Occurrence of the alien crab Percnon gibbesi (H. Milne Edwards, 1853) (Decapoda) and sea hare Aplysia dactylomela Rang, 1828 (Opistobranchia) in shallow marine waters north of Elafonisos Island (Laconian Gulf, Peloponnese, Greece)

Percnon gibbesi (H. Milne Edwards, 1853) (Decapoda, Plagusiidae) and Aplysia dactylomela Rang, 1828 (Opistobranchia, Aplysiidae) are reported for the first time from shallow coastal waters between Pounta and Elafonisos village (Laconia, Greece). The occurrence of the crab unsurprisingly bridges findings from Antikithyra Island to the south and the Mani coast to the west, while the presence of the sea hare links records from the Messinian Gulf, Paros Island, and western Crete.


Introduction
The most recent comprehensive overview of alien taxa in Greek waters lists a total of 237 species (Zenetos et al. 2011), with 60 taxa being recorded from the Ionian Sea and 76 in the south Aegean Sea.Percnon gibbesi (H.Milne Edwards, 1853) (Decapoda, Plagusiidae) and Aplysia dactylomela Rang, 1828 (Opistobranchia, Aplysiidae) were recorded in shallow waters between Elafonisos Island and Pounta beach (Laconia, Greece) in summer 2012.Both species are considered established biota in the eastern Mediterranean by Zenetos et al. (2007Zenetos et al. ( , 2011)).The true provenance of the sea hare is still uncertain (e.g.Yokes 2006;Pasternak and Galil 2010).The mode of introduction of the crab via larval drift, aquarium trade, or ballast water into the Mediterranean also remains controversial (Yokes and Galil 2006;Cannicci et al. 2008;Elkrwe et al. 2008;Katsanevakis et al. 2011).
Percnon gibbesi is widely distributed in the Atlantic, including the Americas, the Caribbean, the African west coast, and most of the Islands (Manning and and Holthuis 1981).The species was initially recorded from the central Mediterranean (Relini et al. 2000) and in Greece from the northeastern Messiniakos Gulf in 2004 (Thessalou-Legaki et al. 2006), showing an eastwarddirected expansion pattern (Katsanevakis et al. 2011).Since then, the species has been reported along the southern Messinian coast, the Mani Peninsula, Antikithira Island, the Cyclades archipelago, and Crete (Thessalou-Legaki et al. 2006;Katsanevakis 2011;Katsanevakis et al. 2010Katsanevakis et al. , 2011;;Nicolaidou et al. 2012;Pirkenseer 2012) (Figure 1, large map).
It is generally assumed that the sea hare Aplysia dactylomela is a circumtropical species (e.g.Eales 1960).However, a recent molecular data set suggests the need to re-evaluate the genus Aplysia, implying independent taxonomic status of Atlantic and Pacific "Aplysia dactylomela" populations, and a possibly synonymy of the latter population with A. pulmonica (Alexander and Valdés 2013).The known distribution of alien Aplysia dactylomela seems to be restricted to the central and eastern Mediterranean (westernmost record Lampedusa Island; Crocetta and Colamonaco 2010;Pasternak and Galil 2010), and might thus be of Indo-Pacific origin.It has been spotted irregularly and mostly in single occurrences in Greek waters, e.g., in the Messinian Gulf, western Crete, and the Cyclades archipelago (Zenetos et al. 2007, Pasternak and Galil 2010, Katsanevakis 2011, Nicolaidou et al. 2012).

Methods and Study Area
The study area is situated west of southern Laconian peninsula between the Island of Elafonisos and the Pounta hamlet, west of the town of Neapolis (Peloponnese, Greece; Figure 1).Data were obtained by in-situ observation and underwater photography during repeated freediving and snorkelling surveys in depths up to 4 m from July to August 2012.Abundance of Percnon gibbesi was estimated along a 100 m section on the coast north of Elafonisos village (Figure 1, please see annex 1) during several sampling events.An outthrust, slightly tilted rock layer (locality 2, Figure 1) runs quasi-parallel to the northern Elafonisos village beach at a distance of 20-40 m from shore.It provides a hard substrate for sessile biota and abundant horizontal crevices as hiding places (Figure 2d) for Percnon gibbesi.The rock layer is overgrown only by a thin algal film, serving as feeding ground for the crab.The site is surrounded by a sandy to slightly muddy soft bottom with sparse seagrass cover (e.g.locality 3 on Figure 1) in water depths between 1-1.5 m.The water is relatively clear, except when turbidity is induced by wave action.Despite intensive human activity, common smaller fish as well as molluscs, Bryozoa and Actinaria were relatively abundant.
Large round boulders densely overgrown by macroalgae (Cystoseira spp.), extremely clear water, strong currents, and small size (approx.420 m) characterize the (artificial?)habitat (Figure 2A) at locality 3 near the southernmost tip of Pounta beach.Except for some small fish (e.g.Diplodus spp., Sarpa salpa) close to the boulders, the surrounding sublittoral sand flats associated to the Pounta beach complex was nearly devoid of macrofauna (Figure 2B).

