After several records of P. gibbesi on different regions of the Algerian coasts: Skikda (Katsanevakis et al. 2011), Algiers (Lamouti and Bachari 2011), Jijel (Noël and Prouzet 2017), Oran (Hussein et al. 2020) and also in the western region of the Annaba Gulf (Menail et al. 2019), our observations by visual surveys carried out on a total linear transect of 16.2 km for two successive years (summer 2017 and 2018) confirm not only the establishment of this alien species in the Gulf of Annaba, but also its expansion along the Eastern Algerian coasts, both inside and outside of the Gulf. The presence of this subtropical crab on the Algerian coasts from East to West is an obvious and logical extension of its distribution area along the countries of North Africa (Azzuro et al 2011; Sghaier et al. 2011; Grimes et al. 2018) and throughout the Mediterranean Sea (Katsanevakis et al. 2011).
Recently, another alien crab has appeared on Algerian coast, the American blue crab: Callinectes sapidus, is know present from the Eastern (Benabdi et al. 2019; Hamida and Kara 2021) to the Western coasts of Algeria (Ragkousis et al. 2020). In the Mellah lagoon (East, Algeria), the invasion of this non-native species, were recorded in mass (Kara and Chaoui 2021). In the Gulf of Annaba, this species has been recently observed invading the extreme East of Sidi-Salem beach, near the mouth of a freshwater stream (B.N., personal observation). The recent presence of this species has even been reported in Lake Tonga (Wilaya El-Tarf), a freshwater lake classified as a Ramsar site since 1982 (Tahri and Boutabia 2022).
The main driver of the P. gibbesi intrusion at the Eastern Algerian coasts would probably result from the migration of adult individuals from the bordering Tunisian coast or by accidental transport through shipping. The Western current of the Mediterranean, which runs along the North African coasts (Moroccan, Algerian and Tunisian), then rises towards Sicily, runs along the north-eastern Italian coast, then along the French coast, and drops down along the Spanish coast, could facilitate and play a role on the transport of planktonic larvae along the Algerian coast from west to east. The relatively long larval phase of P. gibbesi (Paula and Hartnoll 1989) would facilitate the subsequent colonization of post-larvae and megalops (Hartnoll 1992) along east Algerian coasts. It is very likely that there is a time lake between the apparition of this species along the eastern coasts of Algeria and its records, since the population observed in free diving seemed well established. Monitoring exotic fauna, in particular by visual surveys, is often a delicate task, hence the importance of developing medium and long-term programs between the different countries of the Mediterranean coast to collect information on these changes in biodiversity. At the present time, it is difficult to determine exactly the ways arrival of P. gibbesi in the eastern Algerian waters. This massive proliferation on shallow rocky bottoms (< 10 cm), especially in sector 2, was probably favored by the ideal physico-chemical conditions of the area (especially edaphic) and by the rarefaction of predators on this prey. Indeed, the studies carried out on the feeding habits of many coastal teleost in the study area, as is the case of Sparidae (Benchalel et al. 2010), Scorpaenidae (Omri et al. 2019), Labridae (Boughamou et al. 2016), Mullidae (Broudraa et al. 2018) and Serranidae (Zaidi et al. 2017; Rachedi et al. 2018) did not record the presence of this prey in the stomach contents of ichthyologic predators cited previously. The absence of P. gibbesi in the stomacal content of teleosts fish could be explained by the non-identification of the species in view of the highly digestible state of the prey in which they are generally found. Indeed, it is known that the P. gibbesi crab is strictly subtidal and limited to the uppermost reaches of infralittoral zone characterized by rocky shores (Pipitone et al. 2001; Deudero et al. 2005; Thessalou-Legaki et al. 2006) and occasionally the wide bare rock ledges or in crevices on vertical rock walls with a moderate algal cover (Deudero et al. 2005; Sciberras and Schembri 2008). The absence of this species on sandy or homogeneous rocky bottoms or in P. oceanica seagrass at both sectors has also been observed in other regions of the Mediterranean (Sciberras and Schembri 2008; Félix-Hackradt et al. 2018). We suppose that the density of P. gibbesi is closely linked to the availability of algae and adequate rocky structure. Pipitone et al. (2001), revealed that the strong affinity of P. gibbesi for rocky bottoms result probably of the security hidden protection of predation and that availability of food resources.
The present comparative study revealed a very high significant differences in the abundance of P. gibbesi in two different sectors, one exposed to anthropogenic actions (sector 1) and the other far from any source of human pressure (sector 2). We can suppose that this numerical disparity in abundance P. gibbesi is due principally to the distance between the studied sectors and their proximity to the Annaba City and thus, the increasing of anthropogenic activities, represented particularly by the impact of balnear tourism, coastal urbanization and industrialization which are often accompanied by domestic and industrial pollution problems. Indeed, a total of 7 beaches (Lever de l’Aurore, Saint Cloud, Chapuis, La Caroube, Toche, Belvedere, Ain Achir), several hotels and balnear and water sports resorts are located along sector 1, which could explain the low abundance of P. gibbesi observed in all the sampled stations. The homogeneous topographical nature of the substrate in sector 1, such as the presence of large sandy areas, scattered P. oceanica seagrass and flat boulders, certainly constitute unfavorable habitats for the establishment of this species in this sector.
In the two sampled sectors, the three size categories (S, M and L) are significantly represented but always with numerical proportions in favor of individuals of medium and large size. While free diving, small crabs could escape the observer when counting, hence their number is underestimated within established populations. Around Balearic Islands, Cannicci et al. (1999) revealed that the medium individual’s size category of P. gibbesi predominated the distribution of the population. They have also reported that the small individuals were less abundant and they have supposed that was related to the sampling period and recruitment.
The highest mean densities never exceed 3 ind/m², a maximum value observed in station C in sector 2. Overall, the mean density is always less than 1 ind./m². Within "Capo Gallo-Isola delle Femmine" MPA, in the north-west of Sicily, the mean density (ind./100 m² ± se) of the whole area was estimated to 42 ± 6.67 individuals, i.e. less than 1 ind./m² with an almost similar population size class frequency distribution (S: 37%, M: 35%, L: 28%) (Agnetta et al. 2012). Around Dragonera Islands (Spain), the highest densities are recorded on pebble substrates, reaching up to 3 ind./m² (Deudero et al. 2005). However, the rapid expansion of this crab in Mediterranean waters with its ability to occupy disturbed areas (Cannicci et al. 2006) and its resistance to temperature changes (Calado 2006) have shown that their densities can be very high, reaching up to 11.9 ind./m² (Sciberras and Schembri 2007). On the basis of our freediving visual surveys, we found that this invasive species has a preference for rocky and isolated environments where its chances of establishment and survival would be higher compared to homogeneous habitats and areas of high anthropic pressure.
Concerning the behaviour of P. gibbesi we observed that when there is a hydrodynamic tension, this flat crab becomes more active it will be able to makes jumps between cracks to collect suspended algal particles. The behaviour of young individuals observed is slightly different compared to the other larger individuals, the small individuals seem to be more sensitive to the presence of observer, they are perfectly concealed in the cracks and crevasses and their presence are always accompanied by larger individuals.
Other interesting aspects in this study deserve to be clarified on this invasive species such as the (i) nocturnal incursions observed out of the water, probably in search of food (ii) the bacterial attacks that we observed at abdominal level and (iii) unexplained mortality in adult individuals during the summer at very shallow depths. The collection of all this information could allow the scientific community to better understand the movements and the establishment of this species on the Algerian east coasts and along the Mediterranean in general.