden, Reference Snowden2008).
Scalibregmatidae (except for Axiokebuita species that are maggot shaped) are characterized by short bodies, anteriorly swollen; their parapodia are biramous with simple capillary chaetae both on noto- and on neuropodia; the presence of furcate chaetae is frequent. Moreover, in some genera it is possible to observe acicular spines. Mobile appendages are absent; branchiae are not always present and in some species they are branched and placed on anterior segments. The prostomium is bilobed and often in the form of frontal horns (Worsfold, Reference Worsfold2006). The genus Scalibregma occurs with two species in the Mediterranean Sea: Scalibregma inflatum Rathke, 1843 and S. celticum Mackie, Reference Mackie1991.
The genus has the characteristic T-shaped prostomium bilobed or two-horned. Eyes could be present or not, depending on species. The buccal segment is achaetous and on anterior segments branched gills are present. Parapodia are small, biramous and on posterior segments dorsal and ventral cirri are present. Concerning the chaetae, they are chiefly capillaries and even if few forked chaetae could be present, crochet are absent (Day, Reference Day1967; Hayward & Ryland, Reference Hayward and Ryland1995).
Only the presence of S. inflatum is reported for Italian seawaters, which occurs in several areas, including the central Tyrrhenian Sea (Castelli et al., Reference Castelli, Bianchi, Cantone, Çinar, Gambi, Giangrande, Iraci Sareri, Lanera, Licciano, Musco, Sanfilippo, Simonini and Relini2008). This species is described as cosmopolitan, exhibiting a wide depth-range distribution, associated with a muddy sand habitat (Day, Reference Day1967).
Scalibregma celticum was described as a new species by Mackie (Reference Mackie1991), studying individuals obtained from the collections of the National Museum of Wales, Cardiff (NMW), the National Museum of Scotland, Edinburgh (NMS), the Zoological Museum, Oslo (ZMO) and the Norwegian Institute of Water Research, Grimstad (NIVA), and using comparative material borrowed from the Smithsonian Institution, National Museum of Natural History, Washington, DC and the Natural History Museum of Los Angeles County. As Mackie (Reference Mackie1991) reports S. inflatum was considered to be the most widespread scalibregmatid but he proved the presence of at least two new species, including S. celticum.
Like S. inflatum, S. celticum is reported as ubiquitous, and it can occur in coarser sediments, including gravels (Worsfold, Reference Worsfold2006). Moreover, the two species are easy to distinguish because of the presence of eyes in S. celticum, a feature absent in S. inflatum (Worsfold, Reference Worsfold2006). Especially the presence of this evident characteristic avoids the possibility of misidentification of these two species.
Concerning the Mediterranean area, S. celticum has been recorded from Cyprus by Çinar (Reference Çinar2005) and the individual he found represents a new record in the Levantine Sea and in the Mediterranean Sea. This work presents the first record of S. celticum in the western central Tyrrhenian Sea, updating the geographical distribution of this species in the Mediterranean Sea.
MATERIALS AND METHODS
A survey was carried out off the Sardinia north-east coast in February 2009 (Figure 1). The sampling plan foresaw 9 stations in an area of 4 km2. Samples were collected using a Van Veen grab with the area of 0.1 m2 and a capacity of 25 l.
Fig. 1. Sampling area.
Samples were immediately rinsed on a sieve with a mesh of 1 mm and the fraction retained (macrozoobenthos according to Stirn, Reference Stirn1982) was fixed in a 4% formaldehyde–seawater solution, buffered with CaCO3, then stored in polyethylene jars. Subsequently, in the laboratory, samples were sorted to different taxa and each specimen identified at the lowest taxonomic level possible. Finally, each taxon was stored in a 70% alcohol–water solution.
The S. celticum individuals were identified using ‘Identification guides for the NMBAQC Scheme: 1. Scalibregmatidae (Polychaeta) from shallow seas around the British Isles' (Worsfold, Reference Worsfold2006) and ‘Scalibregma celticum new species (Polychaeta: Scalibregmatidae) from Europe, with a redescription of Scalibregma inflatum Rathke, 1843 and comments on the genus Sclerobregma Hartman, 1965' (Mackie, Reference Mackie1991).
RESULTS
Six individuals of S. celticum have been found, at a depth of 40 m, where sediment was composed of sand for more than 75%.
In Table 1 the morphological data of the six individuals of S. celticum are summarized. The individuals found are from 3 to 7 mm long (Figure 2A), they are transparent and the surface of the body appears divided in many equal squares. They have a T-shaped prostomium with two reddish eyes (Figure 2B), and, starting from the second or the third segment, arborescent gills are present (Figure 2D, E). The anterior part of the body is expanded and, in this region, parapodia are very small, not clearly noticeable, especially in individuals where the enlargement is particularly marked. The posterior part of the body is tapered and, in this region, parapodia are more developed, bigger, and dorsal and ventral cirri are present. On the pygidium five anal cirri are present (Figure 2C). Concerning the chaetae, on all parapodia blunt chaetae are present (Figure 2F) and, after the first parapodia also appear furcate chaetae. These are smaller than the capillaries and often hard to detect since they are covered by the capillaries.
