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2. THE SPECIES

As reported in the previous chapter, brackish-waters communities are widely differentiated and can exhibit high degrees of biodiversity (Cognetti, 1994; Cognetti & Maltagliati, 2000, Bilton et al., 2002). Many brackish-water environments share the presence of typically euryhaline species but, in addition, locally adapted populations of stenohaline species typical to marine habitats, as well as some recently introduced species, may be found. In the present study three species, one for each of the previously cited categories, are analysed under a phylogeographic perspective. Hediste diversicolor (O.F. Müller, 1776) (Annelida, Polychaeta), is a typical brackish-water species; Mytilaster minimus (Poli, 1795) (Mollusca, Bivalvia), a marine species known for the capacity of establishing populations in some brackish-water habitat; Xenostrobus securis (Lam., 1819) (Mollusca, Bivalvia), originary of southern Australia and New Zealand estuaries, being invasive for the Mediterranean Sea.

2.1. Family Nereididae (Annelida, Polychaeta)

The Nereididae is one of the most diverse polychaete families, comprising over 540 species and 43 genera (Hutchings et al., 2000).Fifty-four species belonging to 11 genera have been recorded along the Italian coasts (Castelli et al., 2008). Nereididae can be diagnosed from other polychaete families as follows: ‘foregut with one pair of lateral jaws; head discrete and compact, dorsal to mouth; prostomial antennae paired arising anterolaterally; capillary chaetae absent; pygidial appendages present’ (Glasby & Fauchald 2002). The major part of nereidid species is commonly found in shallow marine habitats, but members of this family occur in a wide range of environments, from the deep sea to estuaries, to freshwater streams and even to temporary rainwater puddles in moisty terrestrial environments (Wilson, 2000).

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2.1.1. Genus Hediste

Due to great morphological homogeneity almost all of Nereididae species can be included in the ancient genus Nereis (Fauvel, 1923). As a consequence, in the family Nereididae, several groups of sibling species have been reported. Along with the previously cited case of Perinereis cultrifera (Maltagliati et al., 2001), a well-known case is that of the Hediste limnicola (Johnson, 1901) – H. diversicolor (O .F. Müller) – H. japonica (Izuka, 1908) species complex. There has been much confusion about the taxonomic status of the three species since they were clearly distinguished and recognised as true species using allozymes and reproductive traits (Fong & Garthwaite, 1994). Moreover, H. japonica is considered as a complex of three sibling species: H. japonica, H. diadroma and H. atoka (Sato & Nakashima, 2003). For this species complex the genus Hediste (Malmgren, 1867) is preferred over the genus Nereis commonly applied to these species. Currently the occurrence of two sibling species within H. diversicolor is discussed (Audzijonyte et al., 2008; Virgilio et al., 2009). All species of the genus Hediste bear heavily fused falcigers (composite chaetae) in the posterior neuropodia that are lacking in the genus Nereis (Fong & Garthwaite, 1994). Nevertheless, today the majority of ecologists still refer to the genus Nereis for this species, whereas all population geneticists refer to the genus Hediste (Scaps, 2002).

2.1.2. The species Hediste diversicolor

The common ragworm, Hediste diversicolor (O.F. Müller, 1776) (Fig. 2.1), is a polychaete inhabiting the shallow marine and brackish waters in the north temperate zone from both the European and North American Atlantic coasts (Smith, 1977). Given its great ecological tolerance to extreme environments, it is found in all European brackish-water environments, where it plays a fundamental ecological role in the bioturbation of soft sediments (Scaps, 2002). The common ragworm is a polychaete species with commercial interest and it is therefore dug mainly from brackish-water intertidal mudflats or lagoons of France and Italy and sold as bait for recreational fishing (Bellan, 1964; Ansaloni et al., 1986).

H. diversicolor is characterised by an eversible proboscis with small paragnaths on oral and maxillary rings, a subtriangular prostomium with four eyes,

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two large palpi and two short frontal antennae, a peristomium with four pairs of tentacular cirri (Fig. 2.1). For a comprehensive description of the species, see Fauvel (1923).

