Caulleriella mediterranea, a new species of polychaete (Annelida: Cirratulidae) from the central Mediterranean Sea

Abstract A new species belonging to the genus Caulleriella Chamberlin, 1919 (Polychaeta: Cirratulidae) is described from coastal soft bottom sediments off the northwest Italy coast in fine sand at around 8 m depth. Caulleriella mediterranea sp. nov. is characterized by a slightly biannulate elongate peristomium, bidentate hooks with main fang slightly curved, a long apical tooth, and a narrow wing on the convex side and absence of capillary chaetae in neuropodia. A comparison is made with specimens that fit with the congeneric species C. alata, probably confused in the past with C. mediterranea sp. nov. Morphological characters important for differentiation of these species are discussed. The description of C. mediterranea sp. nov. testifies to an increasing trend of knowledge on cirratulid diversity in the Mediterranean Sea, a family characterized by many species considered early indicators of organic enrichment http://www.zoobank.org/?lsid=urn:lsid:zoobank.org:act:227C3794-DD91-4747-900F-9308E59C0D33 http://www.zoobank.org/?lsid=urn:lsid:zoobank.org:pub:0F35B814-A61D-4705-AEE0-E5A5D8C69B0C


Introduction
The Mediterranean Sea represents a biodiversity hotspot with approximately 17,000 marine species and with about of 20% endemism. Polychaete species number about 1100, which contribute 10% of worldwide biodiversity, and count 210 endemisms (Coll et al. 2010). Despite the fact that the basin is one of the best-known geographic areas of the world, it contains many regions and habitats that remain insufficiently studied, and several taxonomic groups in deep-sea areas and portions of the southern region are still poorly known (Coll et al. 2010), so the description of new species is still common and a high priority.
The "extinction" of invertebrate taxonomy specialists coupled with material not being properly identified is leading to a underestimation of biodiversity (Boero 2001;Coll et al. 2010), and undescribed invertebrate species may become extinct before we even know of their existence, resulting in the loss of valuable biological information about taxonomic groups subject to anthropogenic threats. In addition, and paradoxically, some common and accessible ecosystems such as beaches, among other habitats in the Mediterranean, have been poorly studied (Coll et al. 2010) despite the growing need for knowledge and monitoring to protect their ecological status (i.e. Water Framework Directive 2000/60/EC and Marine Strategy Framework Directive 2008/56/ EC). Polychaetes represent one of the best indicators of environmental disturbance and include both sensitive and tolerant species discerning from pristine to heavily disturbed environments. Among them, Capitellidae, Cirratulidae and Spionidae include species that proliferate after inorganic matter increases, as well as other disturbances. Some authors have typified polluted areas by the presence of species belonging to these families (Bellan 1984;Bellan et al. 1988;Dean 2008). Due to their distribution in areas moderately contaminated by organic matter, Cirratulidae are considered early indicators of organic enrichment.

