New Records of the Rare Slender Snipe Eel Nemichthys scolopaceus (Anguilliformes: Nemichthyidae) in the Mediterranean Sea

Two new records of the rare slender spine eel Nemichthys scolopaceus (Anguilliformes, Nemichthyidae) in the Mediterranean Sea are presented in this contribution. The specimens were collected in August 2023 during a scientific bottom trawl survey in the deep central Karpathian Sea (southern Aegean Sea, Greece). These significant findings highlight the importance of exploring the Mediterranean bathyal habitats to stand out their biodiversity and to increase our knowledge on the distribution patterns of rare species.

Keywords:Deep waters; Spine eels; Nemichthyidae; Eastern Mediterranean; Aegean Sea; Greece

Abbreviations: TL: Total Length; MBD: Maximum Body Depth; HL: Head Length; Sn-L: Snout Length, Sn-D: Snout to first dorsal fin Distance; Sn-PEC: Snout to Pectoral Fin Distance; Sn-A: Snout to Anal Fin Distance; ID: Interorbital Distance; ED: Eye Diameter, SL: Standard Length; %HL: % Head Length; %TL: % Total Length; TW: Total Weight

Nemichthyidae is one of the most interesting fish families comprising three genera and nine valid species, being found in deep waters [1]. Although the taxon is small, it is zoogeographically distributed in seas and oceans extended from the equator to latitudes close to the polar circles. The first genus of the family, Nemichthys Richardson, 1848, was described almost 175 years ago, followed by Labichthys Gill & Ryder, 1883 and the most recent Avocettina Jordan & Davis, 1891. The most diverse genus is Avocettina, represented by A. acuticeps (Regan 1916), A. bowersii (Garman, 1899), A. infans (Günther, 1878) and A. paucipora Nielsen & Smith, 1978. The taxonomy of the Nemichthys genus recognizes three species [N. scolopaceus Richardson, 1848, N. curvirostris (Strömman, 1896) and N. larseni Nielsen & Smith, 1878] whereas of the Labichthys reports two species [L. carinatus Gill & Ryder, 1883 and L. yanoi (Mead & Rubinoff, 1966)].

The slender snipe eel Nemichthys scolopaceus Richardson, 1848 is a cosmopolitan species distributed in tropical and temperate seas. It is found in both sides of the Atlantic and the Pacific oceans [2]. Within the western-central Mediterranean Sea N. scolopaceus has been reported off Algerian and northern Sicilian coasts [3], in the Strait of Messina [4], in Sardinian waters [5-7], in southern Adriatic Sea [8] and in Ionian Sea [9]. The first occurrence of N. scolopaceus in the eastern Mediterranean Sea has been reported off Marmaris coast [10]. Additional records come from other localities in southeastern Aegean Sea [11,12] and in Levantine Sea [13-17].

Nemichthys scolopaceus is an oceanic species, which may be encountered pelagically from surface to depths down to 4337m [18], usually between 100-1000m [19]. The maximum known size for this species is 1300mm [20]. Juvenile and adult individuals are characterized by long bodies and are called snipe eels because of their non-occlusible beak body structure [21]. The genus Nemichthys is considered to be the most extreme example of axial extension among fishes and the species vertebral numbers are the highest of the vertebrates, up to 740 vertebral bones or more [21,22]. Eels belonging to the Nemichthyidae family differ significantly from other mesopelagic eel families because of their head and jaw shape and very thin body structures [23]. Herein we report two additional records of the rare N. scolopaceus from the Karpathian Sea, pointing out the emerged necessity of studying the biodiversity of the deep-sea habitats, taking into account that these environments are among the least explored on earth and simultaneously are significantly impacted by humankind.