Percnon gibbesi
The crab Percnon gibbesi (Figure 3) was recorded from the localities 1 and 2 (Figure 1, annex).The morphology of the observed specimens agrees with the taxonomic description in Relini et al. (2000).
Occurrences of P. gibbesi at the northern Elafonisos village beach (locality 2) reached densities (mainly in or close to crevices) of approx.1-2 specimens/m 2 along the outthrust rock layer, which is comparable to the lower end of densities reported from well established Mediterranean localities (e.g. up to 3 crabs/m 2 , Deudero et al. 2005; about 1 crab/m 2 , Thessalou- Legaki et al. 2006;1.6-11.9crabs/m 2 , Sciberras and Schembri 2007; up to 10 crabs/m 2 , Pirkenseer 2012).Occurrence in locations lacking macroalgae and providing abundant hiding space (crevices) as well as the hiding behaviour agree with published reports (e.g., Pipitone et al. 2001;Sciberras and Schembri 2008;Katsanevakis et al. 2010).Due to the relatively low density, the crabs in most cases were observed feeding individually or in pairs on microalgae and retreated quickly into crevices when approached.
A single specimen of P. gibbesi (Figure 3) was observed in the small patch of boulder habitat at the southernmost tip of Pounta beach (locality 1) at about 3 m water depth, a location characterized by a thick growth of macroalgae (Cystoseira spp.).The specimen's isolated occurrence is a bit puzzling, but may represent the first foothold in colonizing the substrate offshore of Pounta beach like the uncolonized rocky area east of the Pounta ferry jetty and approx.100 m west of locality 1 (Figure 2c).

Aplysia dactylomela
A single specimen of the large sea hare Aplysia dactylomela was found in locality 3 (Figure 1, annex 1) on muddy sand with sparse seagrass cover at about 2 m water depth (Figure 4).Its large size (approx.20 cm in contracted state), coloration (black rings connected by a network of black lines on a pale yellowish background), and body morphology agrees with the amended

Summary and outlook
Two non-indigenous species, Percnon gibbesi and Aplysia dactylomela, were recorded for the first time on the coast between Elafonisos Island and Pounta beach (Laconia, Peloponnese, Greece) during the summer of 2012.Their occurrence comes unsurprising, since the observed area represent a convergence point between previous records from localities to the west, east, and south (e.g.Messinian Gulf, Cyclades archipelago, Crete).Due to low numbers of the crab Percnon gibbesi and the single record of Aplysia dactylomela, an additional survey will be carried out in 2013 to verify the establishment of continuous and stable populations, and the possible colonization of adjacent suitable habitats in the area (e.g.Lefki beach area).

Addenum July 2013
Unsurprisingly, the crab Percnon gibbesi has indeed been spotted in the rocky area just adjacent east of Lefki beach in small numbers (see Appendix 1, and locality 4 on large map) on two occasions.The continued presence of the crab at the Elafonisos village beach is confirmed.

Figure 2 .
Figure 2. Boulder habitat at the southernmost tip of Pounta beach (A), extensive sand bottom habitat surrounding Pounta beach (B), uncolonized rock slab habitat approx.100 m west of the boulder habitat at Pounta beach (C), and crevices in colonized rock slab habitat parallel to the Elafonisos village beach (D) (Images by C. Pirkenseer).

Figure 3 .
Figure 3. Single Percnon gibbesi specimen in the boulder habitat at the southernmost tip of Pounta beach; image by C. Pirkenseer.

Figure 4 .
Figure 4. Contracted Aplysia dactylomela specimen in seagrass habitat near the northern Elafonisos village beach; image by C. Pirkenseer.