A STRANGE OCCURRENCE OF PLECTORHINCHUS GATERINUS (ACTINOPTERYGII: PERCIFORMES: HAEMULIDAE) IN THE THRACIAN SEA (EASTERN MEDITERRANEAN)

A juvenile specimen of Plectorhinchus gaterinus (Forsskål, 1775), a fi sh native to the western Indian Ocean, was found in the stomach of a squid caught in the shallow waters of the Thracian Sea (northern Aegean Sea). Apart from the occurrence of the species which is reported for the fi rst time in the Mediterranean, this case is rather interesting but should be considered with some reservation for two reasons: the unusual detection of the species of 10 cm as a prey item of a medium sized cephalopod, and the place of its detection i.e., a marine region that is considered quite inhospitable to alien fi sh native to tropical environment.

The number of alien fi sh in the Hellenic Aegean waters accounts for 35 species, after subtracting from previous inventories the species that extended their distribution range unaided from the Atlantic via Gibraltar (Zenetos et al. 2012) and after the addition of recently recorded species, namely Champsodon nudivittis (Ogilby, 1895) (see Corsini-Foka et al. 2015); Terapon theraps Cuvier, 1829 (see Minos et al. 2012); Bregmaceros atlanticus Goode et Bean, 1886 (see Dogrammatzi and Karachle 2015); Pterois miles (Bennett, 1828) (see Corsini-Foka and Kondylatos 2015); and Scarus ghobban Forsskål, 1775 and Oxyurichthys petersii (Klunzinger, 1871) (see Karachle et al. 2016).In the northernmost sector of the Hellenic Aegean waters four alien fi sh, namely Fistularia commersonii Rüppell, 1838; Lagocephalus sceleratus (Gmelin, 1789); Liza haematocheila (Temminck et Schlegel, 1845); and T. theraps, have been recorded-a number signifi cantly lower compared to the 30 alien fi sh reported from the southern Hellenic Aegean waters (Corsini-Foka et al. 2015, Corsini-Foka andKondylatos, 2015).Only the Lessepsian blue cornetfi sh, F. commersonii, and the silver-cheeked toadfi sh, L. sceleratus, overlap with the alien fi sh list from the southern basin, but their establishment is still under discussion, indicating that the environmental conditions at the northern extremity of the basin are quite unfavourable to fi sh of tropical origin.
The fi rst record of Plectorhinchus gaterinus (Forsskål, 1775) in the Mediterranean Sea is reported here with a reservation and its presence in the Thracian Sea (North Aegean) is discussed.
A small fi sh, preserved frozen, was deposited for identifi cation at the Hydrobiological Station of Rhodes (Hellenic Centre for Marine Research) on 25 June 2015 (Catalogue number HSR118).The sample was found while emptying and cleaning the stomach of a specimen of the European squid, Loligo vulgaris (mantle length of approximately 30 cm), captured by one of the authors (NS) on 15 May 2015.The squid was caught with a spear gun at 8-10 m of depth, on pebbly bottom, in the Thracian Sea, North Aegean, at the northern coast of Thasopoula, an islet at 2 km from the small city of Keramoti and at 5 km from the northern coast of Thasos Island (approximate coordinates: 4049′55′′N, 2442′07′′E; Fig. 1).The approximate surface seawater temperature was about 17-18C and the salinity ranged from 34‰ to 35‰.
Morphometric measurements were taken with a digital calliper under a stereoscope.The identifi cation of the specimen was carried out following McKay (1984) and Smith and McKay (1986).Publications of Ben-Tuvia and McKay (1986), Louisy (2002) and Froese and Pauly (2015) were also consulted.