Samples
Description. Holotype complete specimens, 26 mm long and 0.6 mm wide, with 155 chaetigers. Paratypes complete specimens, 10-28 mm long, 0.5-0.8 mm wide and 85-150 chaetigers. Body elongate, in cross section, anterior end rounded dorsally, flattened ventrally; middle and posterior segments with shallow dorsal groove and cylindrical in cross section, mid-body segments wider than anterior and posterior ones. Pygidium a simple rounded lobe (Figures 4(a) and 5(i)), anus dorsal. Specimens in alcohol white to yellow in color (Figures 2(c) and 4(a)).
Dorsal tentacles situated on the posterior margin of the peristomium. Dorsal tentacles yellow in color extending back to chaetiger 25, and thick as a chaetiger followed by a pair of branchiae (Figures 3(a) and 5(j, k)). Branchiae from the anterior margin of the first chaetiger present throughout, even putative presence of the branchiae up to the end of the body, one pair per segment, arising near to notopodial ridge.  They can be easily lost in the posterior end. Parapodia laterally positioned with notopodia and neuropodia separated in the anterior part of the specimens (Figures 4(a) and 5(h,j)). In the posterior end, the distance between noto-and neuropodia is reduced (Figures 4(a) and 5(i)). Notochaetae of anterior chaetigers, capillaries of two types arranged in two straight-line rows of eight capillaries until 10th chaetiger ( Figure 5(a,b)), after that 1-2 thin capillaries are present throughout ( Figure 5(c)). Bidentate notopodial hooks with main fang slightly curved, long apical tooth, and with narrow flange or wing on the convex side and boldly curved (Figures 4(b) and 5(d-g)).
Bidentate notopodial hooks with companion capillaries from chaetigers 10, up to three hooks arranged in a straight line ( Figure 5(e)). On posterior notopodia, 7-8 bidentate hooks ( Figure 5(g)). Neuropodial hooks similar to notopodial hooks, shorter and larger with a smaller apical tooth, present from chaetiger 1, numbering 10 per fascicle, arranged in a straight line ( Figure 5(d)). Posterior neuropodial hooks similar to notopodial hooks, numbering 6-7 hooks arranged in straight line ( Figure 5(f)). No capillary notopodial chaetae are present.    Methyl green staining pattern. There is no evident pattern; staining is solid and homogeneous throughout the body.
Remarks. Caulleriella mediterranea sp. nov. belongs to the Caulleriella alata group because the convex side hook is distinctively winged. This character is characteristic, other than of C. alata, of Caulleriella acicula Day, 1961and Caulleriella tricapillata Hutchings & Rainer, 1979. Moreover, Caulleriella pacifica Berkeley, 1929 and Caulleriella murilloi Dean & Blake, 2007 have a hooded hook but with a different pattern, not present on the convex side (in C. pacifica) or with encircling hood around the distal tooth (in C. murilloi). Caulleriella mediterranea sp. nov. differs from C. alata, C. acicula and C. tricapillata in the presence of an elongate and weakly biannulate peristomium. The peristomium of C. acicula and C. tricapillata is strongly triannulate. Although the original description of C. alata does not mention the presence of peristomial rings, they are illustrated in the original description plates (Southern 1914, pl. XII, fig. 27). Moreover, the peristomial rings of C. alata are mentioned in the redescription by Fauvel (1927). An examination of specimens from Belfast Lough, on the coast of Northern Ireland, whose traits fit with the original description of C. alata, shows three annulation rings (Figure 6(a,c)).
Differences have been observed in the general aspect of the worms, which appears thick in C. alata and filiform in C. mediterranea sp. nov. Furthermore, the examined C. alata specimens show chaetae with fine neuropodial hooks and companion capillaries ( Figure 6(b)) that are mentioned in the original description and are not present in C. mediterranea sp. nov.
The other two species of Caulleriella recorded in the Mediterranean Sea (C. bioculata and C. viridis) differ from C. mediterranea sp. nov. mainly in the presence of capillary neurochaetae in the first chaetigers and the absence of hooded hooks.
Moreover, C. mediterranea sp. nov. has a dorsal crest, and this character is reported from a number of species. For instance, it is present in Caulleriella cabbsi Pocklington and Coates, 2010, Caulleriella bremecae Elias and Rivero, 2008, Caulleriella murilloi Dean and Blake, 2007and Caulleriella venefica Doner and Blake, 2006. A comprehensive table of all presently known species of Caulleriella from the Atlantic and the Mediterranean Sea has been assembled in order for readers to understand and use the characters to identify species and species groups (Table I).
Etymology. The specific epithet refers to the first description of a Caulleriella in the Mediterranean Sea.
Ecology and biology. Caulleriella mediterranea sp. nov. was collected in sandy soft bottom sediments at about 8 m depth. Its occurrence is associated with the tubicolous annelid Owenia fusiformis which characterizes a fine sand "facies" (sensu Peres & Picard 1964). Other abundant species were Dialychone usticensis, Paradoneis armata, Prionospio caspersi, Nephtys cirrosa, the mollusks Tellina fabula and Thracia papyracea and the anfipod Perioculodes longimanus. In some locations the new species was relatively abundant (up to 20 individuals) representing 6% of the benthic community.

Discussion
The description of Caulleriella mediterranea sp. nov. testifies to an increasing trend of knowledge on worldwide cirratulid diversity. In the last few years a relatively large number of papers that describe new cirratulid species have been published (i.e. Chambers & Woodham 2003;Dean & Blake 2007, 2009, 2016Elías & Rivero 2008, 2009Çinar & Petersen 2011;Magalhaes & Bailey-Brock 2013;Magalhães et al. 2014;Blake 2015Blake , 2016Elias et al. 2016;Lezzi et al. 2016), evidencing that cirratulid diversity is as yet underestimated. In the Mediterranean Sea, new cirratulid species and genera have been described (Çinar & Petersen 2011;Lezzi et al. 2016) and, until now, no Caulleriella endemisms were described in the basin. Caulleriella mediterranea sp. nov. represents the first species of the genus described in the Mediterranean Sea. Caulleriella bioculata and C. viridis were described in Dinard (North of France) and Madeira (Canary Islands), respectively.
Fauvel, in Faune de France (1927), a widely used guide for Mediterraean polychaete identification, explains that the winged hook is a characteristic of C. alata, causing considerable confusion for the correct identification of the species. The described species C. mediterranea sp. nov. is probably widespread in the Mediterranean Sea, and confused with the congeneric C. alata which shares the peculiar hooded hooks. In particular, most of the identified C. alata from fine sand in the Tyrrhenian Sea (Lardicci et al. 1991;Somaschini 1993) and in other Mediterranean localities such as the northeast coast of Tunisia and Cyprus (Çinar 2005;Zaâbi et al. 2012) may be referred to C. mediterranea.
Moreover, other records of C. alata coming from Lago Fusaro (Naples) coastal lagoon (Sordino et al. 1989), and other biotopes such as rocks, seagrasses and muds from northen Cyprus and the Aegean Sea, western Turkey (Çinar et al. 1998, 2008; Çinar 2005), require particular attention during the examination as suggested by Dr M.E. Petersen in Çinar et al. (1998), who emphasized that C. alata is a complex of species and, despite being described from Ireland, has been