The first record of the brown shrimp Penaeus aztecus Ives , 1891 in the central Adriatic coast of Italy

Seven specimens of the northern brown shrimp Penaeus aztecus, a western Atlantic species, were collected in December 2016 and in November 2017 by bottom trawlers off Termoli, on the central Adriatic coast of Italy. The various colonization scenarios put forward as explanations for the species’ sudden, near concurrent presence in distance sites within the Mediterranean Sea and nowhere else, are discussed. The species is already established as a valuable fishery resource in southeastern Sicily, and is likely to do well in the Adriatic Sea, once its population increases.


Methods
On December 2, 2016, several specimens of P. atzecus were caught on a muddy bottom, at depths of 50-60 m, by the M/P Marpesca, a 28 m long commercial trawler equipped with 50 mm net, off Termoli (42° 07' 0122"N 15° 00' 03.90"E), on the central Adriatic coast of Italy; the shrimps were photographed but not preserved.Nearly a year later, on November 13, 2017, seven more specimens were caught by the same vessel at a nearby site (42° 07' 59.69"N 15° 03' 15.81"E), at depth of 50 m.These specimens were photographed and measured (total length, wet weight), preserved in 80% ethanol, and deposited in the Natural History Museum of Comiso, Ragusa, Sicily (MSNC-4560).
The Museo Civico di Storia Naturale of Comiso has engaged fishermen in Sicily and southern Italy in an effort to monitor rare and alien species.Interviews with Apulian fishermen confirm that several large sized specimens P. aztecus have been captured since 2015 between Mola di Bari and Bari (Apulia) and Ortona and Chieti (Abruzzo), at 40-50 m depth.

Discussion
Several colonization scenarios have been put forward as explanations for the sudden, near concurrent records from distant locations within the Mediterranean Sea, and nowhere else (Galil et al. 2016;Scannella et al. 2017).The first, an expansion scenario, posits that P. aztecus was introduced with ballast water to Antalya, Turkey (sometime prior to 2009, Deval et al. 2010), and then by current-driven post-establishment spread along the Turkish Mediterranean coast (Gökoğlu and Özvarol 2013), the western Aegean Sea (Nikolopoulou et al. 2013;Kevrekidis 2014;Minos et al. 2015), and up to Montenegro, in the eastern Ionian Sea (Marković et al. 2014;Kapiris et al. 2014;Crocetta et al. 2015) in less than three years (December 2010-September 2013).This scenario is based simply on the first collection dates, lacking documentation of previous absence at each site, and fails to account for the relatively short planktonic larval stage (less than 15 days) (Cook and Lindner 1970), and mark-recapture experiments that indicated that adults spread parallel to the coastline, with most individuals remaining within 50 km of their release sites (Klima 1963;McCoy 1968).Nikolopoulou et al. (2013: 372) observed that a progressive dispersal from the southern Turkish coast to the northern Aegean Sea "should have required longer period of time than the 2-3 years" and posits multiple introduction events, either additional transfers in ballast waters from the population established in the Turkish Mediterranean or from the native populations in the western Atlantic (Scannella et al. 2017).Yet, it is highly unlikely that ballast introductions are the cause for the burst of records across the Mediterranean Sea.A more plausible explanation for this pattern of spread is that many of the Mediterranean populations issue from direct human introduction (Galil et al. 2016).Mediterranean countries have been notably careless about biosecurity risks due to intentional "unofficial" introductions, movement of stock, feed, and equipment that may result in introduction of marine species (CIESM 2007).The bilaterally ablated female banana prawn, Penaeus merguiensis de Man, 1888, collected in 2006 in the Bay of Iskenderun, Turkey, is certainly an escape or an inadvertent release from a nearby aquaculture facility, since eyestalk ablation is commonly used in aquaculture for inducing maturation of gonads (Özcan et al. 2006)  when specimens infected with White Spot Syndrome (WSD) Virus were submitted to the Office International des Epizooties (OIE) Reference Laboratory for WSD at the University of Arizona, USA, by industry representatives in 1997 (Stentiford and Lightner 2011).None of these introductions were reported to the competent authorities of the respective countries.Thus, the absence of official records of importation of P. aztecus into the Mediterranean Sea does not rule out direct human introduction, particularly as several of the recorded specimens were collected in the vicinity of fish and shellfish farms, including the Italian Tyrrhenian and Adriatic records (Sanna 2010, fig. 1).Progressive dispersal with the prevailing current of larvae and adults may be responsible for some adjacent records (e.g.SE Levant, S Turkey, SE Adriatic Sea).In time, population genetic analyses may clarify the colonization history of P. aztecus in the Mediterranean Sea (Darling et al. 2017).
Invasive Erythraean prawns, chiefly P. pulchricaudatus (misidentified as P. japonicus Spence Bate, 1888) and its relatives -P.semisulcatus De Haan, 1844, Metapenaeus monoceros (Fabricius, 1798), M. stebbingi Nobili, 1904 -are highly prized and are considered a boon to the Levantine fisheries (Kumlu et al. 1999;Can et al. 2004;Duruer et al. 2008).It seems that P. aztecus is likely to join them as a valuable fishery resource.Along the southern coast of Turkey, large numbers of commercial-sized individuals were collected by trawling and trammel netting within three years of the initial record (Bilecenoglu et al. 2013).Along the Egyptian coast, where it was first noted in the Damietta branch of the Nile delta in 2012, it has since appeared annually between February and mid-June and the plentiful wild fry is welcomed by fishers (Sadek et al. 2018).Along the Ionian coast of Calabria, Italy, it is commonly trawled at 90-100 m depth and marketed locally (Mytilineou et al. 2016).Off the central Adriatic coast of Italy it is fished in small quantities and offered for sale mixed with other penaeid species.Off southeastern Sicily in past years it fetched 10-12 euros/kg, but its value rose recently, and present prices in the fish markets of Catania and Scoglitti (Figure 2) rose to 15-20 euros/kg as compared to 30-40 euros/kg for the native prawn P. kerathurus (Forskål, 1775).

Figure 2 .
Figure 2. A crate of Penaeus aztecus Ives, 1891, offered for sale in February 2018, in the fish market of Scoglitti, Sicily, Italy.Photograph by B. Zava.