Sharks, rays and chimaeras of the Seine and Unicorn seamounts (NE Atlantic Ocean)

Seamounts are underwater mountains which typically rise rather steeply at least several hundred meters above the deep-sea floor. These geological features interrupt water flow and hence may induce changes in the circulation of different water masses, in turn causing different physical and biological effects. For this reason, seamounts are biodiversity hotspots, housing a wide number of species, as is with the case of the Seine and Unicorn seamounts, which are a part of the Madeira-Tore seamount chain located between Portugal mainland, southwestern Europe and Madeira archipelago (NE Atlantic). Fisheries independent surveys allowed the collection of Chondrichthyes specimens from the Seine and Unicorn seamounts. Individuals were caught over the course of two research cruises, first in 2004 and later in 2017, with species distribution ranging from the summit down to 2500 m of depth. Fifteen species belonging to 7 different taxonomical families were collected in the two surveyed areas. Two species were recorded for the first time and added to the checklist of the Seine seamount (Centrophorus granulosus and Somniosus rostratus), and three species for the Unicorn seamount (C. granulosus, Centroscymnus coelolepis and Centroselachus crepidater). Distribution and frequency of occurrence for the collected species were evaluated in relation to depth. This work is a valuable contribution to the knowledge of seamount-associated fish fauna. Moreover, the checklist of sharks, rays and chimaeras was updated for the Seine and Unicorn seamounts, summing up 20 species.

Despite seamount fish fauna having been previously described in several studies, few works have included species captured below 600 m (Tracey et al., 2004). Therefore, many of these deep ocean fish communities remain uncovered and descriptions are restricted to sampling effort, which is primarily associated with fishing activities. Consequently, there is still a knowledge gap regarding seamount associated elasmobranchs (Menezes et al., 2009;Christiansen et al., 2015;Vieira et al., 2018).
The Seine and Unicorn seamounts belong to the Madeira-Tore seamount chain located between Portugal mainland and Madeira archipelago. These seamounts are of special interest and Portugal plans to classify them as Marine Protected Areas (MPAs) in the next years under the European Marine Strategy Framework Directive (EU-MSFD), in light of the fact that these areas often play an important role in species biogeography and connectivity between key stone habitats (Koslow et al., 2016).
This study aims to contribute to the knowledge of fish diversity of the Seine and Unicorn seamounts at different depth strata down to 2500 m. Data on the occurrence of Chondrichthyes species for these ecologically important areas is provided with an update of the existing checklist.

Materials and methods
Specimens were collected in 2004 and in 2017, within the scope of RECPROFMAD and BIOMETORE research surveys. Both surveys were conducted for biodiversity assessment of seamount-associated fish fauna. Fishing sets were carried out from the top of the seamounts to around 2500 m depth. Drifting and bottom longlines, as well as two types of baited traps were used at Unicorn (34°35′N, 14°28′W) and Seine seamounts (33°45′N, 14°22′W) .
All specimens were identified to species level, counted and weighed on board. Additionally, they were photographed before being preserved. Once at the laboratory, all individuals were measured (total length TL, in mm), weighed (total weight TW, in g) and sexed. Voucher specimens from the 2004 and 2017 cruises were deposited in museum collections (Funchal Natural History Museum -MMF and The National Museum of Natural History, Lisbon).
A new complete annotated checklist for the Seine and Unicorn seamounts was produced. In order to revise the list of Chondrichthyes species previously recorded for these areas, MMF collections and scientific literature were assessed. Species frequency of occurrence was classified into four classes, to which different colours were assigned: redrare (1 specimen); yellowoccasional (2 to 4 specimens); light greencommon (5 to 10 specimens) and dark greenvery common (> 10 specimens).
Furthermore, the Food and Agriculture Organization (FAO) species catalog for fisheries purposes for the North Atlantic (Ebert and Stehmann, 2013) was consulted for geographic distribution confirmation and previous records for the archipelago of Madeira.
For each species depth range and maximum size are listed according to Ebert and Stehmann (2013). In addition, species susceptibility to locally used fishing gear and corresponding IUCN Red List status in Europe (Nieto et al., 2015) is provided.
Occurrence data for the collected species was plotted against depth to illustrate the dominant depth strata for each species. This allowed the addition of new information on the vertical distribution of these species at the Seine and Unicorn seamounts.
For the Seine seamount, 66 specimens belonging to 14 different species were identified in the present study (Table 1). Two species (Centrophorus granulosus and Somniosus rostratus) are recorded for the first time for this seamount, increasing the total number of species to 17.
Regarding the Unicorn seamount, 8 species from a total of 68 specimens were reported in the present study. Three (Centrophorus granulosus, Centroscymnus coelolepis and Centroselachus crepidater) of which were recorded for the first time for this seamount, consequently increasing the total number of species to 11. Finally, frequency of occurrence of cartilaginous fish species is presented in Table 2. The most frequently observed species were D. calcea, Etmopterus princeps, Etmopterus pusillus, C. coelolepis and Centroscymnus owstonii, with more than 10 specimens captured each. In turn, the less frequently observed species were Prionace glauca, S. rostratus and Tetronarce. Nobiliana, with only 1 specimen collected.
Detailed information regarding the species collected during the present study is given below:
First published record for Madeira archipelago in Lowe (1838).
This species is a common oceanic shark usually caught with pelagic longlines, hook and lines, pelagic trawls and bottom trawls near the coast (Davidson et al., 2015).
One specimen with 1910 mm TL was caught at 750 m at Unicorn seamount.
Recorded for Madeira in FAO Species Catalogue (Ebert and Stehmann, 2013): Yes.  (1870). This species occurs off the outer continental shelves and upper slopes, frequently over or near the bottom.
Nine specimens ranging from 474 to 1540 mm TL were caught at Seine (3) and Unicorn (6) seamounts between 750 and 1000 m depth, using bottom longlines. Table 2 Frequency of occurrence of cartilaginous fish species caught at the Seine and Unicorn seamounts during the present study. Red -rare (1 specimen); yellowoccasional (2 to 4 specimens); light green -common (5 to 10 specimens); dark green -very common (> 10 specimens) Frequency of occurrence: Light green (common). Recorded for Madeira in FAO Species Catalogue (Ebert and Stehmann, 2013): Yes.
First published record for Madeira in Lowe (1852). This species is a large deep-water gulper shark that occurs on the continental slopes and in the epipelagic or mesopelagic zone.
Five specimens ranging from 1062 to 1275 mm TL were caught at Seine (4) and Unicorn (1) (1839) as Acanthidium calceus. This is the largest and probably the most common of the three Deania species occurring in Madeira (see remarks) and is caught as by-catch of the black scabbardfish fishery. The biology of this species is poorly known.
Fifteen specimens ranging from 579 to 1033 mm TL were caught at Seine seamount at 1000 m depth, using bottom longline.
Frequency of occurrence: Dark green (very common). First published record for Madeira and Seine seamount in Freitas & Biscoito (2007).