Fig. 2. 1. H. diversicolor. View of anterior end. (Courtesy of Emidio Machado).

The common ragworm tolerates great variations of temperature (Ivleva, 1970; Wolff, 1973) and salinity (Wolff, 1973; Neuhoff, 1979) and it is capable to cope with drastic conditions of hypoxia (Wells & Dales, 1951; Kristensen, 1983). As a consequence, this species is able to settle in naturally-fluctuant environments, such as the upper waters of estuaries (Scaps, 2002). Studies by Smith (1955, 1956) suggested that larval stage might be more subject to limitation by salinity than the adults. Adults can withstand a salinity range from freshwater to at least 70‰ (Oglesby, 1969). However, reproduction cannot take place in salinities below about 4‰, because larvae cannot develop in media of such low salt concentrations (Bogucki, 1954, 1963; Smith, 1964). Adults are ionic (Na+, Cl−) conformers at salinities above 25–35‰ of normal seawater and hyperionic in lower salinities (Oglesby, 1970). H. diversicolor is an infaunal species which inhabits sandy and muddy bottoms but also gravels, clays and even turf (Clay, 1967) where it builds U or Y-shaped burrows (Dales, 1950; Lambert & Retière, 1987; Esselink & Zwarts, 1989; Gérino & Stora, 1991; Davey, 1994).

H. diversicolor is gonochoristic and lacks the epitokous phase; it is monotelic and the reproduction is followed by the death of mature worms (Dales, 1950). Reproduction is controlled by temperature and lunar phase. The spawning occurs within a temperature range of 5° C - 11 °C (Scaps, 2002). Literature reports a large variation in the breeding season of field populations of H. diversicolor based on direct

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observations. The reported variation includes a single spawning season (sometimes very short) during spring or summer, an extended breeding season with one or two spawning peaks, or spawning throughout the whole year (Scaps, 2002).

According to experimental observations (Bartels-Hardège & Zeeck, 1990) carried out on individuals collected on the tidal flats of the Jadebusen (North Sea, Germany), eggs are laid by the female inside the burrow before the male ejects his sperm in the water in front of the gallery. Subsequently, the female intensifies its ventilory activity and brings back the sperm into the burrow in a kind of feeding behaviour. Fertilised eggs remain in the maternal burrow, to be brooded by the female. Hatching occurs at the trochophore stage. Larvae crawl on the bottom or remain in the maternal burrow until it reaches the 7 or 8 segments stage. Then the female dies and juveniles, at the end of their “semi-pelagic” phase, reach the 3-setiger erpochaete stage and become sedentary (Scaps, 2002). Very shortly after emergence from their parental gallery, juveniles can be considered miniature adults because of their morphology and behaviour (Marty & Retière, 1999). In most of known populations, maturity is reached between one and two years before spawning (Scaps, 2002).

Divergengence among different population of H. diversicolor on morphological basis has been highlighted in the late sixties by Muus (1967), and was lately confirmed by many authors (Gillet 1990; Garcia-Arberas & Rallo 2000; Khlebovich & Komendantov 2002; Maltagliati et al., 2006). The presence of genetic structuring among geographically isolated population of H. diversicolor is well known since mid nineties (Fong & Garthwaite, 1994; Abbiati & Maltagliati 1996; Rohner et al., 1997) when it was studied mainly with allozyme electrophoresis. Structuring on different spatial scales has been further confirmed by the study of mtDNA sequence (Breton et al., 2003; Virgilio et al., 2006; Audzijonyte et al., 2008; Virgilio et al., 2009). Moreover, the presence of two sibling species was proposed for the North Sea and the Baltic Sea (Rohner et al., 1997; Audzijonyte et al., 2008; Virgilio et al., 2009).

