Rayed pearl oyster Pinctada radiata (Leach, 1814) (Bivalvia: Pteriidae) in the eastern Adriatic Sea – recent observations Zrakasta bisernica Pinctada radiata (Leach, 1814) (Bivalvia: Pteriidae) u istočnom dijelu Jadrana – novija opažanja

* Adriatic Sea as a part of the Mediterranean is settled by many introduced species which extend their range northward. Among them is the rayed pearl oyster Pinctada radiata , a mollusk of Indo-Pacifi c origin that colonises habitats throughout the Mediterranean. P. radiata established a population in a fi sh farm on the island of Mljet in the Adriatic Sea, which was observed in 2017. To determine the status of the population, 48 specimens of P. radiata were randomly sampled from the ropes of a fi sh farm in 2019. Morphometric parameters were determined using a digital calliper. The recorded parameters showed that shell height ranged from 50.55 mm to 72.98 mm, shell length from 46.66 mm to 71.15 mm, and shell width from 19.98 mm to 58.54 mm. During additional visual survey in 2020 the presence of the spat


INTRODUCTION
The construction of the Suez Canal in 1869 meant not only a shortened voyage for ships, but also a passage for allohtonous organisms. The removal of natural barriers allowed the spread of marine organisms from the Pacifi c/Indo-Pacifi c/Red Sea origin to the Mediterranean [1]. Once the organism arrives in a new, suitable habitat, it may establish a population and possibly expand its range further. To date, nearly 1,000 species have been introduced from the Red Sea to the Mediterranean, and it appears that nearly half of them have established populations and continue to spread [2].
Five years after the opening of the Suez Canal, in 1874, the malacologist Monterosato fi rst described the pearl oyster Pinctada radiata, native to Indo-Pacifi c region as adapted in the Alexandria area (Egypt). P. radiata, most likely a circumtropical species is considered the fi rst Lessepsian migrant [3] and is very abundant in the Levant Basin [1]. Due to its good adaptability and tolerance to a wide range of environmental factors as well as, its ability to disperse by ocean currents, P. radiata has successfully spread throughout the northern and western Mediterranean [4]. Humans have also contributed to the spread of P. radiata, either by deliberate introduction for aquaculture purposes, as in Greece [5,6], or by accidental introduction through ship fouling in France, and Italy (Gulf of Trieste, Adriatic Sea) [7,8]. The P. radiata is widely distributed in Greece and Italy and is considered an established species [9,10]. Since 2015, numerous detections of pearl oysters have been recorded in Albania [11], and a successfully colonised pier was also discovered in the port in Montenegro [12]. The fi rst record of P. radiata in Croatia consisted of two specimens found in the northern Adriatic Sea in 2006 [13]. The fi rst established population was recorded in a fi sh farm on the island of Mljet, one of the southernmost Croatian islands [14]. The morphometric characteristics of the Mljet island P. radiata population are presented in this paper as well as recent records of the P. radiata spat in the Lokrum Island waters and earlier observations in the Bay of Mali Ston.

MATERIALS AND METHODS
The Adriatic Sea is the northernmost arm of the Mediterranean Sea, where waters from the Mediterranean Sea enter through the Strait of Otranto and continue along the eastern coast, spreading many organisms northward by passive transport. The infl uence of the waters of Otranto is greatest in the southernmost areas of the eastern Adriatic.
In June 2019, 48 specimens of P. radiata were taken at random from the ropes of a fi sh farm on the island of Mljet ( Figure 1) at a depth of 4-6 metres. The samples were stored in a refrigerator, and morphometric parameters were measured after 48 hours. The morphological measurements were preformed according to [15]. The morphometric parameters were determined using a digital calliper (accuracy 0.1 mm) and included shell length (SL), shell height (SH), shell width (SW) hinge length (HL), while wet weight (WWTOT), including shell weight and soft body weight, shell weight (WS), wet tissue weight (WWT), and dry tissue weight (WDT) were measured using digital balance (accuracy of 0.001 g). Dry weight was determined after 24 hours of drying the wet soft tissue in a dryer at 110 °C. Data were entered into tables, and averages were calculated using MS Excel.
To determine the presence of the spat within population at Mljet fi sh farm, an additional underwater visual census was conducted in June 2020. Figure 1 Geographical position of Pinctada radiata fi ndings in Adriatic Sea:2008 Survey-two juvenile specimens [13], Brijesta -adult specimens on two occasions (2009 Onofri, 2021 Franušić -oral communications), Mljet -established population [14], Lokrum -spat on more than one occasion, Tivat -established population [12] In the waters of the island of Lokrum, 46 km southeast of the fi sh farm Mljet, sets of collectors for bivalve spat were anchored at three locations, in July 2021 and May 2022 ( Figure 1). The collectors consisted of four 60×40 cm mesh polypropylene bags for vegetable storage and were attached to a rope in of two-meter intervals. In October 2021 and October 2022, the collectors were retrieved from the sea and immediately examined. Organisms settled on the collector bags were collected, fi xed in ethanol and preserved. Species identifi cation was performed in the laboratory.
On two occasions, divers found single specimens of P. radiata among other bivalves in the shellfi sh farms in Brijesta, Maloston Bay (Figure 1)