The juvenile specimen, 105.9 mm of total length, was in good status of conservation and did not show signs of digestion (Fig. 2), though leading to the conclusion that the fi sh was very recently captured and swallowed by the squid.Description.Dorsal fi n rays: XIII + 19; Anal fi n rays: III + 7 (second spine strong and longer than third); Pectoral fi n rays: 15; Pelvic fi n rays: I + 5. Chin with 6 pores not followed by median groove.Gill rakers of fi rst arch: 18 on lower limb, 1 at corner, and 8 on upper limb.Preopercle with 22 spines.Mouth reaching anterior margin of eye, but not overpassing it.Scales ctenoid, rough to touch and extending to nostrils.Palate red-orange.Caudal fi n truncate.Selected proportions: head length 29.28, body depth 35.31 as % of standard length, eye diameter 32.26, interorbital distance 22.58, snout length 35.03 all as % of head length (Table 1).Colour of defrosted specimen.Pale silver-grey with six dark, longitudinal bands (from above, fi rst four black, the fi fth paler greyish and last, ventral shorter and vanishing) (Fig. 2), bending downward and converging to snout (Fig. 2, detail).Pectoral fi ns yellowish transparent, pelvic fi ns yellowish with dark shadings, anal fi n yellowish with more intense dark shadings.Dorsal fi n yellowish, whole upper margin blackish, black band at base of spinous dorsal fi n and fi rst third of soft dorsal fi n, continuing horizontally in middle of the remaining soft dorsal fi n (Fig. 2).Caudal fi n with evident dark band crossing transversally upper lobe and dark large blotches in rest of fi n.
Characteristics and coloration of the young specimen were fully consistent with the specimens of Plectorhinchus gaterinus (Forsskål, 1775) (Haemulidae) described in McKay (1984), Smith and McKay (1986), and Froese and Pauly (2015), apart from the pelvic fi ns length, being, in the presently reported specimen, only slightly longer than pectoral fi ns (Table 1).
Similarly to its confamiliar species, adults of Plectorhinchus gaterinus differ in colour compared to juveniles.Adults are yellowish grey, back greenish, and belly yellowish, while the black stripes bands of juveniles break into lines of dark grey or black-brown spots on body and fi ns at 10-12 cm of standard length (thus, the common name blackspotted rubberlip) (McKay 1984).Up to date, only one species congeneric to Plectorhinchus gaterinus is known in the Mediterranean Sea, namely the native Plectorhinchus mediterraneus, from which our sample differs in meristic counts and colour pattern.Following Ben-Tuvia and McKay (1986), gill rakers of P. mediterraneus are 19-20 on lower arch, dorsal fi n is XI-XII + 17-19 and anal fi n III + 8-9 (gill rakers 18 on lower arch, dorsal fi n XIII + 19, and anal fi n III + 7, respectively in our sample), while, according to Louisy (2002) juveniles of P. mediterraneus have two longitudinal stripes (six in our sample).
The distribution range of Plectorhinchus gaterinus is the Western Indian Ocean: Red Sea, Gulf of Aqaba, Gulf of Aden, Persian Gulf, south to Natal, South Africa, Mauritius, Madagascar, Comoro and Reunion Islands (McKay 1984, Froese andPauly 2015).It occurs at 5-55 m of depth in coastal reefs, sand banks and near estuaries, often in large groups under ledges or along coral slopes by day (Froese and Pauly 2015).The common size of the Blackspotted rubberlip is 35 cm in length, up to 45-50 cm.It feeds mainly on crustaceans, molluscs, small benthic organisms, and small fi shes.In underwater surveys, it is approached with relative ease.The species is fi shed commercially by hand-lines, gillnets, traps, and spears, but its fl esh is not greatly esteemed due to its iodoform taste (Smith and McKay 1986).
Small juveniles of Plectorhinchus gaterinus are seen in the aquarium trade, being attractive for their dazzling coloration and the interesting, exaggerated, side-to-side swimming motion, but they usually suffer as they are fi nicky eaters.Keeping of P. gaterinus in captivity is suggested only for advanced aquarists, at temperatures between 22C and 28C, in large tanks of 700-1000 L capacity, adequate to house the adults.This species is therefore suitable for large tropical displays of public aquaria, such as the Aquarium of the Oceanographic Museum of Monaco (Pierre Gilles, personal communication) and the Aquarium La Rochelle, France (Pierre Moriniere, personal communication); in the past, it was kept also at the Wilhelma Zoologisch-Botanischer Garten Stuttgart, Germany (Isabelle Koch, personal communication).