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2.2. Family Mytilidae (Mollusca, Bivalvia)

Members of the family Mytilidae are recognized by characteristics of shell shape, sculpture, hinge, and mussel scars . Bivalves placed in this family are equivalved with strongly inequilateral shells, have anterior terminal or subterminal beaks and an elongate ligament, and usually maintain a heavy and often hirsute periostracum (Soot-Ryen, 1955). They are heteromyarian with the anterior adductor small or absent. The gills are filibranch, and a well-developed byssal apparatus is usually present (Newell, 1969). Many species are gregarious and form large colonies, typically attached to hard substrates, such as rocks, wood, and corals. However, other habitat specializations involve a more solitary existence, such as coral or rock boring (Lithophaginae), byssal nest formation (Crenellinae), or living partly buried in soft sediments (many Modiolinae) (Yonge, 1976). Whereas most mytilid species are marine, a few species are found in brackish or freshwater.

Systematics of Mytilidae includes many complex issues. The family includes 250 estimated species within 33 genera (Boss, 1971) that are divided into four subfamilies (Newell, 1969). Subfamilies and genera are assigned largely according to the shape and characteristics of the valves, hinges, and dentition. Mytiliform species with terminal umbones and pointed, triangular to fanshaped shells, are assigned to the subfamily Mytilinae, whereas modioliform species with subterminal umbones close to the anterior end and a more rounded outline are assigned to the Modiolinae. Modioliform species with valves demonstrating pronounced posterior- and anterior-radiating sculpture separated by a smooth central region are assigned to Crenellinae. Finally, species that bore in rock, coral, or mud with elongate cylindrical shells are assigned to the subfamily Lithophaginae (Soot-Ryen, 1955). Some taxa, however, display intermediate forms and habits or may change from modioliform to mytiliform during development. Moreover, shell shape may be extremely dependent on environmental conditions and on the age of the animal (Seed, 1968).

Not surprisingly, there is little consensus on specific taxonomic assignments within Mytilidae. It has been widely recognized that the heavy reliance on shell form in the classification of Mytilidae and the potential for such forms to arise convergently in response to common environmental conditions cast considerable doubt on the phylogenetic consistency of mytilid classifications (Distel, 2000).

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2.2.1. The genus Mytilaster

The genus Mytilaster was firstly introduced by Monterosato (1883) with the following description "species usually small with choppy sculpture or deep accretion signs, ventral margin arcuate; hinge with teeth and correspondent sockets; ligamental margin all distinctly denticulated". The designed type was M. lineatus (Gmelin in L., 1791) synonymous of Mytilus crispus Cantraire, 1835. Three species were assigned to the genus:

- M. minimus (Poli, 1795)

- M. solidus (H. Martin in Monterosato, 1884) - M. lineatus (Gmelin in L., 1791)

In 1884, the year after the institution of genus Mytilaster, Monterosato re-described genus Mytilaster. Here the author divided the genus into two groups:

i) choppy (or typical) species: M. lineatus;

ii) species with accretion marks, not choppy: M. minimus and M. solidus.

In that work the author gave a brief description of M. solidus, previously undescribed. For some authors M. solidus and M. minimus are different forms of the same species, given the relative morphological variability of the two morphs (Bogi et al., 1985). In 1889, Locard described a fourth Mytilaster species, M. marioni.

2.2.2. The species Mytilaster minimus

Mytilaster minimus has an elongated shape, the angle of dorsal margin is obtuse and placed toward the posterior half; ventral margin is straight, umbones are acute and slightly keeled. Valves are smooth, except for numerous growing marks; periostracum is shiny, not perfectly adherent and dark, reddish toward the dorsal part, yellow-green toward the ventral (Fig. 2.2). Ligament margin is lightly curved, internal ligament being narrow and elongated. Between the ligament and the dorsal angle a series of crenulations can be found. Hinge comprehends two or three cardinal teeth, sometimes obsolete. Bissal fissure is small and narrow. The inside of

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the valves are nacreous and brilliant, with a purplish reflex. Bogi et al. (1985) reports a total shell length of 13 to 15 mm.