Recorded for Madeira in FAO Species Catalogue
This is the only Deania species with a subcaudal keel beneath the caudal peduncle. It is caught as bycatch in the demersal fishery and its biology is poorly known.
Nine specimens ranging from 551 to 983 mm TL were caught at Seine (4) and Unicorn (5)
First published record for Madeira and for the Seine seamount in Freitas & Biscoito (2007). This is the largest species of Etmopterus and can be distinguished from other species of the genus by its lateral trunk denticles with fairly thick and stout cusps, forming inconspicuous, regular longitudinal rows on caudal peduncle and caudal-fin base. It has been recorded for both sides of the North Atlantic.
Twenty-five specimens ranging from 230 to 678 mm TL were caught at Seine (4) and Unicorn (21) (1839) as Acanthidium pusillum, although previously recorded by the same author (Lowe, 1834) under the preoccupied name Centrina nigra.
This species is caught as by-catch of the black scabbardfish fishery and Madeira is the locality type for this species (Syntype BMNH 1855.11.19.27).
Sixteen specimens ranging from 350 to 467 mm TL were caught at Seine (4) and Unicorn (12) seamounts between 750 and 2500 m depth, using bottom longlines and traps.
Frequency of occurrence: Dark green (very common).
Recorded from Madeira in FAO Species Catalogue (Ebert and Stehmann, 2013): Yes.
Local gear susceptibility: Bottom longlines and traps.
According to Maul (1948) this species is rare and occurs offshore.
Three specimens ranging from 411 to 461 mm TL were caught at Unicorn seamount at 750 m depth, using bottom longlines and traps.

Recorded for Madeira in FAO Species Catalogue
Depth range (Ebert and Stehmann, 2013) This little-known species occurs on or near the bottom on continental slopes and upper and middle rises.
Twenty-nine specimens ranging from 713 to 1115 m TL were caught at Seine (17) and Unicorn (12) seamounts between 1500 and 2500 m depth, using bottom longlines and traps.

