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Nomeus gronovii (Gmelin, 1789) is a pelagic species widely distributed in tropical and subtropical waters [1,2] of the Atlantic, Indian, and Pacific Oceans [3], The species is commonly known as the man of war fish due to its close ecological association with the siphonophore Portuguese man of war Physalia physalis (Linnaeus, 1758), with which it exhibits a commensal relationship [3]; It is the sole representative of the genus Nomeus Cuvier, 1816 [4].

To date, N. gronovii has not been recorded in the Mediterranean Sea. The present note reports the first occurrence of this species in the Mediterranean basin, based on a specimen collected from Lebanese waters, Levantine Eastern Mediterranean Sea. In addition, potential introduction pathways and ecological implications associated with this record are discussed.

Materials and methods

On 15 February 2026, a single specimen was captured by a local fisherman (©Annous, M.) using a trammel net at a depth of approximately 23 m near Ramkin Island (34°28′50.98″N, 35°46′19.19″E), Palm Islands Nature Reserve (Tripoli, north Lebanon). Photographs and videos of the specimen were provided to one of the authors (SF) as part of an ongoing citizen science initiative. In 2022, two Marine Protected Areas (MPAs) in Lebanon-Tyre Coast Nature Reserve (TCNR) in the south and Palm Islands Nature Reserve (PINR) in the north-launched a collaborative marine biodiversity monitoring program involving fishers, divers, and marine stakeholders. This initiative aims to anticipate the arrival of Non-Indigenous Species (NIS), monitor established populations, and document ecologically significant taxa [5]. Subsequently, the specimen was retrieved from the fisherman by one of us (SF) for detailed examination, and preservation. Identification was confirmed through the analysis of photographic material and direct inspection of the specimen, based on standard ichthyological diagnostic criteria, and morphological characteristics.

The specimen was preserved in 70% ethanol and deposited under the code PINR12 for future reference and genetic analyses. Morphometric and meristic measurements were obtained directly from the specimen and supplemented by calibrated measurements derived from photographic analysis.

Results

The examined specimen (Figure 1) corresponds to a juvenile individual, with a total length (TL) of 10 cm. The body is moderately deep and laterally compressed, exhibiting a relatively robust appearance compared to typical pelagic juveniles. The general coloration is pale whitish to yellowish, overlaid with irregular dark brown blotches and rounded spots distributed along the flanks, forming a characteristic mottled pattern. The head is relatively large, with a short, blunt snout and a terminal, slightly oblique mouth. The jaws bear small, fine teeth. The eye is large and circular, occupying a substantial proportion of the head length.

Figure 1: Specimen of Nomeus gronovii captured from the Lebanese waters. (C) S. Fatfat.

The dorsal fin is distinctly divided into two portions: a short anterior section composed of four spines, followed by a long second dorsal fin with approximately 32 soft rays. The second dorsal fin is elevated anteriorly and gradually decreases in height posteriorly. The anal fin mirrors the second dorsal fin in both shape and extent, comprising approximately 17 soft rays. The pectoral fins are well developed, fan-shaped, and inserted low on the body. The pelvic fins are relatively large, triangular, and composed of approximately 16 rays. The caudal fin is forked, with moderately elongated lobes and darker pigmentation towards the tips. The caudal peduncle is short and robust. The body surface appears smooth, with fine, inconspicuous scaling.

Meristic counts and morphometric measurements are summarized in (Table 1). The observed counts (D1: IV; D2: 32; A: 17; V: 16) and body proportions are consistent with the diagnostic characteristics of juvenile Nomeus gronovii.

Table 1: Meristic counts and morphometric measurements of Nomeus gronovii specimen from Lebanese waters. TL = total length; D1 = first dorsal fin (spines); D2 = second dorsal fin (soft rays); A = anal fin (soft rays); V = pelvic fins (rays). Values are expressed in centimeters (cm) and as percentages of TL.

Overall, the morphological and meristic features observed in the present specimen are in agreement with previous descriptions of Nomeus gronovii [2], particularly those reported from the Florida Current [6], the North Pacific [7], southern Japan [8], and more recent observations from the southwestern Atlantic [3].

Discussion

The presence of Nomeus gronovii in Lebanese waters represents the first record of this species in the Mediterranean Sea. However, the exact pathway of its introduction remains uncertain, and multiple mechanisms may be considered. The proximity of Lebanese waters (~400 km) to the Suez Canal-a major corridor for Lessepsian migration-suggests a potential route of entry, particularly given that the majority of non-indigenous fish species recorded along the Lebanese coast originate from the Indo-Pacific and have entered via the canal [9]. In addition, recent Lebanese studies have also documented an increasing number of newly recorded Non-Indigenous Species (NIS) in Lebanese waters [10,11].

Nevertheless, considering the pelagic nature of N. gronovii and its well-documented ecological association with the siphonophore Physalia physalis [3], dispersal through surface currents and rafting mechanisms also represents a plausible pathway. Although P. physalis has been reported in the Mediterranean Sea, its occurrence is primarily restricted to the western and central basins and remains rare in the eastern Mediterranean [12]. This suggests that rafting-mediated transport to Lebanese waters, while possible, is likely infrequent. Therefore, the introduction pathway of N. gronovii remains unresolved, and further investigations, including continued monitoring, are strongly recommended.

Ecologically, N. gronovii is known to feed on small pelagic fishes, crustaceans, and planktonic invertebrates [6,3]. Its presence may potentially influence local pelagic communities through competition with native predators and modifications in trophic dynamics [6]. However, the current ecological impact is likely negligible, given that only a single juvenile specimen has been recorded and catched. Continued monitoring is therefore essential to detect potential establishment and assess long-term ecological consequences.

This finding also highlights the importance of citizen science in marine biodiversity monitoring. The detection of this species was made possible through collaboration with local fishers, who act as key observers of environmental change. Initiatives implemented by Tyre Coast Nature Reserve and Palm Islands Nature Reserve demonstrate the effectiveness of engaging stakeholders in reporting unusual catches in the Lebanese waters [5]. Such participatory approaches significantly enhance early detection capacity and provide a cost-effective tool to anticipate the introduction of non-indigenous or range-expanding species, thereby supporting timely scientific and management responses in the Mediterranean region. This is particularly relevant in the eastern Mediterranean, where increasing maritime traffic, ongoing geopolitical pressures, and the expansion of the Suez Canal may further facilitate the introduction and spread of non-indigenous species.

Acknowledgements

The authors would like to thank the local fisherman, Mr. M. Annous, for providing the specimen. This work was carried out within the framework of the MedFund project.

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