Percnon gibbesi (H. Milne Edwards, 1853) and Callinectes sapidus (Rathbun, 1896) in the Ligurian Sea: two additional invasive species detections made in collaboration with local fishermen

The non-indigenous crabs Percnon gibbesi (H. Milne Edwards, 1853) and Callinectes sapidus (Rathbun, 1896) (Crustacea, Decapoda, Portunidae) are reported from Genoa and La Spezia, respectively, in the Ligurian Sea at the northern rim of the Mediterranean Sea. Both specimens were collected by fishermen who then brought them to the attention of professional scientists. This illustrates the importance of engaging local communities in detecting non-indigenous species and monitoring


Introduction
The grapsoid crab Percnon gibbesi (H.Milne Edwards, 1853) (Percnidae) is widely distributed on shallow, infralittoral, rocky shores from: California to Chile in the Pacific Ocean; North Carolina to Brazil in the Western Atlantic Ocean; and the Azores to Angola in the Eastern Atlantic Ocean (Katsanevakis et al. 2011).In the Mediterranean Sea, the species was likely introduced through shipping (Galil et al. 2008), although aquarium releases (Katsanevakis et al. 2011) and larval drift through the Strait of Gibraltar (Pipitone et al. 2001) cannot be excluded as potential pathways.In 1999, the species was first detected almost simultaneously from the Island of Linosa (Sicily) (Relini et al. 2000) and the Balearic Islands (Garcia and Reviriego 2000).It has since spread rapidly throughout the Mediterranean.In Italian waters, the species spread north from the first record in Linosa (Relini et al. 2000), through Sicily (Pipitone et al. 2001;Mori and Vacchi 2002;Catalano 2004), the Adriatic Sea (Gravili et al. 2010), the Ionian Sea (Faccia and Bianchi 2007;Crocetta and Colamonaco 2010), and the southern (Bellantoni and Corazza 2002) and central Tyrrhenian Sea (Russo and Villani 2005).North of Naples, the distribution of this species is deemed as "not continuous" (Katsanevakis et al. 2011) and, until recently, there were no observations of P. gibbesi north of the Tuscan Archipelago (Stasolla and Innocenti 2014).The species was considered absent from the Ligurian Sea (Katsanevakis et al. 2011); nevertheless, in 2016 high densities of P. gibbesi were reported from Elba and Capraia Islands, just south of the Ligurian Sea (Stasolla et al. 2016).Here we report a specimen from Genoa, at the northernmost extent of the Ligurian Sea.
The blue crab Callinectes sapidus (Rathbun, 1896) (Crustacea: Decapoda: Portunidae) is a commercially important species, widely distributed in estuaries and lagoons along the entire Western Atlantic coast from southern Nova Scotia, Canada, to Argentina (Nehring 2011).It is an euryhaline crustacean commonly found on muddy and sandy bottoms, generally at depths < 35 m (Hill et al. 1989).The earliest confirmed record in the Mediterranean is from 1948 when two specimens where found in the Northern Adriatic (Giordani Soika 1951), although its presence in the Aegean Sea was suspected as early as 1935 (Nehring 2011).Since then, C. sapidus has been widely recorded in Mediterranean coastal waters (Galil et al. 2008;Galil 2011;Daban et al. 2016) and in Atlantic Europe (reviewed by Manfrin et al. 2016;Mancinelli et al. in press).In Italy, there now are established populations of blue crab in the Adriatic Sea (Cilenti et al. 2015;Manfrin et al. 2016).Elsewhere from Italian seas, there are only a few reports from the Ionian and Tyrrhenian Seas (Cavaliere and Berdar 1975;Bisconti and Silvi 2005;Gennaio et al. 2006;Stasolla and Innocenti 2014;Sperone et al. 2015).In the Ligurian Sea, the only records are of two specimens collected in the the port of Genoa (1962) and a large male caught near La Spezia in 1965 (Tortonese 1965).Here we report another specimen from the eastern part of the Ligurian Sea.

