Galeus melastomus
Galeus melastomus
The Blackmouth catshark (Galeus melastomus Rafinesque, 1810), is a small deep-sea shark belonging to the Scyliorhinidae family.
Systematics –
From a systematic point of view it belongs to:
Eukaryota domain,
Kingdom Animalia,
Phylum Chordata,
Subphylum Vertebrata,
Superclass Gnathostomata,
Chondrichthyes class,
Subclass Elasmobranchii,
Superorder Selachimorpha,
Order Carcharhiniformes,
Family Scyliorhinidae,
Genus Galeus,
G. melastomus species.
The terms are synonyms:
– Galeus melanostomus Rafinesque, 1810;
– Pristiurus melanostomus Lowe, 1843;
– Pristiurus melastomus (Rafinesque, 1810);
– Pristiurus souverbiei LaFont, 1868;
– Scyllium artedi Risso, 1820;
– Scyllium artedii Risso, 1820;
– Scyllium melanostomum Bonaparte, 1834;
– Squalus annulatus Nilsson, 1832;
– Squalus delarochianus Blainville, 1816;
– Squalus prionurus Otto, 1821.
Geographic Distribution and Habitat –
Galeus melastomus is a fish widely distributed in the northeastern Atlantic Ocean, from southwestern Iceland and Trondheim, Norway south to Senegal, including the Faroe Islands, British Isles, the Azores, and the northern part of the mid-ridge -Atlantic.
In the Mediterranean it is present in all seas with the exception of the northern waters of the Adriatic and the Aegean. It is absent from the Black Sea.
Its marine habitat is mainly on continental slopes at depths of 150–1,400 m, however, it has been documented in shallow waters down to 20–25 m in Norway and down to 2,300–3,850 m in the eastern Mediterranean.
The depth at which it lives varies between various regions; for example 300–500 m in the Bay of Biscay, 400–800 m off Portugal, 500–800 m in the Strait of Sicily, 1,000–1,400 m in the Sea of Catalonia and 1,500–1,830 m in the eastern Mediterranean.
This fish generally prefers a muddy habitat. Numerous studies in the northern and western Mediterranean have reported that adults are found deeper than juveniles, while according to other authors the correlation does not exist.
It is possible that areas such as the waters off southern France offer suitable habitat for sharks of all ages. Another explanation with some scientific support is that adults are more common at intermediate depths, while juvenile sharks are restricted to shallower waters, and both adults and juveniles are found in deeper waters. If true, the age and depth inconsistencies observed from previous research could be the result of incomplete depth sampling.
However, water temperature does not appear to be an important factor in determining the distribution of this species.
Description –
Galeus melastomus is recognized by its elongated body, with a flattened head, prominent and pointed snout, and nasal openings placed in the lower part of the snout, near the mouth.
It measures at most 90 cm but it is very rare to find individuals longer than 50 cm and heavier than 400 g.
The skin scales have three fairings and three points, and behind the eye, which is large and elongated, there is a small spiracle. It has five gill slits, the last one above the pectorals.
The mouth is V-shaped and carries small sharp teeth in the jaws, generally with two lateral cusps. The inside of the mouth is black, hence the name melastomus.
The first dorsal fin is small and placed beyond the middle of the body. The second dorsal is similar to the first and is placed near the tail. The anal is long, with a straight edge and ends close to the caudal. The latter is almost a quarter of the length of the body and above, thanks to bony scales, it takes on the appearance of a serrated keel. The pectoral fins have almost straight margins and poorly rounded apices.
It normally has a grayish brown colour, but specimens with reddish gray or yellowish red tones can also be seen, with irregularly arranged darker spots and a whitish belly.
Biology –
Galeus melastomus is an oviparous species which lays 2 to 4 eggs, rarely 8.
In this period the females carry two to four eggs in the uterus, contained in horny capsules without filaments and cirri, of an amber colour, which darkens more and more with the maturation of the embryo.
Unlike most members of its genus, G. melastomus exhibits multiple oviparity, where more than one egg can mature within each oviduct at the same time. Females may contain up to 13 developing eggs, although 1-4 per oviduct is typical.
