The presence or even the blooms of gelatinous zooplankton (cnidarians, ctenophores, and pelagic tunicates) can have several negative impacts on ecosystems, on the economy (e.g., tourism, fisheries, aquaculture) and human health (Boero et al., 2008; De Donno et al., 2014; Bosch-Belmar et al., 2017; Marambio et al., 2021). About this latter aspect, it is well-known that zooplanktonic cnidarians, commonly named jellyfish, can generate serious wounds and poisoning to humans, and in some cases death (Burnett and Gable, 1989; Burke, 2002). However, since this gelatinous zooplankton is not usually targeted by fisheries and not easily surveyed or collected by research activities, data on dispersal and impact on human life are sometimes lacking (Condon et al., 2012; De Donno et al., 2014; Rossi et al., 2019; Gravili and Rossi, 2021). Hence, we cannot state with absolute certainty for several species that a general increase of their abundance is occurring or will occur in a specific region. In this regard, Citizen Science and local public networks can be of great help in collecting data on a large spatial-temporal scale in order to better understand the dynamics of jellyfish populations, as demonstrated by several projects (Boero et al., 2010; Baumann and Schernewski, 2012; Fleming et al., 2013; Bellido et al., 2020; Gutiérrez-Estrada et al., 2021; Marambio et al., 2021), though some species remains overlooked, such as Physalia physalis (Linnaeus, 1758).

One of the most dangerous pelagic species in the Mediterranean Sea is the Portuguese man-of-war, P. physalis, a pleustonic colonial organism rarely observed in the open Mediterranean basin, whose direct observations mainly concern stranded specimens (Berdar and Cavallaro, 1980; Deidun, 2010; Boero, 2013; Prieto et al., 2015; Castriota et al., 2017; Mghili et al., 2020; Deidun et al., 2020; Macías et al., 2021). This species, belonging to the Cnidarian group—order Siphonophores, is part of the floating community of ocean organisms that live at the interface between water and air (Boero, 2013; Munro et al., 2019). Though the whole body seems a single individual, it is a colony in which small individuals (zooids) perform different functions such as floating, feeding, defence and reproduction (Lane, 1960). The structure of the colony is complex; on the whole, it can be described as composed of two portions: a portion visible floating on the ocean surface—pneumatophore—and a not visible tentacular portion expanding below the water surface—the complex of zooids (Munro et al., 2019).

Physalia physalis is easily recognized by its bluish pneumatophore, an air bladder developed from one of the zooids that can reach 30 cm in length (Bardi and Marques, 2007; Munro et al., 2019), surmounted by a crest that acts like a sail that helps to navigate and float on the surface of the sea (Lane, 1960; Iosilevskii and Weihs, 2009). The tentacles of this species, maybe up to 30 m long and containing more than 750,000 stinging cells each (Munro et al., 2019), contain the “physaliatoxin” (Bardi and Marques, 2007) that is the cause of its painful sting, which causes a series of symptoms, ranging from local skin necrosis to neurological and cardiorespiratory problems that can cause death (Labadie et al., 2012). Furthermore, unlike other cnidarian species, the Portuguese man-of-war nematocysts seem to penetrate surgical gloves (Pierce, 2006). Physalia physalis behaves like a predator, plays an important ecological role and influences the abundance of populations of small animals, in particular juveniles of fishes and larvae that it captures with tentacles (Purcell, 1984). On the other hand, the species can be preyed upon by fishes, turtles, lepadomorph barnacles, and glaucid nudibranchs (Wangersky and Lane, 1960; Bieri, 1966; Thompson and Bennett, 1970; Arai, 2005), and other taxa not disclosed yet (Arai, 2005).

Physalia physalis is a cosmopolitan species, distributed from tropical to temperate waters roughly comprised from 51°N to 38°S (Shannon and Chapman, 1983; Kirkpatrick and Pugh, 1984; Haddad et al., 2002; Elston, 2007; Pontin et al., 2011; Oliveira et al., 2016). The species is surface-dwelling thanks to the chamber filled with air and carbon monoxide which represents a sail, thus the colony moves with the prevailing wind (Mapstone, 2014).

It is interesting to underline that the cohorts of P. physalis originate from unknown marine georeferenced points (Graham et al., 2001; Ferrer and Pastor, 2017; Ferrer and González, 2021). These colonies move toward coastal areas, where they find optimal trophic conditions, and float along the shoreline, as a consequence of the direction and energy of prevailing winds and surface currents (Ferrer and Pastor, 2017). Thus, the continuously updating of records is much relevant to foresee the dispersal routes.

