The genus Sabellaria (Annelida, Sabellariidae) includes at least 39 species worldwide (Kirtley 1994, Nishi et al. 2010, Hutchings et al. 2012). As in all the sabellariid genera, the anterior part of the body shows a truncated surface forming an operculum used to seal the tube entrance to avoid desiccation and predation, and as a tool in shaping and arranging sand grains during tube construction (Eckelbarger 1975). Opercular features are of paramount taxonomic importance in Sabellariidae, but their taxonomy and phylogeny are not yet solved (Capa et al. 2012).

In Sabellaria, the operculum consists of three rows of chaetae called paleae. An outer row formed during ontogenesis by the pair of outer chaetigerous sacs, and middle and inner rows formed by inner chaetigerous sacs (Eckelbarger 1975). Opercular paleae can be highly variable in form and size, and their morphology is of particular interest in systematics because of the relative uniformity of other structures. However, the opercular paleae may show ontogenetic changes, with the “intermediate” forms of paleae differing from those of adult worms (Kirtley 1994). Moreover, the paleae can break and be altered by epifaunal growths and abrasion, increasing their morphological variability.

Three species of Sabellaria are currently known for the Mediterranean Sea: S. alveolata (Linnaeus, 1767), S. spinulosa (Leuckart, 1849), and S. alcocki Gravier, 1906 (Castelli et al. 2008). Sabellaria alveolata forms extensive long-lasting (4 to 9 years) intertidal biogenic reefs along most Atlantic European coasts (Wilson 1971, Achary 1974, Gruet 1982, 1986). Along the Italian coast, S. alveolata reefs occur in the Ligurian and the Tyrrhenian Sea (Dubois et al. 2002, La Porta and Nicoletti 2009).

Sabellaria spinulosa and S. alcocki are smaller than S. alveolata and inhabit the subtidal and lower intertidal/sublittoral fringes. Sabellaria spinulosa has been described from the North Sea (Leuckart 1849) and S. alcocki from the Indian Ocean (Gravier 1906). In Mediterranean areas, S. spinulosa occurs solitary or forming small clumps (Wilson 1970). By contrast, dense aggregations forming reefs up to 60 cm high and extending over several hectares can be found in areas with very high levels of suspended sediment in the North Sea (Wilson 1970, 1971) and around the British Isles (Pearce et al. 2011). Reefs may persist for many years, although individual clumps may be regularly formed and disintegrated (Jones et al. 2000). In fact, sabellarian reef structure and morphology are mainly influenced by currents, among other environmental conditions, with cyclical phases of construction and destruction (Gruet 1982, 1986, Porras et al. 1996, Dauvin 1997, Dubois et al. 2002).

Sabellaria spinulosa seems to be a fast growing annual form, which is able to quickly recolonize areas lost due to winter storms. Many thin crust-like structures formed by S. spinulosa probably correspond to these annual or transient constructions, but more stable communities have also been reported (Linke 1951). Sabellaria spinulosa shows a great deal of variability in the opercular features. There may be a single kind of middle paleae, alternate long and short, or several different lengths. The outer paleae are reported to have 2-3 teeth on the distal margin, with a plume bearing 5 to 12 denticles on each side. By contrast, the middle paleae in S. alcocki are all geniculate and have a long distally erect blade, while the outer paleae have three distal teeth and a long plume with up to 18-20 denticles on each side. Sabellaria spinulosa and S. alcocki are also distinguished by the absence of nuchal spines in the latter (Kirtley 1994).