0.5 cm

A B

Fig. 2.2. Mytilaster minimus. A) Left and right valves, B) internal view of the right valve.

M. minimus can be found in the Mediterranean Sea and on the eastern coasts of Atlantic Ocean, from notrhern Morocco to the southern side of the Bay of Biscay, especially on exposed rocky shores in the midlittoral fringe (Parenzan, 1974). M. minimus settles in small natural crevices, where waves use to impact harder, in the upper midlittoral fringe. In these areas chemical-physical conditions tend to vary greatly due to effects of the weather and tides. On calcareous shores M. minimus can live in endolithic algae cavities. In the trottoir formed by Lithophyllum tortuosum the species can live in deep siphons, anchored at the base of the algae. Individuals use to live in dense clumps of both adults and juveniles, attached to the bottom or each other with the bissum, sometimes living exposed only the posterior margin for water circulation. The space left between different shells is usually filled up with sediments coming from the erosion of surrounding rocks. These sediments can retain enough moisture so that the animals can survive while not submerged and help them in regulating temperature. M. minimus is well adapted to recurrent emersion, currents, waves, freshwater flowing, drying, high temperatures, sun radiation and can survive for long time within serrated valves slowing down its metabolism. Survival is possible at salinities of 5 to 60 ‰, but long term tolerance is demonstrated between 20 and 40 ‰ (Bouchet, 1961).

Mytilaster minimus is typical of marine water, but it is commonly observed in brackish waters, where it can be found on rocks or on artificial hard substrates (Camilli et al., 2001). In particular, along Italian coasts, M. minimus was recorded in the brackish waters of the Sacca di Goro, in the Fusaro Lake, in the Orbetello Lagoon, and in the coastal pond of Casaraccio, Calich, Corru S’Ittiri and Santa

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minimus associated with marine vegetation in Mazoma Lagoon (Amvrakikos Gulf, Grece). In the Diana coastal pond (eastern Corsica, France) individuals were observed attached on leaves of Zostera sp. (F. Maltagliati, personal communication). Bouchet (1961) identified species’ lower limit on the brown alga Cystoseira, on which the mussels stick. This Author stated that larvae have a stronger affinity for algal coverage rather than rock, but when they have to settle, in late spring, the space is already occupied by more praecox species such as Mytilus edulis or M. galloprovincialis which partly share the habitat. The extreme lower limit of M. minimus is the upper infralittoral, but the species is more commonly found between Mytilus spp. (on the lower limit) and Chtamalus spp. (on the upper limit).

Very scarce information is available about the life-cycle of M. minimus. Some observations along the rocky shore of Calafuria (Livorno, Tuscany, Italy:) showed that the species can produce two generations per year (Curini-Galletti & Galleni, 1984), as most mytilids of the northern hemisphere (Bayne, 1976). For the Atlantic Ocean one generation per year is reported (Bouchet, 1961). The species seems to be sensitive to nitrate concentration and, despite its abundance along the Calafuria shore, where water is not affected by significant organic pollution, it is fairly rare to observe it in the northernmost areas near the port of Livorno (central Italy: Curini-Galletti & Galleni, 1984). Similar patterns are reported for the populations living in Marseille (France: Bellan-Santini, 1965) and Biarritz (France: Bouchet, 1961). To the very best of my knowledge, no studies have been conducted on M. minimus larval life and life-span. Mytilids may have lecitotrophic larvae, but some species show direct development yet the majority have a planktotrophic larva (Bayne, 1976).

Previous studies conducted by Camilli et al. (2001) using allozymes electrophoresis detected high levels of genetic divergence between the population occurring inside the Orbetello lagoon (Tuscany, Italy) and that present in adjacent marine sites. Similar results were obtained from the analysis of other populations living inside and outside several Italian brackish-water environments (Camilli, 2000). Such findings suggested the possibility of genetic structuring for M. minimus related to the colonisation of brackish-water biotopes (Camilli 2000, Camilli et al., 2001).

