Introduction

The oarfish Regalecus glesne Ascanius, 1772 also known as the king of herrings, is a slender, greatly elongated and ribbon-like fish of the family Regalecidae, order Lampriformes. This family has undergone many rearrangements over time that have added or deleted species and genera considered synonyms or sisters (Albano et al. 2022). Currently, the accepted classification attributes two genera to this family, Agrostichthys Phillipps, 1924, with Agrostichthys parkeri (Benham, 1904) as unique species, and Regalecus Ascanius, 1772 with two species Regalecus russellii (Cuvier, 1816) and R. glesne Ascanius, 1772. Roberts (2012) provides a diagnosis of both Regalecus species which can be differentiated mainly by the number of total gill rakers and dorsal-fin rays. The oarfish, known as the longest teleost (maximum recorded length of about 17 m) (Robins et al. 1986), is a mesopelagic fish occurring in most tropical and temperate areas of the planet (Froese and Pauly 2010). It is mainly found at depths from 300 to 1000 m (Roberts 2012), only rarely observed or caught alive in surface waters (Whitehead et al. 1986). Despite having a cosmopolitan distribution (Roberts 2012), reaching great depths, it is rarely caught by common fishing gearsand probably this contributes to its rare reports from fishermen (Schmitter-Soto 2008; Dragičević et al. 2011). In the Mediterranean Sea, R. glesne was first recorded in 1826 by Risso under the name Gymnetrius gladius. It is considered a very rare species (Quero et al. 2003), despite its presence has been reported throughout the entire basin (Whitehead et al. 1986; Kova et al. 2021). The information on its biology is scarce and based mainly on stranded or dying specimens (Schmitter-Soto 2008; Dulčić et al. 2009). In particular, this teleost has been reported in Liguro-Provençal waters between the Ligurian Sea in Italy (Ariola 1904; Mezzana 1909; Vinciguerra 1918; Guiglia 1950; Tortonese 1970; Psomadakis et al. 2008, 2012) and the Gulf of Lion in France (Risso 1827; Cuvier and Valenciennes 1835; Giglioli 1880; Damiani 1913; Vayssiére 1917; Quero et al. 2003; Psomadakis et al. 2008; Gicqueau et al. 2018), where its strandings are quite frequent (Psomadakis et al. 2008). It has also been reported in Spanish waters of the western Mediterranean (Lozano-Cabo 1969) and south of the Balearic Islands in the Algerian basin (Moranta et al. 1998), which currently remain the only two records from the western Mediterranean Sea. It has been recorded off Gennadi (Greece) (Corsini-Foka 2009) and in the Aegean Sea (Ondrias 1971; Papaconstantinou 1988), in the Egypt waters (Haroun et al. 2017), in the Levantine Sea along the Turkish coasts (Bilecenoglu et al. 2014), in the Maltese waters in the central Mediterranean Sea (Jennings 1996; Lanfranco 1993) and in Croatian waters of the central Adriatic Sea (Dulčić et al. 2009; Dragičević et al. 2011). In the Italian seas, apart from the Ligurian Sea mentioned above, the oarfish has been observed in the waters surrounding the island of Elba (Damiani 1913), in the northern Tyrrhenian Sea (near the shore of Castiglioncello) (Vinciguerra 1918), in the Gulf of Naples (Tortonese 1970) and in Terracina (Psomadakis et al. 2012). It was also captured in the southern Tyrrhenian Sea, in the area near the Strait of Messina in Sicily (stranded in Oliveri), where eggs and young individuals at different stages of development were also collected (Berdar et al. 1975; Cavallaro and Cavaliere 1980) and a single juvenile specimen was also reported from Capo Peloro, in Sicily (Cavallaro and Cavaliere 1980). Its presence has also been documented in the northern Adriatic Sea (off the coasts of Rimini) (Padovani 1933), while to date, no findings have been recorded in the Ionian Sea. All mentioned occurrences are shown in Fig. 1.

This work updates the information on the distribution of R. glesne in the Mediterranean basin, reporting the finding of two specimens of this rare teleost in the north-western Ionian Sea. Morphometric measurements and information on the sex and gonad maturation stage of the two captured individuals are also reported.

Fig. 1
figure 1

Previously recorded occurrence of R. glesne in the Mediterranean Sea (●)

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Materials and Methods

Two specimens of R. glesne were captured in the north-western Ionian Sea (Fig. 2; Table 1). The first (CODE 1), during an experimental haul carried out in September 2021 at a depth of 525 m off the coast of Maruggio in the province of Taranto, Apulia (Italy) (40.15016° N, 17.51416° E) within the MEDITS survey (International Bottom Trawl Survey in the Mediterranean) (Spedicato et al. 2019) The second specimen (CODE 2) was accidentally caught in March 2022 at a depth of 350 m, always in the Gulf of Taranto, Apulia (Italy) (40.40900° N, 17.00066° E), by a professional fishing vessel engaged in commercial trawling. After the capture, both specimens were placed individually in plastic container and kept frozen (-20 °C) until the time of analysis. The main morphometric measurements were taken for both specimens, in particular, the two lengths were measured to the nearest mm, using a millimetric ichthyometric table with an accuracy of 1 mm, while the two body weights were measured using a digital balance with an accuracy of 0.1 g. Furthermore, information on sex and maturity stage of the gonads was also recorded. Since the specimens were captured with the terminal part of the body truncated, the Total Length (TL) was estimated considering that the Pre-Anal Length (PAL) represents approximately 30% of the TL, as reported by Psomadakis et al. (2008).