Although S. alcocki has been considered as exclusive of the Indian Ocean (Nishi et al. 2010, Hutchings et al. 2012) and has not been reported in the Mediterranean Sea (Coll et al. 2010), the species was included in the checklist of the Italian coast (Castelli et al. 2008) and continues to be reported in Mediterranean faunistic papers (Parapar et al. 1992, Çinar et al. 2006, 2008). As a consequence, the geographical distribution of S. spinulosa and S. alcocki remains unclear (Read 2014). This is probably due to the above-mentioned high morphological variability of S. spinulosa. Fauvel (1914) considered S. alcocki as one of the varieties of S. spinulosa summarized in Fauvel (1927). Accordingly, S. spinulosa var. alcocki differs from S. spinulosa in having the middle opercular paleae with alternating long and short falciform blades. However, no mention of nuchal spines (“soies aciculaires” sensu Fauvel 1927) is present in Fauvel’s description of S. spinulosa var. alcocki. Following Fauvel (1927), “S. spinulosa alcocki” has been reported worldwide (e.g. Kirtley 1994, Fauchald and Bellan 2013). More recently, however, S. alcocki started again to be reported as a separate species, leading to several erroneous reports around the world (e.g. Hartman 1944, Achary 1974, Fauchald 1977), including the Mediterranean coast (Parapar et al. 1992, Castelli et al. 2008, Çinar et al. 2005, 2008).

Within the framework of a multidisciplinary project on the functional and ecological role of marine bioconstructions, large biogenic formations of S. spinulosa were found along the Adriatic coast, offering an opportunity to examine the opercular variability of this species. In the present paper, we report the analyses of this variability in adults, coupled with a laboratory study on larval development to describe changes during the opercular formation. As mentioned above, the opercular paleae undergo ontogenetic changes in shape (Eckelbarger 1975). Also, during the development of S. spinulosa, the “primary paleae” are lost and replaced by “secondary paleae” progressively resembling the outer opercular paleae of the adult (Wilson 1929, 1970). Such a loss and progressive formation of the opercular paleae in post-settlement stages has only been reported for S. vulgaris Verril, 1873, although the size required to reach the stage having the adult paleae was not defined.

Taking into account that the development of S. spinulosa refers to populations from northern latitudes (Wilson 1929, 1970), we present herein the first ones for a Mediterranean population.

The specimens were identified as S. spinulosa according to the shape of the middle paleae, the dentition pattern of the outer adult paleae, and the presence of nuchal spines, which allow them to be distinguished from S. alveolata and S. alcocki (Table 1).

A typical individual collected in October 2013 (Fig. 2) measured 10 mm in body length and had up to 25 abdominal segments (Fig. 2A, B). It has a medial organ with eyespots and 3-4 nuchal spines. Its posterior segments lack branchiae and the operculum is formed by 25-29 pairs of outer paleae (having a distal margin with 2-4 teeth and a central denticulate plume with about 6 denticles on each side, Fig. 2D), 10-12 inner paleae (geniculate, with shallow excavate base on upper surface, Fig. 2E), and 10-12 middle paleae (slightly geniculate, with a variable distal blade and an excavate base, Fig. 2F, G).

The present work represents the first formal report of S. spinulosa reefs in the Mediterranean Sea, where, until now, the species was reported as solitary or as forming small clumps (Wilson 1970). These three-dimensional biogenic structures are hotspots of biodiversity and are listed as threatened habitats according to the OSPAR Convention (the Convention for the Protection of the Marine Environment of the North-East Atlantic) (Benson et al. 2013) due to their fragility and vulnerability, and we strongly encourage this status to be extended to the Mediterranean reefs.

The reef-building Mediterranean S. spinulosa were ripe in all sampled periods, with a maximum of ripe females in April in all years and in June 2013. Reproduction may therefore extend during the spring-summer period, with females making up 65% of the population. Considering the state of maturity and the size of individuals within the population, we also agree with Linke (1951) that the species is an annual form reproducing once in its life. Specimens from April 2013 belong to the 2012 generation, which mainly reproduces in spring. This generation possibly continues to grow and reproduce, but with a low survival of their larvae because, according to our experiments, complete development was only attained by the April and June 2013 individuals. Most individuals from October 2013 belonged to a new generation, which was still growing. Though some of them appeared ripe, the eggs were unable to complete development after fertilization and their reproduction could probably be expected in the following April.

The number of eggs for each specimen appears low if compared with S. alveolata (Ayata et al. 2009), in line with its low recruitment (Rees and Dare 1993); this, coupled with the influence of currents for larval retention, could explain the absence of reefs in most of its distributional range, as previously demonstrated for the S. alveolata population from the Mont Saint Michel Bay (Ayata et al. 2009).