On 23 August 2023, two slender snipe eels Nemichthys scolopaceus were caught during an experimental bottom trawl survey (Fisheries & Sea Operational Program 2014 - 2020, «Innovation in Fisheries» project) at 640m of depth in the central Karpathian Sea (southern Aegean Sea; coordinates: 36°14.579’N, 026°24.976’E) (Figure 1). Individuals were preserved in 98% ethanol and kept at -20 °C on board fishing vessel prior to examination. In the laboratory, morphometric measurements were taken on the left side of the specimens to the nearest 0.01mm using digital calipers. The list of morphometric measurements includes: TL: Total Length; MBD: Maximum Body Depth; HL: Head Length; Sn-L: Snout Length; Sn-D: Snout to first dorsal fin Distance; Sn-PEC: Snout to Pectoral Fin Distance; Sn-A: Snout to anal fin Distance; ID: Interorbital Distance; ED: Eye Diameter; SL: Standard Length. Morphometric measurements are expressed as percentages of Head Length (%HL) or Total Length (%TL). Total Weight (TW, g) was recorded as the total weight of each specimen. Both specimens were photographed and stored in the marine fauna collection of Dr Anastasiadou in the FRI’s premises (Catalogue No: NemSCO1 and NemSCO2. For the species identification, all the relevant literature was used [20,24,25].

Figure 1:Map showing the sampling localities of N. scolopaceus published records in the Aegean-Levantine Sea. PS: present study; Numbers correspond to references.

Herein we report for the second time the presence of N. scolopaceus in the Karpathian Sea, following one specimen of 1204mm (TL) captured between the islands of Saria and Rhodes on 13 September 2002, by longline, at 640m depth [12]. Morphometric measurements of N. scolopaceus specimens are presented in Table 1. The specimen NemSCO1 reached 582.0mm (TL) and weighed 2.78 g (TW). The specimen NemSCO2 was not found in good state and only MBD, HL and SnL were recorded (Figure 2). Based on both specimens, the species is characterized by a long and strongly compressed body with caudal fin ending as filament, mouth extremely long, pointed beak-like, and well-developed eyes. The posterior part of the head is deeper than the rest of the body. Both dorsal and anal fins are long and confluent with caudal fin. Dorsal fin origin is located in front of pectoral fins. Anus is located below pectoral fins. Anal fin begins just behind the anus. Pectoral fins are present. Teeth are small with recurved tips and closely set in diagonal rows. Morphometric measurements of N. scolopaceus have also been recorded in one specimen from the central Mediterranean Sea [26], and from four specimens from the eastern Mediterranean Sea [10,11,14,15]. At this point it should be noted that the review of the relevant literature showed variations in the body measurements of the studied individuals of the species, which could be attributed to the frequent non-integrity of the specimens, especially in what concerns the caudal part (filament ending).

Table 1:Morphometric measurements expressed in mm and as percentages of total length (%TL) or head length (%HL) of Nemichthys scolopaceus specimens (NemSCO1 and NemSCO2) from the Karpathian Sea (southern Aegean Sea). Unavailable measurements are represented with an en-dash (-).

Figure 2:Nemichthys scolopaceus preserved specimens (NemSCO1 and NemSCO2) caught by bottom trawler in the central Karpathian Sea (southeastern Aegean Sea, Greece) at 640m depth on 23 August 2023.

The slender snipe eel belongs to a group of offshore pelagic species that can be found worldwide because the relative homogeneity of the deep oceans gives them access to a much larger area of suitable habitat [27]. Although N. scolopaceus is rarely caught in the Mediterranean Sea, the species is probably more common than the data suggests mainly due to insufficient exploration of deep-sea marine habitats and due to the fact that it has an extremely elongate slim body that makes it difficult to capture. Overall, the species might be less rare than thought and its presence is probably underestimated.

Herein, we report two specimens of the slender snipe eel Nemichthys scolopaceus from the southeastern Aegean Sea, Greece. It is important to underline that in a set of 12 bottom trawls carried out in an extended geographic area (Crete Sea, Libyan Sea, Karpathian Sea and Rhodes Sea) and in a depth range from 610 to 800m only these two individuals of the species were found. The new records of this rare species increase our knowledge about the deep-sea ichthyofauna, which remains limited in the largest part of it. On the basis that deep-sea expeditions are extremely timely and costly, and consequently limited in number, any relevant information on such species remains valuable and interestingly completes the puzzle of their zoogeographical distribution both locally and globally.

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