Fig. 2
figure 2

Records of two specimens of Regalecus glesne, CODE 1 and CODE 2 captured in the north-western Ionian Sea (▲)

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Table 1 Historical records of Regalecus glesne in the Mediterranean Sea from 1826 and new findings in the north-western Ionian Sea (°)
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Results

Historical Records

In Table 1 are shown the geographical area of the records, the year of occurrence, the total length, when observed, and the reference of the records of R. glesne reported in the Mediterranean basin. It can be observed that most of the R. glesne findings are distributed in the western part of the Mediterranean basin, between the Ligurian Sea (Italy) and the Gulf of Lion (France), with 22 and 17 findings, respectively. The TL recorded in the Ligurian Sea ranges from 86.5 to 433 cm, and in the Gulf of Lion from 88 to 446 cm. Most of the specimens were caught by local fishermen or observed alive swimming near the water surface. Some specimens, instead, were found stranded. Anyway, ancient reports present some problems related to the different nomenclature of the species used in past centuries (Albano et al. 2022). Also, possible errors in the previous morphological identification of specimens are not to be excluded, considering that some reports are attributed to fish observed alive and not caught, especially considering the lack of scientific knowledge and tools available in the past.

New Findings

The new records of R. glesne, captured in the north-western Ionian Sea, extend the distribution range of the species in the Mediterranean Sea. All recorded morphometric measurements are reported in Table 2, where the estimated TLs for two specimens (CODE 1 and CODE 2) were 650 and 2700 mm, respectively. The individual CODE 1, identified as sexually indeterminate, had body damaged mainly along the dorsal and pectoral fin rays and altered body colour, due to poor recovery status. The specimen CODE 2 showed some visible marks probably due to the mesh of the fishing net and some damages on dorsal and pectoral fins (Fig. 3A). Only a few of the long anterior rays of the dorsal fin, which form the characteristic “plume”, have been preserved intact (Fig. 3B). The mouth was protruding, probably due to pressure changes, from the bottom to the surface, during the capture. Macroscopic examination of the gonads identified this specimen as a female in the early stage of gonad maturity. The skin, without scales, was covered by typical dermal tubercles with circular bases and the body colouration was silvery bluish grey with visible small scattered dark spots and longitudinal dark bands along the sides. The damaged membrane of the dorsal fin was red/orange in colour, transparent in the basal part and surrounding the white rays, while the rays of the plume were bright red/orange. The anal fin was absent. The large ocular globes were cream-coloured with a dark iris and clear pupils. The features described, for both captured specimens, are in agreement with the species description reported by Whitehead et al. (1986) and Roberts (2012). Other taxonomic features, which could not be observed in our two specimens due to the damage, are the number of the dorsal fin rays (usually 400–450), caudal fin rays (3–4) and the total gill rakers on first gill arch (37–47) (Roberts 2012).

Table 2 Morphometric measurements of two specimens of Regalecus glesne caught in the north-western Ionian Sea (central Mediterranean Sea)
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Fig. 3
figure 3

Specimen CODE 2 of Regalecus glesne: visible marks on body A; detail of the head region and some of the long anterior dorsal rays of the plume B. Specimen CODE 1 C and detail of the head region D

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Discussion

The two specimens of R. glesne captured in the Gulf of Taranto represent the first occurrence of this rare species in north-western Ionian Sea (central Mediterranean) and they also update its distribution in the Mediterranean Sea, with a total of 60 oarfish observations recorded since 1826. The TL recorded for adult specimen ranges from 86.5 to 446 cm (average TL = 221 cm), while for juvenile specimens the verified SL ranges from 15 to 25.2 cm (average SL = 20 cm). The largest number of observations occurred between the Ligurian Sea and Gulf of Lion, 36.6% and 28.3% respectively, on the total recorded observations.

For both new specimens, the total lengths were estimated as the posterior section of the truncated body, probably due to serial autotomy. This process involves just the posterior part of the body over the anus and caudal fin, without involving any vital organs and it may occur several times during life (Roberts 2012). This mechanism could represent a defence strategy against predators, as reported by Chakrabarty (2013). The reasons for the occasional stranding and death of oarfish are unclear, but according to Schmitter-Soto (2008) and Dulčić et al. (2009) there are several possible causes, such as shark attacks or large bony fish predators, red tide, confluence of sea currents, earthquakes, etc.

The pelagic habit of oarfish regarding a wide depth range could explain its low availability to common fishing gear and its perceived rarity. However, this species could be really rare or uncommon.

Given the supposed rarity of R. glesne, the present findings provide a new contribution to the knowledge on the distribution of this teleost fish in the Mediterranean.