Frequency of occurrence: Dark green (very common).
Recorded for Madeira in FAO Species Catalogue (Ebert and Stehmann, 2013): Yes.
This little-known dogfish occurs off the outer continental shelves and upper continental slopes, on or near bottom.
Eleven specimens ranging from 711 to 1096 mm TL were caught at Seine (7) and Unicorn (4) seamounts between 1000 and 1500 m depth using bottom longlines and traps.
First published record for Madeira in Günther (1870) as Centrophorus crepidater. This deep-water shark occurs along the upper continental and insular slopes on or near the bottom.
Five specimens ranging from 558 to 668 mm TL were caught at Seine (2) and Unicorn (3) seamounts at 1500 m depth using bottom longlines.
First published record for the Madeira of two specimens in Maul (1955). This deep-water shark rarely occurs in the region and only t a few records exist on the outer continental shelf and upper slope of the northeast Atlantic and western Mediterranean Sea.
One specimen with 1120 mm TL was caught in Seine seamount at 1000 m depth using bottom longline.
Recorded for Madeira in FAO Species Catalogue (Ebert and Stehmann, 2013): Yes.
First published record for the Madeira by Lowe (1938) as Torpedo hebetans and confirmed as T. nobiliana by Biscoito et al. (2018).
Adults are frequently pelagic or semi-pelagic whereas juveniles are mainly benthic living on soft-substrate and coral reef habitat in shallow water. The biology of this species is poorly known.
One specimen with 972 mm TL and 510 mm DL was caught at Seine seamount at 200 m depth using bottom longline (MMF 47181).
Recorded for Madeira in FAO Species Catalogue (Ebert and Stehmann, 2013): No.
First published record for the Madeira by Lowe (1938). Madeira is the locality type of this endemic species for Madeira and the Azores archipelagos (Portugal) in the northeastern Atlantic. The biology of this species is poorly known.
Two specimens with 680 mm TL (355 mm DL) and 700 mm TL (393 mm DL) were caught at Seine seamount at 200 m depth using bottom longline.
First published record for the Madeira in Freitas & Biscoito (2007).
It is a benthopelagic species which occurs on continental slopes and deep-sea plains, feeding on small fishes and invertebrates.
Two specimens with 1075 and 1268 mm TL were caught at Seine seamount at 2500 m depth using bottom longline.
Recorded for Madeira in FAO Species Catalogue (Ebert and Stehmann, 2013): No.
On one hand, Centrophorus granulosus and C. squamosus occurred at 1000 m in both seamounts. On the other hand, C. coelolepis mainly occurred between 1500 and 2000 m deep. However, one specimen of this species was captured at 2500 m deep at the Seine seamount. Furthermore, D. calcea was collected at 1000 m deep only at the Seine seamount.
E. princeps occurred in both seamounts, though more frequently at 1500 m, within a depth range between 750 and 2000 m of depth. Nonetheless, E. pusillus was the species that presented the widest depth range. Despite being more dominant between the 750 to 1000 m of depth, it also occurred at 2500 m. Moreover, E. spinax and P. glauca were exclusively collected at the Unicorn seamount. Contrarily, H. affinis, R. maderensis, S. rostratus and T. nobiliana were only captured at the Seine seamount.

Discussion
Twenty cartilaginous fish species were confirmed in the present checklist for the Seine (17) and Unicorn (11) seamounts, corresponding to approximately 31% of the total valid chondrichthyan species reported for Madeira archipelago (Biscoito et al., 2018), and to 2.3% of the total species worldwide (Weigmann, 2016). Despite having been previously reported for Madeira archipelago, some of the present study's species were reported for the first time for both the Seine (C. granulosus and S. rostratus) and Unicorn (C. granulosus, C. coelolepis and C. crepidater) seamounts.
Only one specimen of P. glauca, S. rostratus and T. nobiliana was collected over the course of this study, while C. coelolepis and E. princeps were revealed as the most common species.
Regarding the collected specimens' total length, individuals of D. profundorum presented a slightly larger size than the maximum length recorded by Ebert and Stehmann (2013). All other species were within their known size range.
Among the most frequent species D. calcea and C. coelolepis present a similar depth range when comparing with previous works (Menezes et al., 2009). However, three species (E. pusillus, R. maderensis and H. affinis) were collected at greater depths than those recorded by Ebert and Stehmann (2013). In the particular case of E. pusillus, a wider depth range was observed in the present study in contrast with Menezes et al. (2009). In fact, the widest depth range was recorded for the Etmopteridae family (750-2500 m), while Centrophoridae and Somniosidae species were captured mostly between 1000 and 1500 m deep, respectively. The large-scale hydrographic regime characteristic of the Northeast Atlantic might explain the similarity among species composition associated with seamounts located within the same geographical region.
In Europe, 40.4% of Chondrichthyes are listed as Threatened (Nieto et al. 2015). In Madeira archipelago, 35.8% of the total recorded species are labelled under the same category (Biscoito et al., 2018), which is only slightly more than the 28.9% species recorded for the Seine and Unicorn seamounts. Additionally, and when considering that Chondrichthyes are a group particularly sensitive to threats posed by overfishing (Dulvy et al., 2014), this study may be of importance when aiming to improve fisheries regulations currently in place, as shedding light on these species' occurrence at different depths in commercial-fishing dominated seamounts might be of help in shark conservation. Finally, this work's relevance is also marked by the updated annotated checklist contribution to the knowledge of seamount-associated fish biodiversity, and may serve as a strengthening tool for a future proposal for the creation of an MPA within the Madeira-Tore region.