Up to date, the first documented record of P. physalis for the Mediterranean Sea dates back to 1980 (Berdar and Cavallaro, 1980). The species has been subsequently reported sporadically, in the western and central basins, mostly around Sicily and Malta (Deidun, 2010; Deidun et al., 2020; Boero, 2013). The latest records in Sicily date back to 2019 and 2021, in the Aegadian Sicilian Islands (Deidun et al., 2020) and Donnalucata (Sicily), respectively.

Gathering the baseline information on this species records over time and space constitutes an important task for the elaboration of management programs aimed to alert the local human population and touristic structures (Macías et al., 2021). Physalia physalis represents an important species to be aware of, and for which dissemination of data about its presence should be improved.

Here we introduce a dataset of sightings, strandings, and museum records of the species in the Mediterranean Sea, and the first documented Mediterranean records, originating from the southern Tyrrhenian basin. Relevant biodiversity information can be obtained from materials and data collected in the past and preserved in the Natural History Museums, which can provide overlooked and, in many cases, still unpublished data (Tedesco et al., 2020; Lo Brutto et al., 2021). Additionally, the sightings and strandings herein reported highlight the potential role of the citizens’ participatory science. The data from social media and local Citizen Science projects can have a key role if well-managed; the information is often uploaded on the web but not archived in the scientific literature or summarized in any report, thus risking being lost. The case of P. physalis fills such a gap.

The paper gives an overview on (i) the first records of P. physalis in the Mediterranean Sea through analysis of zoological collections; (ii) the recent records detected in the literature and data from civil society; (iii) the recent records in Italian waters and an assessment of risk for humans.

All data here reported can represent an important reference point for the development of models and predictive tools based on the presence, abundance, and jellyfish trends over a large timeframe.

A survey on the presence of P. physalis in the Mediterranean Sea was carried out. Information was obtained from scientific and grey literature, from the web and social media (mainly Facebook, Instagram and Twitter) and analysed.

As the zoological records of museum collections are not reported in the literature, a further search of historical museum specimens in Italy and Malta was integrated to find out old data never published. In particular, information was acquired from the Natural History Museum of the University of Florence, Italy, the “Darwin Dohrn” Museum of the Anton Dohrn Zoological Station of Naples, Italy, the Museum of Zoology “P. Doderlein” of the University of Palermo, Italy, the Museum of the Department of Biology of the University of Malta, Malta, and the Museum of Zoology, University of Cagliari, Italy. A very recent record of the species is preserved at the Civic Museum of Natural History of Comiso, Italy.

For each source, only information documented with images or collected specimens were validated for the aims of the present study. The records were assigned to the main Mediterranean sectors based on the sampling locality; from west to east: Strait of Gibraltar, Alboran Sea, Algerian-Balearic Sea, Western Mediterranean, Liguro-Provençal basin, Central Tyrrhenian Sea, Southern Tyrrhenian Sea, Channel of Sicily, Strait of Messina.

In accordance with the results by Macías et al. (2021), apart from the Spanish coast, the colonies in the summer period are more frequent in the southern Italian zone. Consequently, the records from the Italian maritime regions were subdivided into three “categories of risk” according to the season of occurrence. The records of P. physalis detected in February-March were included in the low-risk category; the records occurred in April-May in the medium-risk one; the records happened in June-August in the high-risk category. These categories were created to designate a level of danger for swimmers in such a particular area, specifically it was assumed that the sightings of P. physalis in the summer period (June-August) could be considered the most dangerous (high-risk), according to the higher number of swimmers than the remaining months of the year; while medium and low-risk were associated to a lower frequency of human activities accordingly. The records from this zone belonging to the three categories of risk were counted and grouped following the biogeographical sectors proposed by Bianchi (2004). A final map showed such clustering.

We can assert that at least 8,744 colonies of P. physalis entered the Mediterranean Sea, in a timescale comprised between 1850 and 2021 (August). Thirteen colonies were counted in the museum collections explored, 8,346 colonies were counted from scientific literature, information about 16 colonies was found on the web, and 369 records are herein published for the first time. Most of the colonies were reported stranded. The highest incidence was recorded in the Alboran Sea, in a limited area extending from the Strait of Gibraltar to the Algerian-Balearic Sea; while the central Mediterranean area was the one where P. physalis mainly spread (Table 1). None information was found from the Eastern Mediterranean, Adriatic Sea and Aegean and Levantine basin.

