The brown shrimp Penaeus aztecus Ives, 1891 (Crustacea: Decapoda: Penaeidae) spreading northern in the Adriatic Sea: a first record from Croatia

The brown shrimp Penaeus aztecus Ives, 1891, a well - established alien species in the Mediterranean Sea, is here recorded from Croatia (central - northern Adriatic Sea) on the basis of 18 specimens found since November 2016. Such numbers triple the number of confirmed specimens known from the basin so far and testify its northern spreading in the Adriatic Sea. There are no certainties regarding possible pathway(s) of arrival of this species in the area. However, unaided dispersal from neighbouring countries constitutes the most probable one, although introduction through ballast water or deliberate introduction or escape from aquaculture facilities cannot be ruled out. All the specimens were captured while trawling on muddy substrates at depths ranging from 50 to 200 m, and were communicated to the authors of the present note by local fishermen. This confirms the importance of fishermen knowledge to detect the occurrence of non - indigenous megafauna in the Adriatic Sea as a complementary tool to proper scientific surveys.


Introduction
The Mediterranean Sea hosts a variegate biota composed by ~ 17.000 species, which include a high proportion of endemic taxa (25%) but also of alien species (Coll et al. 2010;Zenetos et al. 2017). Among them, alien decapods account for slightly less than 100 species, the majority of which have an Indo-Pacific origin and are widespread in the eastern Mediterranean, having entered the Mediterranean Sea through the Suez Canal (Klaoudatos and Kapiris 2014;Zenetos et al. 2017;Manfrin et al. 2018). Nevertheless, a small proportion of them were also introduced via shipping in ballast water or within fouling communities and through aquaculture, whether intentionally or not (Klaoudatos and Kapiris 2014;Manfrin et al. 2018). To this latter category belong species that may alter ecosystems, modify habitats, change community structure or food web properties, and generally compete with local fauna and affect biodiversity maintenance, like the portunid crab Callinectes sapidus Rathbun, 1896, the panopeid crab Dyspanopeus sayi (Smith, 1869), and the Chinese mitten crab Eriocheir sinensis H. Milne Edwards, 1853 (Galil et al. 2002;Mizzan 2005;Crocetta et al. 2020). However, this category also includes species whose impacts are presumably negligible or poorly known, and that at the same time may also constitute a possible boon to Mediterranean fisheries due to big sizes and possible high market prizes, like several shrimp species, such as the brown shrimp Penaeus aztecus Ives, 1891 and the Japanese tiger prawn Penaeus japonicus Spence Bate, 1888 (Kumlu et al. 1999;Duruer et al. 2008;Kleitou et al. 2021).
Among them, P. aztecus is native and commonly distributed throughout the north-western Atlantic Ocean (from Massachusetts to the Gulf of Mexico and the north-western Yucatán) (Pérez-Farfante 1969, 1988Pérez-Farfante and Kensley 1997), and was first recorded as an alien species in the Mediterranean Sea from Antalya Bay (Levantine Sea, Turkey) (Deval et al. 2010). Within few years, it became among the most widespread alien decapod species in the Mediterranean Sea, with records spanning from the eastern to the western parts of the basin (e.g. Corsini-Foka et al. 2015;Crocetta et al. 2015;Zenetos et al. 2015Zenetos et al. , 2018Mytilineou et al., 2016;Giovos et al. 2019;Özcan et al. 2019;Servello et al. 2019;Stern et al. 2019;Katsanevakis et al. 2020;Kondylatos et al. 2020;Abdulrraziq et al. 2021;El Zrelli et al. 2021). Moreover, this taxon is also expanding in the Adriatic Sea, where it was so far recorded up to the southern and mostly the centralwestern Adriatic (Marković et al. 2014;Karachle et al. 2017;Kampouris et al. 2018;Zava et al. 2018). Based on recent records obtained through the analysis of the bycatch of commercial trawling activities, we hereby widen its distribution in the invaded range and first record the species from the Croatian part of Adriatic Sea.

