First Records of a Hydrolagus Species ( Holocephali : Chimaeridae ) from Reunion Island and Mayotte (Southwestern Indian Ocean)

: Two specimens of large blackish chimaeras of the genus Hydrolagus were caught, one off Reunion Island and the other off Mayotte in the southwestern Indian Ocean. The specimens, an adult male of 710 mm BDL and a female of 870 m BDL, are described, compared to similar species (i


Introduction
Chimaeras or ghost sharks of the family Chimaeridae are cartilaginous fishes (Chondrichthyes) characterised by a very large head (Holocephalii), a soft cone-shaped body, a tapering tail that ends as a caudal filament, a conical snout, short to moderately long, a single external gill opening, conspicuous mucous canals and sensory pores on the head, teeth fused in pairs of dental plates forming a beak, males that have bifurcate or trifurcate claspers, and supplementary clasping organs on the forehead (frontal tenaculum) and a pair in front of pelvic fins (prepelvic tenacula).Sizes of Chimaeras vary from small (38 cm LT) to large (147 cm LT); they live mostly in the deep seas (200-3000 m) of the world ocean.The biology and the ecology of the chimaeras are largely unknown, and their taxonomy is still unclear.A few species are exploited for their meat and liver oil.
Globally, chimaeras (Chimaeridae, Callorhinchidae, and Rhinochimaeridae) have rarely been observed since the description of the first species by Linnaeus [1].Only 30 species were described for about 250 years (from 1758 to 2000), then 24 new species were described during the last two decades thanks to the development of deep-sea explorations all around the world and the specific works of a number of ichthyologists on this group of fishes [2][3][4][5][6][7][8][9][10][11][12].
Traditionally, two genera were recognized in the family Chimaeridae, Chimaera and Hydrolagus, distinguished by the presence (Chimaera) or the absence (Hydrolagus) of an anal fin.Today, 42 species are recognized as valid; 21 in the genus Chimaera and 21 in the genus Hydrolagus (Table 1), plus a doubtful species from Japan ( [13]; FishBase and the Eschemeyer's Catalogue of Fishes consulted in 2023).Despite recent studies devoted to these fishes, the taxonomic status of some species is still pending.Furthermore, recent genetic analysis does not support the traditional distinction in two genera and seems in favour of a single genus.However, even if the morphologic criteria "presence/absence of an anal fin" is proved to have no generic significance, it is still useful at the species level.In the course of longline fishing operations, two specimens of large blackish chimaeras were caught off La Reunion (two females were caught, but only one was preserved) and off Mayotte in the southwestern Indian Ocean.They represent the first records of chimaeras for these remote islands in the southwestern Indian Ocean.
Because of their deep-sea habitat offering relatively stable environmental conditions, chimaeras are likely to have a very large distribution in the world ocean; thus, comparisons of the specimens from Reunion and Mayotte have been conducted with all sibling species.These specimens are herein described, compared to the species with the same characteristics (i.e., large size, over 110 cm total length (TL), dark blackish colour, rather long conical snout) and identified as Hydrolagus affinis.The discovery of the specimen from Reunion was briefly reported by Séret et al. (2013) [44] without species assignment in the bulletin of a Mauritius conservation association.

Materials and Methods
The two specimens were incidentally caught on swordfish longlines during professional fishing operations off Reunion and Mayotte Islands (Figure 1).They were frozen and sent to the first author, who preserved them in formalin, then in ethanol, and deposited them in the collections of the Museum National d'Histoire Naturelle (MNHN), Paris.

Materials and Methods
The two specimens were incidentally caught on swordfish longlines during professional fishing operations off Reunion and Mayotte Islands (Figure 1).They were frozen and sent to the first author, who preserved them in formalin, then in ethanol, and deposited them in the collections of the Museum National d'Histoire Naturelle (MNHN), Paris.Morphometric measurements were taken on preserved specimens according to Didier [3] and Didier and Séret [4] and expressed as percentages of the body length (BDL).Measurements of sibling species were extracted from their original descriptions and from subsequent re-descriptions and new records.Terminology and measurements of canals of the lateral line system on the head follow that of Didier [3] and Didier and Séret [4].
Material: MNHN 2015-97, adult male, 1120 mm TL, 710 mm BDL, Mayotte, 20 September 2014, off Bouéni reef, 400-500 m depth, by longliner Bambou of Frederic Cierco fishing company.MNHN 2015-98, adult female (eviscerated), 1300 mm TL, 870 mm BDL, November 2012; caught on a sailfish longline by professional fisherman Charles Delmas, off Sainte-Marie, the longline sunk to about 1000 m depth (Figures 2-5).Morphometric measurements were taken on preserved specimens according to Didier [3] and Didier and Séret [4] and expressed as percentages of the body length (BDL).Measurements of sibling species were extracted from their original descriptions and from subsequent re-descriptions and new records.Terminology and measurements of canals of the lateral line system on the head follow that of Didier [3] and Didier and Séret [4].

