Physalia physalis is among the most dangerous species of cnidarians that threaten public health. This is because the sting of this species is very painful and the venom present in nematocysts can cause health complications for people attacked. In some documented cases, these incidents have had fatal outcomes (Burke 2002; Labadie et al. 2012). Physalia physalis can be harmful also in an indirect way, as prey of neustonic nudibranchs; some planktonic molluscs ingest the cnidarian and undischarged nematocysts as a defensive strategy (Thompson and Bennett 1969). The nematocysts maintain toxicity in the predators (Thompson and Bennett 1969; Ottuso 2009). The same harmfulness has been detected in the stranded P. physalis individuals on a beach, even after several days of dehydration (Tiballs 2006).
The World Health Organization (WHO) included P. physalis in the dangerous aquatic organisms for human health within the “Guidelines for safe recreational water environments” and suggested avoiding bathing in waters where Portuguese man-of-war is concentrated (WHO 2003). The assessment of the risk of contact with the Portuguese man-of-war within the Mediterranean Sea to develop policies for controlling and managing the well-being in water recreation needs a wide spatial and temporal scale of data. Further, the launch of a series of activities, through scientific dissemination campaigns, can allow the local authorities in charge to plan effective actions regarding the protection of public health and tourism activities not as isolated measurements for managing risks (De Donno et al. 2014; Montgomery et al. 2016).
This is particularly relevant within the Mediterranean Sea where dispersal of some cnidarian species need major attention (Rossi et al 2019) and where a great portion of the population still ignore the real hazard of most of the species, even though the high number of sightings and strandings recently detected through the media and social networks have helped to deepen the knowledge of the group.
Thus data were herein collected from different sources: social networks – scientific literature – museum zoological collections, as current and historical information on the distribution of the Portuguese man-of-war in the Mediterranean had not been organized yet and presented in a single article.
Under this operational framework, it was possible to track the putative first record of P. physalis in the Mediterranean Sea. The oldest documented capture of P. physalis occurred in the Gulf of Naples in 1914. Furthermore, it was possible to date a colony of P. physalis, presumably beached, on the North-Western coast of Sicily in a period antecedent to 1980 (Silvano Riggio pers. comm.), even older than the record by Berdar and Cavallaro (1980) in the Strait of Messina.
The role of museums was fundamental for reconstructing the past distribution of P. physalis. A new scenario on its presence in the Mediterranean Sea has been depicted, up to now suspected by the scientific community (Boero 2013). Physalia physalis has always entered the Mediterranean Sea.
The occurrence of P. physalis in the Mediterranean had been considered rare till the last decades, due to the lack of documented records in the literature (Castriota et al. 2017). Only in recent years, an increase of P. physalis sightings in the central Mediterranean Sea was reported. The reasons are attributable to the advancement of technological innovations (smartphones), which make sightings validated with photographs or videos, in the past not allowed, as the case of the Maltese fishermen who were not able to provide evidence of sightings (Calleja 2009).
From 2001 to 2020, several P. physalis strandings have been documented in the Mediterranean Sea (Patrick J. Schembri pers. comm.; Deidun 2010; Focus 2010; Boero 2013; Mare Nostrum Italia 2013; Prieto et al. 2015; Castriota et al. 2017; Deidun et al. 2020; Mghili et al. 2020) and lead us to assess the flow of the colonies in the Mediterranean Sea. The route and the extent of flow depend on the particular climatic-oceanographic conditions which regulate the entry from the Atlantic Ocean towards the Mediterranean, as argued by Poulain et al. (2012).
The data showed massive strandings (hundreds of colonies) recorded in the Alboran Sea (Prieto et al. 2015; Mghili et al. 2020), probably due to the action of the strong gyres in shallow water (Poulain et al. 2012), which behave like a physical barrier, as suggested by Prieto et al. (2015). The Atlantic water in the Alboran Sea describes two anticyclonic (clockwise rotating) gyres that dominate the surface flow pattern: a quasi-permanent anticyclonic gyre in the west (Western Alboran Gyre, WAG) and a more variable circuit in the east (Eastern Alboran Gyre, EAG) (Fig. 1) (Millot 1999). In such a situation, the vein flowing from Spain to Algeria, named ‘the Almeria-Oran jet’ (Millot 1999), stops a large number of colonies, preventing their eastward dispersal, except some colonies that find a favourable oceanographic route and move towards the Western and Central Mediterranean.
As the circulation features of the Alboran sector is largely controlled by meteorological conditions (Macías et al. 2008), some concerns regard climate change which could alter the role of natural barrier in the future, allowing a greater entry of P. physalis from the Atlantic Ocean. Under such a hypothetical scenario of an increasing number of colonies of P. physalis, we should limit human activities eventually exposed to risks in the central Mediterranean.
The pattern described in the map of Fig. 1 gives an idea of how the Portuguese man-of-war circulates in the Mediterranean. The records follow the surface currents and cover a limited geographical range may be due to biological constraints, as it is probable that Physalia physalis completes its life cycle in the Mediterranean Sea.
The tracking of this species in the Mediterranean Sea should not be overlooked, as the species threaten human health. For this reason, we selected some areas of risk by grouping the records of P. physalis in the most concerned four Italian maritime sectors: Sardinian waters; Channel of Sicily; Tyrrhenian Sea; Strait of Messina. The marine sectors most involved by P. physalis passage were identified and associated with levels of danger for swimmers according to the seasons of occurrence of the colonies (Fig. 4). The highest number of colonies were recorded in the Channel of Sicily, probably also thanks to the active monitoring that has taken place in recent years. The area showed records belonging to the three categories of risk, low- medium- and high-risk category; it means that the species occurs from February to August. The Strait of Messina seems to be the area less involved by the passage of the Portuguese man-of-war.
The recent expansion of Citizen Science projects and the use of social media for the detection of “strange-looking species” or charismatic species may have certainly contributed to increasing the number of observations in nature, including P. physalis. As a consequence, the presence of the species seems to have suddenly increased only in recent years.
In this regard, we have observed, especially in the last decade, how Citizen Science is experiencing an upsurge of interest and was demonstrated to be particularly effective for data collection and maritime monitoring (Devictor et al. 2010; Dickinson et al. 2010). In the Mediterranean Sea, several Citizen Science projects are currently active and led to the early detection and monitoring of uncommon or non-indigenous species (Giovos et al. 2019; Tiralongo et al. 2019; Tiralongo et al. 2020). These projects have often benefited from the involvement of the public through social media (Azzurro and Tiralongo 2020; Al Mabruk et al. 2021). Citizen Science could be an efficient tool for information campaigns to introduce the Portuguese man-of-war to the population and better track the early detection of the species, as already done for other harmful organisms (Andaloro et al., 2016).
The results suggest that a national multidisciplinary summer surveillance program in the central Mediterranean Sea coast is required to provide alerts to the public, to better identify patients at risk for developing severe clinical symptoms, and hopefully to improve the quality of health care. Local authorities should set up a monitoring and alert system, with the participation of territorial organizations, and the contribution of the scientific community, as proposed in other seas (Labadie et al. 2012). In this regard, engaging tourists as citizen scientists can be a useful and low-cost method for the monitoring of this and other dangerous species in general on a large spatial-temporal scale (Schaffer and Tham 2019). Continuous monitoring could beneficial for marine tourist facilities.
In general, the importance of obtaining time-series data to monitor biodiversity changes in the Mediterranean Sea is herein demonstrated and acquire relevant role respect the more frequent extreme events and temperature anomalies.