Material and methods
A single specimen of Percnon gibbesi (Figure 1) was captured by hand on 24 October 2016 in Genoa, Italy (44º23′30.1″N;8º57′37.6″E) at 1 m depth by a sport-fisherman.Wishing to document an unusual occurrence, the fisherman contacted professional researchers and provided the specimen.On 11 November 2016, a single specimen of Callinectes sapidus (Figure 2) was captured by artisanal fishermen, near the harbour of La Spezia, in a gill net set at 3-5 m depth (44º04′48.2″N;9º52′17.58″E).Both specimens were photographed and carapace measures (length and width) were taken to the nearest mm.Thereafter, the P. gibbesi specimen was fixed in 75% ethanol and deposited in the biological collection of CNR-ISMAR of Pozzuolo di Lerici, whilst C. sapidus was fixed in 75% ethanol and kept by the collector.S1-S2).

Results and discussion
The P. gibbesi specimen was an adult male with carapace length (CL) of 32.0 mm, carapace width (CW) of 27.5 mm and orbital spine width (OW) of 16 mm (Figure 1).During collection, the 4 th right pereiopod and the left cheliped were lost, however all morphological features fitted very well with the literature descriptions of this species.Water temperature at the time of collection was 21 °C and the bottom consisted of artificial boulders and scattered pebbles with low macro-algal cover.These characteristics of the collection habitat agree with the preferences of P. gibbesi for shallow artificial rocks (i.e., in ports) (Thessalou-Legaki et al. 2006), as also reported by many other observations (e.g.Pipitone et al. 2001;Fanelli and Azzurro 2004;Deudero et al. 2005;Russo and Villani 2005).More importantly, our finding confirms anecdotal reports of the occurrence of this invader in the Genoa Gulf (cited by Stasolla et al. 2016).The distribution of P. gibbesi has therefore expanded up to the Ligurian Sea, at the northernmost rim of the Mediterranean Sea (Figure 3).Noteworthy, the sighting of P. gibbesi in this case, as on other occasions (e.g.Stasolla et al. 2016), was provided first by interested citizens and then confirmed by professional scientists.
The C. sapidus specimen was an adult male 190.5 mm CW.At the time of collection, the surfacewater temperature was 21.2 °C, and the area is mostly characterized by sandy-muddy bottoms.The fishermen immediately recognized the blue crab as an "unusual capture" of a conspicuous species "never seen before", factors that can be key components for the participation of fishermen in the detection of exotic species (Azzurro 2016).Multiple introduction events by ballast waters are considered the most probable initial vector (Nehring 2011), and the finding of this specimen in proximity of La Spezia port is consistent with the role of ships in the spread of this species.
Both, P. gibbesi and C. sapidus are considered among the "worst" invasive alien species in the Mediterranean Sea (Streftaris and Zenetos 2006), but appropriate monitoring protocols do not exist for these species or for most marine invaders.Nevertheless, increasing efforts are being made to engage local communities in the process of detecting and reporting newcomers, representing examples of usefulness of both citizen science and local ecological knowledge (e.g.Azzurro et al. 2011).These participatory approaches provide new and complementary options to detect and investigate biological introductions while promoting awareness among local communities.

Figure 2 .
Figure 2. Callinectes sapidus male specimen collected on November 11 th , 2016 in the Gulf of La Spezia, Italy (Photo by E. Azzurro).

Figure 3 .
Figure 3. Capture locations of Percnon gibbesi (top) and Callinectes sapidus (bottom) in the Mediterranean Sea.Present findings are marked with a black star.Previous records are marked in red and based on the revisions of Katsanevakis et al. (2011) and Stasolla et al. (2016) for P. gibbesi; Galil et al. (2008), Manfrin et al. (2016) and Mancinelli et al. (in press) for C. sapidus (see details in supplementary material TablesS1-S2).