The number of eggs laid annually per female has been estimated at between 60 and 100, increasing with female size.
In mature females only the right ovary is functional. The ovoid case is shaped like a vase and has a slight flange along the lateral margins; the front end is squared off, with a pair of stubby, twisted horns at the corners, while the rear end is rounded. The surface of the case is somewhat translucent, smooth and shiny. The case is a golden brown color when first put down and turns dark brown in seawater.
Egg capsules produced by Atlantic sharks measure 3.5–6.5 cm long and 1.4–3.0 cm, while those produced by Mediterranean sharks tend to be smaller at 4.2 cm long. –5.5 cm and a diameter of 1.7–2.5 cm.
Larger females produce slightly larger egg capsules.
Mating and egg laying proceed throughout the year; reproductive activity is highest in winter and summer, although not all studies have found such a seasonal pattern.
The eggs are laid on muddy substrates in relatively shallow water.
Ripening size varies between geographic regions and is generally larger in the Atlantic than in the Mediterranean.
Lengths at maturity for males and females have been variously reported as 48 to 79 cm and 56 to 79 cm, respectively, in the Atlantic, and 42 to 55 cm and 39 to 61 cm in the Mediterranean.
Ecological Role –
The Galeus melastomus is a small shark that moves with a slow but active swim.
This fish feeds on crustaceans, small cephalopods and fish.
Juveniles consume a greater quantity and variety of crustaceans than adults, including smaller types such as mysids and hyperiid amphipods. Adults prefer relatively large fish prey and have been known to take other sharks and rays and smaller members of the same species. The importance of cephalopod prey over the ages varies from region to region. The stomachs of some blackmouth dogfish have been found to contain chunks of the animal too large for a single shark to overwhelm, suggesting that they may sometimes attack in groups. Scavenging has rarely been documented, even of human waste.
Its visual and electroreceptive systems are adept at tracking moving bioluminescent prey.
Within its range it is a fish of a nomadic nature and can be found alone or in groups.
It often swims just above the seabed, perhaps using ground effect (a reduction in drag on a wing when near the ground) to conserve energy. It has also been seen resting motionless on the bottom.
Known predators of G. melastomus include the shark Dalatias licha and the squid Todarodes sagittatus).
As for its parasites, the tapeworm Ditrachybothridium macrocephalum and the protist Eimeria palavensis have been documented.
Due to its abundance in some seas it forms a substantial part of the bycatch of commercial deep-sea fisheries in much of its range. It has low economic value and is usually discarded, although larger sharks can be traded for meat and leather.
This fish is caught with bottom trawls on muddy bottoms. The meat is not good, but is often sold skinned as dogfish or dogfish.
Some fisheries, such as those off Portugal and Italy, retain and use a small subset of the larger individuals for fresh or dried and salted human consumption and for leather; the fishing fleet in Viareggio, Tuscany, reported landing 700kg in 2005. In the northeastern Atlantic, this shark is increasingly targeted by fishermen following the decline of other deep-sea shark species.
Off Corsica, Sicily and southern Portugal, and in the Ionian, southern Adriatic and Aegean Seas, most of these small shark catches are immature, suggesting that there has been a negative impact of fishing pressure. However, the species remains extremely abundant in a number of areas, and survey and fishery data have shown no evidence of overall population decline. The wide depth range it occupies likely affords it some protection against fishing, particularly given the 2005 ban on trawling deeper than 1,000m in the Mediterranean.
Therefore, the International Union for Conservation of Nature (IUCN) has listed the blackmouth catshark under Least Concern. In European Commission waters, the fishery for this species is managed within the Total Allowable Catch (TAC) for deep-sea sharks.
As far as its interaction with humans is concerned, it is harmless.
Guido Bissanti
Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Global Biodiversity Information Facility.
– Louisy P., 2016. Guide to the identification of marine fish of Europe and the Mediterranean. Il Castello Editore, Milan.
– Nikiforos G., 2008. Fauna of the Mediterranean. Giunti Editore, Florence.