RESULTS AND DISCUSSION
Specimens of P. radiata found attached with byssus threads to the farm ropes along with the native Mediterranean mussels at the Mljet fi sh farm ( Figure 2) were sampled at random and 48 specimens underwent morphometric measurements.
The minimum, maximum and average values of biometric parameters of the specimens of P. radiata samples from Mljet fi sh farm are shown in Table1.
Shell height ranged from 50.55 mm to 72.89 mm and shell length ranged from 46.66 mm to 71.15 mm, corresponding to the shell heights and lengths of the specimens from the 2017 survey at Mljet fi sh farm [14]. However, shell width values, which ranged from 19.98 mm to 58.54 mm, were much higher compared to shell widths from the 2017 survey (12.66-26.45 mm), so it appears that pearl oysters in this area grow much more with increasing shell width. The lack of smaller individuals suggests that there has been no spawning recruitment in the intervening period [14]. During the study around the island of Linosa in the Sicilian Channel (Italy), both adult and juvenile specimens were observed and a maximum size of 78.7 mm was recorded [15]. The average value of shell height (64.78 mm) corresponds to the average shell height of samples collected in 2017 [14] and those from the Aegean Sea [19], as well as samples from Turkey [20,21] and Tunisia [22]. The biometric parameters measured in the samples collected in Montenegro, the nearest observed established population, were lower, the shell height ranged from 32 mm to 52 mm with an average shell height value of 38.3±6.1 mm [12].
The average wet weight of the sampled oysters was 23.33 g (8.90 g -37.30 g), which is signifi cantly lower than the average of 50.09 ± 16.25 g measured for samples from the Gulf of Antalya in May [20].  The size of the sampled individuals, as well as previous studies indicate perennial survival and adaptation of a relatively numerous population P. radiata is a protandrous hermaphrodite, and the fi rst sexual maturity occurs at a size of 17 mm [16]. Since P. radiata has a well-defi ned annual reproductive cycle, in which temperature is probably the most important parameter for maturation and spawning [17] it can be assumed that all observed individuals are sexually mature. Potential off spring were not observed in the collected samples. The reason for the absence of juveniles is not clear. A visual survey of the population was conducted in 2021, and no juveniles were observed either, so it appears that oysters are not reproducing in the Mljet area. Since Mljet is directly exposed to the water masses coming from the Ionian Sea through the Strait of Otranto, bringing with them everything that cannot resist the currents, this could very well be an introduction path of the existing population on Mljet. The larval cycle of Pinctada spp. is 16-30 days, depending on genetics, temperature, food, and substrate availability, so it is possible that the potential spawning source is quite distant [18]. This is confi rmed in October 2021 and in October 2022 during samplings of spat collectors in the waters of Lokrum Island, 46 km southeast of Mljet island. On those sampling occasions 6 specimens up to 10 mm in size were recorded at the collectors immersed from July to October 2021 and one specimen of P. radiata at the collectors immersed from May to October 2022. Pinctada is an epibiont that colonizes the substrate by attaching byssus threads, and it seems that it prefers anthropogenic substrates where it is a pioneer. It is also found in the area of Mali Ston Bay, an area where oysters and mussels are traditionally farmed and which is not directly exposed to incoming Adriatic currents. On two occasions, divers found a 90 mm and a 95 mm specimen amidst the native European fl at oysters Ostrea edulis. The fi rst specimen was found in 2009 (V. Onofri, oral communication), the second in October 2021 (M. Franušić, oral communication). Since these individuals are the oldest found so far in the Adriatic, and since they occur in the area of shellfi sh farming, it can be expected that the spread of the population would not go unnoticed. It appears that spat prefers previously uncolonized substrate [12,14], so newly submerged buoys, ropes, farms, piers, etc. should be observed.
As a fi rst Lessepsian immigrant [3] and highly adaptable and tolerant of a wide range of environmental factors, P. radiata has successfully spread throughout the Mediterranean [4]. It was accidentally introduced into the Adriatic Sea by ship fouling [8], but it appears that the primary route of introduction and spread for P. radiata is inter-basin connectivity, which allows natural dispersal [23]. Established and naturalized populations are expected to expand their range over time. In addition to the high reproductive potential and use of natural pathways and increasingly favourable temperatures, vessels, marine litter, or animals to which P. radiata individuals can attach will also contribute to the eff ective dispersal of the P. radiata and accelerate long distance spread. Impacts to native organisms and ecosystems, as well as to traditional shellfi sh farming should be expected in near future. Funding: The research was initiated during a fi eld trip with Aquaculture students from the University of Dubrovnik and funded by the University of Dubrovnik. It was also supported by the Croatian Environmental Protection and Energy Effi ciency Fund -the project "Noble pen shell preservation in the Adriatic Sea" coordinated by the Croatian Ministry of Economy and Sustainable Development.
Confl ict of interest: The authors declare no confl ict of interest.