All data are shown in Table 1; the records are listed according to chronological order, and each locality is associated with a marine sector indicated by a point in the map of Figure 1 (see the ID numbers). The marine sectors where P. physalis was been recorded are the following: Strait of Gibraltar (corresponding to the ID number 1 on the map of Figure 1), Alboran Sea (ID 2), Algerian-Balearic Sea—Western Mediterranean (ID 3 and 4), the Liguro-Provençal basin (ID 4 and 5), coastline of Corsica—Liguro-Provençal basin (ID 6 and 8), the coastline of Sardinia—Western Mediterranean (ID 7), Tunisian waters—Channel of Sicily (ID 9 and 10), Pelagie Archipelago—Channel of Sicily (ID 11), Northern Tyrrhenian Sea (ID 12), Aegadian Archipelago – Southern Tyrrhenian Sea (ID 13), Southern Tyrrhenian Sea (ID 14), Central Tyrrhenian Sea (ID 15), Maltese Archipelago—Channel of Sicily (16), Strait of Messina (17) (Table 1 and Figure 1).

Physalia physalis is one of 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). P. physalis can be harmful also in an indirect way, as prey of neustonic nudibranchs; in fact, 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 (Tibballs, 2006).

In the “Guidelines for safe recreational water environments” of the World Health Organization (WHO), P. physalis is included among the most dangerous aquatic organisms for human health, suggesting avoiding bathing in waters where Portuguese man-of-war is concentrated (World Health Organization [WHO], 2003). In order to develop policies for controlling and managing the well-being in recreational activities, a wide spatial and temporal scale of data is needed for an adequate assessment of the risk of contact with the Portuguese man-of-war within the Mediterranean Sea. Further, the launch of a series of activities based on scientific dissemination campaigns can allow the local authorities in charge to plan effective actions regarding the protection of public health and tourism (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 animal 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 records of P. physalis in the Mediterranean Sea. The oldest documented capture of P. physalis occurred in Nice in 1850. Furthermore, it was possible to date further colonies of P. physalis, preserved in museums and captured in a wide area of the Western Mediterranean region in a period antecedent to 1980, even older than the record attributed as the first 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). Indeed, based on these new data and those reported in the literature and here examined, it is very likely that P. 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 (not possible in the past) 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), Marambio et al. (2021), and Macías et al. (2021)] and lead us to assess the pathway 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 toward the Mediterranean, as argued by Poulain et al. (2012).

The data showed massive strandings (thousands of colonies) recorded in the Alboran Sea (Prieto et al., 2015; Mghili et al., 2020; Macías et al., 2021), 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) (Figure 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 toward the western and central Mediterranean.

As the circulation features of the Alboran sector is largely controlled by meteorological conditions (Macías et al., 2021), 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 Figure 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 maybe due to biological constraints, as it is probable that P. physalis dies during its “travel” in the Mediterranean Sea and new colonies enter through the Strait of Gibraltar the successive year. Based on this, at least for now, we cannot consider this species as an invasive or non-indigenous one.

The tracking of this species in the Mediterranean Sea should not be neglected, 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 (Figure 5). 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. Concerning some records here reported, such as the specimen of the Gulf of Castellammare (Balestrate, southern Tyrrhenian Sea), the particular morphology of gulfs or bays with their currents regime could contribute to maintaining the colonies of P. physalis within the bay once has entered, with no or little chance to leaving the area. Hence the need for a better understanding and prediction through models of the currents and wind regime in these particular areas is auspicate.

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 Physalia physalis. In this regard, we 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 contribute to the early detection and monitoring of uncommon or non-indigenous species (Giovos et al., 2019; Tiralongo et al., 2019, 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 public 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, 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 and on a large spatial-temporal scale (Schaffer and Tham, 2019). Continuous monitoring could beneficial for marine tourist facilities.

In general, the data reported can be considered an important reference point for the development and calibration of predictive models on jellyfish abundance and distribution over a large timeframe. The importance of obtaining time-series data to monitor biodiversity changes in the Mediterranean Sea is crucial and acquire a relevant role in respect to the more frequent extreme events and temperature anomalies.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

SL and FT conceived the project and contributed to final review and editing of the manuscript. LP provided some data. SL, LP, and RB analysed the data. SL, FT, and RB drafted the manuscript. SL, LP, FT, and VA reviewed the manuscript. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.