Description of the Specimens
Both specimens are adult: the male has fully developed claspers, and the female has ripe oocytes.Their morphometric measurements are given in Table 2.They have large robust bodies (1120 and 1300 mm TL, 710 and 870 mm BDL), globulous heads with rather long conical (bluntly pointed) snouts (Figures 6 and 7), and prenarial length 7.8-12.1% BDL.The eyes are large, 6.0-6.1% BDL.The trunks are massive.The tails are tapering, ending as short filaments (probably damaged) (Figures 8 and 9).The skin is naked, rather firm, and not deciduous.

Description of the Specimens
Both specimens are adult: the male has fully developed claspers, and the female has ripe oocytes.Their morphometric measurements are given in Table 2.They have large robust bodies (1120 and 1300 mm TL, 710 and 870 mm BDL), globulous heads with rather long conical (bluntly pointed) snouts (Figures 6 and 7), and prenarial length 7.8-12.1% BDL.The eyes are large, 6.0-6.1% BDL.The trunks are massive.The tails are tapering, ending as short filaments (probably damaged) (Figures 8 and 9).The skin is naked, rather firm, and not deciduous.Both specimens are adult: the male has fully developed claspers, and the female has ripe oocytes.Their morphometric measurements are given in Table 2.They have large robust bodies (1120 and 1300 mm TL, 710 and 870 mm BDL), globulous heads with rather long conical (bluntly pointed) snouts (Figures 6 and 7), and prenarial length 7.8-12.1% BDL.The eyes are large, 6.0-6.1% BDL.The trunks are massive.The tails are tapering, ending as short filaments (probably damaged) (Figures 8 and 9).The skin is naked, rather firm, and not deciduous.The first dorsal fin is triangular, pointed, and preceded by a long spine exceeding the fin tip in the female (spine broken in the male).There is no interdorsal space.The second dorsal fin is long and evenly high in its whole length.The pectoral fins are large, triangular, and with a rather pointer apex, reaching the origin of pelvic fins when depressed on the body.The pelvic fins are well developed, triangular, and with a rather pointer apex.There is no dorsal-caudal space, but instead a long pelvic-caudal space.The caudal fin is leaf-like, with almost symmetrical oblong lobes, with the ventral lower lobe being a little larger than the dorsal lobe; the origin of the dorsal lobe is behind the level of that of the ventral lobe.There is no anal fin; no notch is visible at the origin of the lower caudal lobe.The mouth is small, and the jaws have 7-8 vomerine tritors (upper tooth plates) and four incisor-like mandibular tritors (lower tooth plates) that look like a beak (Figures 10 and 11).The first dorsal fin is triangular, pointed, and preceded by a long spine exceeding the fin tip in the female (spine broken in the male).There is no interdorsal space.The second dorsal fin is long and evenly high in its whole length.The pectoral fins are large, triangular, and with a rather pointer apex, reaching the origin of pelvic fins when depressed on the body.The pelvic fins are well developed, triangular, and with a rather pointer apex.There is no dorsal-caudal space, but instead a long pelvic-caudal space.The caudal fin is leaf-like, with almost symmetrical oblong lobes, with the ventral lower lobe being a little larger than the dorsal lobe; the origin of the dorsal lobe is behind the level of that of the ventral lobe.There is no anal fin; no notch is visible at the origin of the lower caudal lobe.
The mouth is small, and the jaws have 7-8 vomerine tritors (upper tooth plates) and four incisor-like mandibular tritors (lower tooth plates) that look like a beak (Figures 10  and 11).Lateral line sensory pores and mucous canals are very conspicuous on the head; their measurements are given in Table 2.The preopercular canal (POP) shares a common branch with the oral canal (O), which is connected to the otic (OT) and infraorbital (IO) canals.There are numerous pores on the head, some more conspicuous along or on Lateral line sensory pores and mucous canals are very conspicuous on the head; their measurements are given in Table 2.The preopercular canal (POP) shares a common branch with the oral canal (O), which is connected to the otic (OT) and infraorbital (IO) canals.There are numerous pores on the head, some more conspicuous along or on cephalic canals or forming groups close to the canals: a group of 11/11 pores in front of the occipital canal (OC) in MNHN 2015-0097/MNHN 2015-0098), a line of 7-8/9 below the posterior branch of the infraorbital canal (IO) and 10-11/10 pores on the anterior branch of IO, 6/10 larger pores on the angular canal (AN), a line of 14/14 pores along the mandibular canal (M), and a group of 12/15 pores in the concavity of the suborbital canal (SO); no conspicuous pores along the otic canal (OT) and the preopercular canal (POP).The lateral line on the body starts at the junction of the otic (OT) and occipital (OC) canals, smoothly sloping on the trunk and then running almost straight on the tail to its tip (Figures 6 and 7).
Claspers of the male are robust, slightly extending over the pelvic rear tip, with a muscular base and fleshy bulbous tips covered with fine denticles; they are bifurcate, divided distally for 1/3 their length, with a third fleshy lobe that lies along the dorsal side of the medial cartilaginous arm (Figure 12).The frontal tenaculum is thumb-shaped and spinulous with about 40 clow-like denticles; its dorsal surface is smooth (Figure 13).There is a pair of prepelvic tenacula retractable in slit-like pouches in front of the pelvic-fin base; they are spatulate, with a row of five strong hooked denticles on the inner edge.The distal edge is indented but covered by an integument so that it appears straight (Figure 14).
Coloration.Freshly caught specimens were plain dark brown to blackish, with some purple reflections; fins darker, almost jet black.The body is chocolate brown in preservation.The tooth plates are yellowish.The eyes are opalescent.The cephalic lateral line sensory pores are whitish.The claspers are brownish with a purple tint; terminal fleshy bulbs are whitish.The prepelvic tenacula are creamy white.