The analysis of the opercular structures in the field population showed a great deal of variability, especially in the middle paleae, with alternating and equal-size patterns each present in half of the specimens and lacking correlation with body size. This intrapopulation variability largely covers the differences between S. spinulosa and S. spinulosa alcocki, thus preventing the erection of several varieties within S. spinulosa. We therefore hypothesize that the varieties were erected in the past based on a few specimens.

Larval development of S. spinulosa is here described for the first time for a Mediterranean population, allowing a comparison with the non-Mediterranean studies. Regarding the egg diameter, we found 100 μm against 50 μm by Pearce et al. (2011) and 150 μm by Wilson (1929). Developmental time was also different: we found only 2 weeks of pelagic development against 4 weeks found by Pearce et al. (2011) and 6-8 weeks by Wilson (1929). By contrast, our data on developmental time are similar to those observed experimentally for other species: S. alveolata and Phragmatopoma californica (Fewkes, 1889) (Cazaux 1964, Pawlik 1988). However, egg size estimates could be influenced by operational procedures, while growth rate and settlement time of sabellariid larvae under laboratory conditions can be greatly influenced by the culture technique (Eckelbarger 1977). For example, the long developments recorded for S. alveolata and S. spinulosa (up to 32 weeks in the former) are probably not representative of the species in nature (Wilson 1970), while the early settlement at 6 weeks experimentally recorded by Wilson (1929) most likely represents the real field time. Thus, caution must be taken when comparing laboratory and field larval development times.

In our study, the development of S. spinulosa seems to be influenced by the temperature only during the pelagic stages, with a faster growth at 21°C. However, after settlement, specimens from this treatment delayed metamorphosis by about 8 days compared with those reared at 14°C, so no difference in the total developmental time could be identified. By contrast, a delay in metamorphosis of about 10 days has been experimentally demonstrated for S. vulgaris (Eckelbarger 1975) and P. californica (Eckelbarger 1977).

Mortality during the pelagic life in laboratory conditions was not specifically measured, but it seems to be higher during settlement, as previously observed in experimental conditions (Wilson 1929, Cazaux 1964, Eckelbarger 1975), where it seems to depend on the presence of stirring and available sand. During settlement, only some degree of gregariousness characterizing other sabellarian species (Pawlik 1988) was observed. However, the lack of gregarious behaviour could also be due to the particular rearing conditions in our experiments.

Morphological changes in larval development of S. spinulosa specimens from the Mediterranean are largely indistinguishable not only from non-Mediterranean S. spinulosa populations, but also from those described in other sabellariids (Wilson 1929, 1968 a, b, Hartman 1944, Dales 1952, Cazaux 1964, Eckelbarger 1975), confirming a noteworthy similarity overall. As observed by Wilson (1929), the early developmental stages of S. spinulosa and S. alveolata are indistinguishable. These two species become easily distinguishable only when the adult primary opercular paleae appear among the long provisional bristles of the first parapodia. As already pointed out, however, during opercular formation settling chaetae or “primary paleae” are lost and replaced by “secondary paleae” showing the final adult shape (Wilson 1929, Eckelbarger 1975). We also observed marked ontogenetic changes during the post-settlement period, with several different forms of each palea type present within the juvenile operculum, which become progressively more adult-like as paleae are lost and replaced. Consequently, in juvenile S. spinulosa the length of the denticulate distal plume in outer paleae and the shape and length of the middle paleae are similar to those in S. alcocki. As in the case of S. alveolata, S. alcocki becomes easily distinguishable from S. spinulosa only when the operculum achieves the adult features.

Data obtained from laboratory rearing combined with observations from a field population led us to hypothesize that the final opercular configuration was achieved when the worm is about 4 mm long. Thus, the records of S. alcocki in the Mediterranean area could be partially attributed to the examination of S. spinulosa specimens at different stages of development. Therefore, we strongly support the hypothesis that Sabellaria alcocki is not present in the Mediterranean and that special care must be taken in identifying small or young individuals of S. spinulosa. Particular attention must be addressed to the presence of nuchal spines, as the young specimens lacking these spines closely resemble S. alcocki both in the outer denticulate distal plume of the outer paleae and in the shape of the middle paleae.