Materials and methods
The data reported in the present paper fall within the framework of a project held by the authors that aims at monitoring the decapod biodiversity in Croatia by integrating information gained from field surveys and the bycatch analysis of commercial fishing activities. In the present case, one of the authors of the present note was informed by local fishermen about the presence of a big-sized pink shrimp since November 2016 [see Figure 2A, with a specimen of the native Parapenaeus longirostris (H. Lucas, 1846) used for comparison to highlight different total lengths]. Then, once the species was properly identified as the brown shrimp P. aztecus [mostly based on absence of stipes or blotches, presence of an epigastric tooth, presence of abdominal cicatrices, well developed cervical sulcus on the carapace, well developed dorsolateral sulcus on the last abdominal segments, and by the overall uniform creamy-beige or partially orange body coloration: see also Pérez-Farfante (1988) and Pérez-Farfante and Kensley (1997)], and it resulted to be a new arrival in Croatia, local fishermen were   Table 1).
engaged to communicate each subsequent finding to the authors and deliver a photograph of the specimen(s) found each time, together with a ruler or an object.

Results and discussion
Our research effort yielded 18 records until March 2021, all accounting for single adult specimens (≥ 20 cm in total length). They were all taken while trawling on muddy substrates at depths ranging from 50 to 200 m (Table 1, Figures 1, 2). Such numbers almost triple the number of confirmed specimens known from the Adriatic Sea so far, which accounted for a single specimen Figure 2. Examples of photos received by the Croatian fishermen involved in the present study. Full data and copyright as in Table 1.
found in Montenegro in 2013 (Marković et al. 2014), seven specimens found in Italy till 2016 (Zava et al. 2018), and two specimens found in Albania in 2018 (Kampouris et al. 2018) (Figure 1). However, Zava et al. (2018) reported that Italian fishermen communicated to the authors that several specimens were found along the southern and central parts of the Adriatic coast of Italy till 2015, which suggests that the numbers reported above are presumably an underestimation of its presence in the area. A similar situation also holds true here, with fishermen reporting it as usually fished in the area, although we have no proof and thus cannot confirm that the specimens were properly identified. Notwithstanding that, the present record first testifies the common occurrence of P. aztecus in Croatia and its northern spreading in the Adriatic Sea.
The new records reported here immediately raise the question of the origin of these specimens. Indeed, the most plausible hypothesis is that P. aztecus may have recently spread to Croatian waters from nearby detected or yet-undetected populations in the Adriatic Sea. In fact, this taxon displays a relatively short planktonic larval stage that lasts less than 15 days (Cook and Lindner 1970), and therefore other populations in the Mediterranean Sea are too far away. Another possible explanation may be connected to ballast introduction, a vector that was already speculated by some authors for its first record in the Mediterranean Sea (Deval et al. 2010), although it was then considered as unlikely by others (Zava et al. 2018). Finally, the brown shrimp may also have been illegally introduced or may have escaped from nearby aquaculture facilities, although we have no concrete evidences that this species is cultured in Croatia. Penaeus aztecus is already high-priced and sold as a delicacy in some localities of southern Italy (e.g. Mytilineou et al. 2016;Zava et al. 2018), and a similar scenario is also expected in the Adriatic Sea if the populations will increase in number.
Whatever is true, further fieldwork is necessary to evaluate the current status of this alien species in the Adriatic Sea. In fact, taking into account the present records in its Croatian part, the species may be already widespread along the entire coastline, but undetected due to absence of search efforts. Azzurro et al. (2019) recently highlighted the importance of fishermen knowledge to detect the occurrence of indigenous and non-indigenous megafauna in the Adriatic Sea as a complementary tool to proper scientific surveys, and P. aztecus perfectly fits such statement, with all Adriatic records held so far through bycatch analysis and with the collaboration of local fishermen.