Fig. 2. Scalibregma celticum: (A) entire body; (B) prostomium with eyes; (C) pygidium; (D, E) arborescent gills; (F) chaetae.
Table 1. Morphological characteristics of individuals analysed.
DISCUSSION
The presence of S. inflatum is reported in Italian sea waters (Castelli et al., Reference Castelli, Bianchi, Cantone, Çinar, Gambi, Giangrande, Iraci Sareri, Lanera, Licciano, Musco, Sanfilippo, Simonini and Relini2008). According to Mackie (Reference Mackie1991), S. celticum can be identified by the presence of some features: the most obvious is the presence of eyes. Scalibregma celticum has a single pair of subdermal eyes (Fig. 6), ranging from dark brown to black, placed on posterolateral margins of the prostomium. This feature distinguishes it from S. inflatum, in which eyes are lacking, and that is present in central Tyrrhennian Sea (Castelli et al., Reference Castelli, Bianchi, Cantone, Çinar, Gambi, Giangrande, Iraci Sareri, Lanera, Licciano, Musco, Sanfilippo, Simonini and Relini2008). Then it is possible to observe short blunt spines in parapodia of both setigers 1 and 2, while in S. inflatum there are no blunt chaetae in anterior parapodia (Worsfold, Reference Worsfold2006). Other differences are an expanded hood-like peristomium obscuring eyes and the posterior part of prostomium, an upper lip of the mouth that is medially folded and the presence of smaller epidermal pads above the notopodia and across the dorsum of setigers 1–5. With regard to the size, S. celticum is smaller than S. inflatum: S. celticum lengths are reported from 2.5 to 22 mm for 30–48 segments, for S. inflatum the length-range reaches 37 mm for 23–64 segments (Mackie, Reference Mackie1991). The features observed in the individuals found by us perfectly match the description proposed by Mackie (Reference Mackie1991), allowing their identification as S. celticum.
Both the European Register of Marine Species (ERMS) and the World Register of Marine Species (WoRMS) reported the distribution of S. celticum in the European area that includes the Mediterranean Sea Area (Costello et al., Reference Costello, Bouchet, Boxshall, Arvantidis and Appeltans2008; Appeltans et al., Reference Appeltans, Bouchet, Boxshall, Fauchald, Gordon, Hoeksema, Poore, van Soest, Stöhr, Walter and Costello2011). Nevertheless the species is not reported in the ‘Italian Checklist of Marine Fauna' (Castelli et al., Reference Castelli, Bianchi, Cantone, Çinar, Gambi, Giangrande, Iraci Sareri, Lanera, Licciano, Musco, Sanfilippo, Simonini and Relini2008) and our record can be reported as the first record in Italian sea waters.
The worldwide distribution of S.celticum is reported in Figure 3. In this image yellow dots are related to the known distribution of S. celticum, as reported on the Ocean Biogeographic System website (Vanden Berghe, Reference Vanden Berghe2007). Red dots update the known distribution because they are related to Çinar's (Reference Çinar2005) record in Cyprus sea waters and to our record in Italian sea waters.
Fig. 3. Worldwide distribution of Scalibregma celticum: yellow dots represent the distribution as reported on the iOBIS website (Vanden Berghe, Reference Vanden Berghe2007); red dots represent Çinar's (Reference Çinar2005) record in the Levantine Sea and our record in the Tyrrhenian Sea.
Probably the species is not mentioned in the Italian checklist for several reasons including difficulty in finding individuals, as well as a lack of investigation, particularly in the area analysed in this work. The two species of Scalibregma are easy to distinguish mainly due to the presence/absence of eyes as well as the presence/absence of blunt chaetae in anterior parapodia. For these reasons we can suppose that misidentification is to be excluded. Besides, the examination of the individuals found confirms the original description provided by Mackie (Reference Mackie1991). All the distinctive characteristics of S. celticum have been identified in order to allow a clear distinction between this species and S. inflatum already reported in the ‘Italian Checklist of Marine Fauna' (Castelli et al., Reference Castelli, Bianchi, Cantone, Çinar, Gambi, Giangrande, Iraci Sareri, Lanera, Licciano, Musco, Sanfilippo, Simonini and Relini2008). It is premature, at this stage, to give an opinion about the origin of the species or its possible dynamics of colonization/expansion. The record of other individuals, eventually coming from other areas of the Mediterranean, will enable more detailed studies, possibly using also genetic techniques, in order to make hypotheses about the origin and the colonization pattern of the species.