Comparison with H. affinis
Few morphometrical measurements of H. affinis are available in the scientific literature.The measurements used for comparisons (Table 2) were calculated from data expressed in percentage of PCL in Bigelow and Schroeder [48] for two specimens, a male of 797 mm PCL and a female of 973 mm PCL.These calculated values are mostly in the ranges of the specimens from Reunion and Mayotte, except for the pectoral anterior margin (P1A) 34.5-35.2(measured) versus 41% BDL (calculated) and eye height (EYH) 4.0-4.1 (measured) versus 5.3% BDL (calculated).Despite these slight differences, the specimens from Reunion and Mayotte are tentatively identified as H. affinis as they share similar prepelvic tenacula with 4-8 denticles in H. affinis and five strong denticles in the male from Mayotte and a distinct indentation on the distal edge covered by integument.This later feature was already observed by Hardy and Stehmann [37] for H.affinis, which seems unique.Coloration.Freshly caught specimens were plain dark brown to blackish, with some purple reflections; fins darker, almost jet black.The body is chocolate brown in preservation.The tooth plates are yellowish.The eyes are opalescent.The cephalic lateral line sensory pores are whitish.The claspers are brownish with a purple tint; terminal fleshy bulbs are whitish.The prepelvic tenacula are creamy white.Coloration.Freshly caught specimens were plain dark brown to blackish, with some purple reflections; fins darker, almost jet black.The body is chocolate brown in preservation.The tooth plates are yellowish.The eyes are opalescent.The cephalic lateral line sensory pores are whitish.The claspers are brownish with a purple tint; terminal fleshy bulbs are whitish.The prepelvic tenacula are creamy white.Hydrolagus affinis is mainly a North Atlantic species [37] but extends to Uruguay in the western Atlantic, to Angola in the eastern Atlantic, and possibly in South Africa and Mozambique [49].Hydrolagus affinis was also tentatively reported from Tanzania [50] based on Baited Remote Underwater Video (BRUV) photos and videos taken during the deep-sea expedition SERPENT off Tanzania (https://serpentproject.com/, Accessed on 17 November 2023).With the dentification on photos and videos being delicate, the same observed specimen is legended Hydrolagus sp. in Gates et al. [51].
Furthermore, H. melanophasma has prepelvic tenacula with only 3-4 denticles and without indentation on the distal edge; a straight trunk lateral line without regular undulations versus trunk lateral line starting with a strong curve then slightly wavy rearward in the specimens from Reunion and Mayotte.
Furthermore, H. homonycteris has a distinctly rounded, almost fan-like pelvic fin versus pointed distally in specimens from Reunion and Mayotte, with prepelvic tenacula with only 3-4 denticles and without indentation on the distal edge.The frontal tenaculum of H. homonycteris has more denticles (50 versus 40) than that of the male from Mayotte.