At this stage of knowledge, the pathway/vector of introduction of Plectorhinchus gaterinus in the Thracian Sea is unknown.A possible introduction of the species through the Lessepsian migration process seems, at the moment, unlikely because of The environmental conditions in the marine area under study, as mentioned above, appear unfavourable to the establishment of species of Red Sea/Indo-Pacifi c origin; There have been no other records of P. gaterinus along the pathway of colonization usually followed by Lessepsian immigrants along the Levantine coasts, after their arrival into the Mediterranean via the Suez Canal.
Some Lessepsian immigrant fi sh such as Lagocephalus sceleratus and Tylerius spinosissimus (Regan, 1908) were fi rstly recorded in the Mediterranean basin from the southeastern Aegean Sea (Golani 2010), a region characterized by a subtropical environment suitable for alien biota of Red Sea origin (Corsini-Foka et al. 2015).The possibility that the fi sh under study was undetected by divers or fi shermen seems also improbable: its size is not irrelevant, its typical and evident dark stripes attract attention and do not allow it to be confused with other species, it dwells in shallow coastal waters and it is easily approached.
Plectorhinchus gaterinus is not listed as a pet fi sh in aquarium stores in Greece (Papavlasopoulou et al. 2014) but today an additional source of ornamental aquatic organisms is the online trade, an emerging commerce, poorly regulated and probably overlooked in many countries (Mazza et al. 2015).The presently reported P. gaterinus may be either a very recent juvenile escapee from aquarium trade or accidental release or discard by aquarists (Zenetos et al. 2012).Furthermore, introduction of P. gaterinus via a cargo ship in its ballast waters should not be excluded, as hypothesized for Terapon theraps by Minos et al. (2012), after its fi nding in Chalkidiki waters, Northwest Aegean Sea.
The majority of the newcomers are recorded for the fi rst time in the Mediterranean through fi shery activities (professional, amateur, research) or underwater biological surveys.Plectorhinchus gaterinus was fi rstly detected as a prey item of the European squid, Loligo vulgaris, common in the study area.Although unusual, this case is not unique: Bregmaceros atlanticus was found for the fi rst time in the stomach of a lizardfi sh, "Saurida undosquamis (Richardson, 1848)" (see Yılmaz et al. 2004), a Lessepsian immigrant recently described as the new species Saurida lessepsianus Russell, Golani, Tikochinski, 2015.The modality of the present fi nding, although it is a single and casual one, could indicate, however, that, in addition to the rather adverse environmental characteristics of the northernmost sector of the Aegean Sea when the introduction of exotic fi shes is considered, predation performed by native biota may be an effi cient natural control factor of newcomers at the initial stage of their possible attempts to adapt to the new ecosystem.
Based on the comments and suggestions of renowned Mediterranean cephalopod experts, the presently reported fi nding of an intact specimen of Plectorhinchus gaterinus of the length of about 10 cm, in the stomach of a medium sized squid (approximately 30 cm) is extraordinary and should be approached with a reservation.However, due to the ongoing changes in marine biodiversity in the Mediterranean, it is believed that every record, despite of how strange it might appear, should be reported for the sake of future scientifi c fi ndings.

Fig. 2 .
Fig. 2. The specimen of Plectorhinchus gaterinus (total length 105.9 mm), found in the stomach of the squid Loligo vulgaris, in the Thracian Sea (Detail: frontal view, showing the trend of dark bands on snout)

Table 1
Principal morphometric measurements of Plectorhinchus gaterinus specimen from the Thracian Sea (northern Aegean) SoDFB = soft dorsal fi n basis, SpDFB = spinous dorsal fi n basis.