Speckled smooth-hound

Mustelus mento is a shark of the family Triakidae, order Carcharhiniformes ^[1]. It is a coastal shark, endemic to the southeastern Pacific, living from northern Peru to south-central Chile ^[2]. In Peru it is known as «tollo fino» or «tollo con bandas» ^[3]. The IUCN classifies it as Critically Endangered (CR) ^[4].

Cope described it in 1877 under the name it still bears, from a 300 mm holotype from Peru that is kept at the Academy of Natural Sciences of Philadelphia ^[5]. It later received other names now treated as synonyms, among them Mustelus edulis and Mustelus abbotti ^[5],[1].

It is a stout-bodied smooth-hound with a short snout, angular and blunt in profile, and a short tail stalk ^[5]. Its teeth are molar-like and symmetrical, without a cusp and with a broad, rounded crown, a trait that sets it apart from almost every other Mustelus ^[5]. The rear edge of its dorsal fins is covered with denticles and leaves no fin rays exposed ^[5].

The back is a uniform grey or grey-brown and the belly pale; it carries many white spots and no dark ones, and individuals under 35 cm also show dark bars ^[5]. Embryos close to birth already have dark bands on the back and fins edged in dark brown or black ^[2].

The FAO catalogue gave it a maximum size of 130 cm ^[5]. However, between 2021 and 2023 females of up to 159.3 cm and males of up to 165.0 cm total length were measured at Antofagasta, northern Chile ^[2].

Similar species

• Mustelus whitneyi: It is another smooth-hound of the genus, common off Peru, but it has neither white nor dark spots, its teeth carry a distinct cusp and the rear edge of its dorsal fins is bare and dark; it also lives deeper, mostly between 70 and 100 m ^[5].

It has been reported from the Galápagos Islands, Peru, Chile and the Juan Fernández archipelago; old records from Argentina, in the Atlantic, may belong to other species of the genus ^[5]. A recent study regards it as endemic to the southeastern Pacific, between northern Peru and south-central Chile ^[2]. It appears in the updated checklist of cartilaginous fishes of the Peruvian sea ^[6].

It is a bottom shark of temperate waters, found on continental and island shelves both near the coast and offshore, at depths of 16 to 50 m ^[5]. In San Jorge Bay, Antofagasta, it is caught with gillnets at 25 to 35 m over sand and mud, in an upwelling area where sea surface temperature ranges from 16 to 22 °C with the season ^[2].

Its reproduction had not been described at the population level until 925 individuals landed at Antofagasta were examined between November 2021 and October 2023 ^[2]. The size at which half of the individuals are mature turned out to be 53.6 cm in females and 48.7 cm in males, far below the 86 to 90 cm and 65 to 76 cm given before ^[2],[5].

It is viviparous without a placenta: each embryo grows inside a thin, transparent capsule and feeds on its yolk sac, which stays attached throughout development, correcting the idea that it was placental ^[2]. Nine pregnant females, 106 to 139 cm long, carried 71 embryos in all, in litters of 4 to 12 young split evenly between the two uteri ^[2].

Embryos appeared only in summer: the smallest, averaging 6.79 cm, in January, and the largest, averaging 12.97 cm, in March ^[2]. It is estimated to be born at 13 to 14 cm, to give birth in early austral autumn and to have a gestation of about one year ^[2]. An earlier source spoke of 7 young per litter and about 30 cm at birth ^[5].

In northern Chile both sexes were much more frequent in summer landings, from December to February, than in winter and early spring ^[2]. The summer abundance of pregnant females suggests that San Jorge Bay serves it in that season as a breeding or nursery area ^[2].

It shares the coastal bottoms with other elasmobranchs caught in the same nets, such as Mustelus whitneyi, Triakis maculata and Galeorhinus galeus ^[2]. Because its range is small and it is highly vulnerable to gillnets, the authors of that study propose monitoring it as a sentinel species for the effect of fishing on coastal sharks ^[2].

The IUCN classifies it as Critically Endangered (CR) ^[4]. The reasons given are that it is fished hard and that no management measure targets this particular species anywhere it occurs ^[2]. Chile also considers it Critically Endangered in its official classification ^[2].

It is landed under the market category «tollo», which lumps it with Mustelus whitneyi, Triakis maculata and Galeorhinus galeus, so catch data specific to the species are missing ^[2]. In Peru, IMARPE reported smooth-hound landings as a single group until 1996, without separating them by species ^[7].

Most of what was landed in the Antofagasta sample were juveniles: 70.6% of the females and 66.0% of the males had not yet reached their size at maturity ^[2]. In Peru, the 2001 regulation on minimum sizes sets 60 cm total length for the species, with a tolerance of 20% juveniles ^[8],[2].

It is regularly caught together with other smooth-hounds for human consumption ^[5]. Peruvian landing statistics for 1996 to 2010 include records of the species ^[7].

In northern Chile it is caught mainly with coastal gillnets set for reef fishes such as Sarda chiliensis, Paralabrax humeralis and Paralichthys adspersus; in that fleet, sharks and rays make up about 60 to 75% of the yearly landings ^[2].

Its skin has also been studied as a raw material: researchers from universities in Trujillo obtained gelatin from it by alkaline hydrolysis and found that the best combination, 220 to 240 minutes at 60 to 65 °C, gave 27 to 28% gelatin; more time meant more yield, and a lower temperature a firmer gel ^[9].

In Peru it is known mostly from fishing: official landing statistics for 1996 to 2010 count it among the sharks landed in the country ^[7]. The 2015 checklist of cartilaginous fishes of Peru documents its presence in Peruvian waters with publications from 1973, 2001 and 2014 ^[6]. The FAO catalogue places it on bottoms at 16 to 50 m depth ^[5].