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2.2.3. The genus Xenostrobus

The generic name Xenostrobus was introduced by Wilson (1967) for Modiolus inconstans Dunker (type species), Modiola securis Lamarck, 1819, and Modiola pulex Lam., 1819. The new genus was based on several major anatomical differences between these species and the genus Modiolus Lamarck, 1810, as typified by M. modiolus (L., 1758). All three species are typical of southern Australasia. X. pulex is an intertidal marine species, X. inconstans is found in estuaries while X. securis lives in the tidal mouths of rivers and the upper reaches of estuaries where the salinities may be very low (Wilson, 1968).

2.2.4. The species Xenostrobus securis

As reported by Wilson (1968), X. securis occurs throughout southern Australia from the Swan estuary in Western Australia to Rockhampton in Queensland. Moreover, it is also found in New Zealand and the Chatham Islands (NZ). In Australia it has often been considered an estuarine form of X. pulex. Perna confusa Angas, 1871 (type locality Lane Cove River, New South Wales) is a synonym. In New Zealand and the Chatham Island the species has been known as Modiolus fluviatilus Hutton, 1878.

The shell is equivalve, subcylindrical, with the ventral margin straight or slightly arched and the umbones nearly terminal. The shell is dark brown on the outside while internally is usually pearly, purplish above and white below the umbonal keel; periostracum is shiny and hairy in younger specimens (Garci et al., 2007) (Fig 2.3).

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X. securis is euryhaline and eurythermal and its resistance to variation in salinity is probably due to valve closure. The salinity tolerance range of adult stage is between 2 ‰ and 56 ‰. At the lower limit animals are vital but metabolism is blocked (Wilson, 1968). Larvae can survive between 16 ‰ and 31 ‰. As a consequence, drafting of adults individuals rather than larval migration trough open sea accounts for colonization of new brackish-water sites (Wilson, 1968).

X. securis can be found at high density on different substrates, natural or artificial, hard or soft, ranging from rocks, shells, mud, plants to pipelines, pylons, buoys and boats (Garci et al., 2007). Mussel beds reach their higher densities for the deeper colonies in the infralittoral fringe, while lower densities are reported for shallower colonies at the intertidal level (Garci et al., 2007). Salinity seems to influence mean sizes of individuals. As reported for Mytilus edulis in the Baltic Sea larger sizes can be reached because of the adaptation to local lower salinity levels (Pearse & Gunter, 1957).

In recent years, X. securis has been recorded in a number of brackish-water biotopes along the Italian coasts of the Adriatic Sea (Lazzari & Rinaldi, 1994; Sabelli & Speranza, 1994; Russo, 2001), in the lagoons of the western Mediterranean coasts of France (Zenetos et al., 2004), and in the Ria de Vigo in Galicia (Atlantic Spain) (Garci et al., 2007). In addition, since the 1970s, X. securis was observed in Japanese waters, where it was incorrectly re-described as Limnoperna fortunei kikuchii (Habe, 1981) (Kimura et al., 1999). Giusti et al. (2008) found individuals of X. securis in a brackish-water canal located in proximity of the port of Leghorn (Ligurian Sea, central Italy). Recently, we found that the species was spread across several coastal brackish-water environments located between the cities of Leghorn and Pisa in central Italy. This bivalve possesses many bio-ecological characteristics of invasive species and it is, therefore, considered a potential pest (Garci et al., 2007).

Pascual et al. (2008) tried to infer the colonization routes of X. securis analysing sequences of nuclear and mitochondrial genes. Samples analysed in Pascual et al. (2008) were collected in Spain, France, Italy and Australia, yet contrasting signals were detected with the different markers, maybe due to very small sample size (2 < n < 4 for each locality).

Figura

Fig. 2. 1. H. diversicolor. View of anterior end. (Courtesy of Emidio Machado).
Fig. 2.2. Mytilaster minimus. A) Left and right valves, B) internal view of the right valve
Fig. 2.3. Xenostrobus securis, right valve

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