Discussion
A specimen preserved in the MNHN collections, MNHN 2004-0820, adult male of 1065 mm TL and 700 mm BDL, is similar to the specimens from Mayotte and La Reunion and is here identified as the same species.It was caught during a fishing cruise of the commercial longliner "Croix du Sud" off Saint-Paul Island by 37 • 36 7.2 S, 78 • 4 12 E, and 1035-1045 m depth, on 8 June 2001.This specimen is mentioned here because it has a particularity: on the left side of the head, the preopercular canal (POP) shares a common branch with the oral canal (O) that is connected to the otic (OT) and infraorbital (IO), but on the right side of the head, they have no common branches and are connected separately, POP to OT and O to IO canals (Figures 15 and 16).This means that this character (POP and O canals commonly branched or not) has less taxonomic significance than usually thought.
The present assignment of the specimens from Reunion and Mayotte to H. affinis is provisional, pending a revision of the group is completed.Comparative anatomy seems insufficient to solve the complex situation of this group of chimaeras, so genetic analysis should be obtained for all nominal species.However, it seems that many sequences in genetic banks are extracted from specimens whose species identification remains uncertain and hardly verifiable, as most are not illustrated or described.1035-1045 m depth, on 8 June 2001.This specimen is mentioned here because it has a particularity: on the left side of the head, the preopercular canal (POP) shares a common branch with the oral canal (O) that is connected to the otic (OT) and infraorbital (IO), but on the right side of the head, they have no common branches and are connected separately, POP to OT and O to IO canals (Figures 15 and 16).This means that this character (POP and O canals commonly branched or not) has less taxonomic significance than usually thought.The present assignment of the specimens from Reunion and Mayotte to H. affinis is provisional, pending a revision of the group is completed.Comparative anatomy seems insufficient to solve the complex situation of this group of chimaeras, so genetic analysis should be obtained for all nominal species.However, it seems that many sequences in 1035-1045 m depth, on 8 June 2001.This specimen is mentioned here because it has a particularity: on the left side of the head, the preopercular canal (POP) shares a common branch with the oral canal (O) that is connected to the otic (OT) and infraorbital (IO), but on the right side of the head, they have no common branches and are connected separately, POP to OT and O to IO canals (Figures 15 and 16).This means that this character (POP and O canals commonly branched or not) has less taxonomic significance than usually thought.The present assignment of the specimens from Reunion and Mayotte to H. affinis is provisional, pending a revision of the group is completed.Comparative anatomy seems insufficient to solve the complex situation of this group of chimaeras, so genetic analysis should be obtained for all nominal species.However, it seems that many sequences in Trials of genetic analysis have been performed with tissue samples from the specimens of Reunion and Mayotte, thanks to Dr. G. Naylor (Florida Museum of Natural History).The provisional results are not herein included because of the uncertainty of the identification of the species whose sequences are stored in genetic banks; similarities have been found with H. affinis but also with other species (G.Naylor person.com, accessed on 8 June 2017).It would be highly valuable that the sequences are deposited in genetic banks supplemented by illustrations of the sampled specimens.This will greatly help taxonomic revisions.

Conclusions
Deep-sea chimaeras are globally poorly known.Although several new species have been described in recent years, their inventory is still to be established with accuracy, and the status of some nominal species should be confirmed.The present records contribute to improving our knowledge of this group of ghost fishes that is still somewhat mysterious to scientists.Any piece of information collected on these fishes is useful.

Figure 1 .
Figure 1.Map showing the localities where the two specimens of Hydrolagus affinis were caught off Reunion and Mayotte Islands in the southwestern Indian Ocean.

Figure 1 .
Figure 1.Map showing the localities where the two specimens of Hydrolagus affinis were caught off Reunion and Mayotte Islands in the southwestern Indian Ocean.

Figure 15 .
Figure 15.Hydrolagus affinis from Saint-Paul Island, MNHN 2004-0820, lateral line canals on the left side of the head showing the common branch shared by POP and O canals.

Figure 16 .
Figure 16.Hydrolagus affinis from Saint-Paul Island, MNHN 2004-0820, lateral line canals on the right side of the head showing the direct connections of POP and O canals to OT and IO canals.

Figure 15 .
Figure 15.Hydrolagus affinis from Saint-Paul Island, MNHN 2004-0820, lateral line canals on the left side of the head showing the common branch shared by POP and O canals.

Figure 15 .
Figure 15.Hydrolagus affinis from Saint-Paul Island, MNHN 2004-0820, lateral line canals on the left side of the head showing the common branch shared by POP and O canals.

Figure 16 .
Figure 16.Hydrolagus affinis from Saint-Paul Island, MNHN 2004-0820, lateral line canals on the right side of the head showing the direct connections of POP and O canals to OT and IO canals.

Figure 16 .
Figure 16.Hydrolagus affinis from Saint-Paul Island, MNHN 2004-0820, lateral line canals on the right side of the head showing the direct connections of POP and O canals to OT and IO canals.

Table 1 .
List of Chimaera and Hydrolagus species, with distribution, depth range, size (TL and BDL), and IUCN Red List status (consulted in 2023).Species presented by genus and chronological order.

Table 2 .
[43,45,46]ic measurements of the two specimens of Hydrolagus affinis from La Reunion and Mayotte expressed as percentage of the body length (BDL) and closely related species with data from the literature[43,45,46].