Mexican hogfish

Bodianus diplotaenia is a fish of the wrasse family (Labridae) that Gill described in 1862 under the name Harpe diplotaenia, from specimens taken at Cape San Lucas, Baja California ^[1],[2]. In Peru it is known as «gallo» and as «vieja negra» ^[3]. It lives on rocky bottoms and reefs of the eastern Pacific, from the Gulf of California to Peru and the region's oceanic islands ^[2]. Its global category on the IUCN Red List is Least Concern ^[4].

The epithet diplotaenia joins the Greek diploos, meaning double, and tainia, meaning ribbon; it refers to the 2 dark stripes running along the body of juveniles and initial-phase adults ^[2].

The dorsal fin has 12 spines and almost always 10 soft rays, and the anal fin 3 spines and 12 rays; the pectoral fin carries 15 branched rays and the lateral line 31 pored scales ^[2]. Large adults grow a fleshy hump on the forehead, between the eyes, which becomes exaggerated in the biggest fish; with size, the tips of the dorsal and anal fins stretch into a filament, and the top and bottom rays of the tail fin extend into 2 long points ^[2]. The largest specimen Gomon examined measured 447 mm standard length ^[2], and the maximum recorded size is 76 cm total length and 9 kg in weight ^[5].

Its colouring changes with age. Juveniles are a uniform lemon yellow on which broken black stripes appear, along with 2 black spots at the base of the tail ^[2]. Initial-phase adults are red or reddish brown, with 2 broken black stripes on the upper half of the body, a white underside to the head and a black spot between the first dorsal spines ^[2]. Terminal-phase adults are red, bluish or brownish grey, with a narrow yellow band on the flank below the fifth to seventh dorsal spines and a black blotch near the upper rear corner of the pectoral fin ^[2].

The forehead hump is not simply a male badge: Gomon found that many small fish with terminal-phase colouring had ripe ovaries, and neither the function nor the cause of the swelling is known ^[2].

Similar species

• Bodianus eclancheri: It is the only other species of the genus that grows a hump on the forehead. Bodianus diplotaenia has canines at the front of the jaws, whereas Bodianus eclancheri has incisor-shaped teeth there ^[2]. Adults of Bodianus eclancheri are also very variable in colour, with combinations of red, orange, yellow, brown, black and white that often do not match from one flank to the other ^[2].

It ranges along the Pacific coast of the Americas from the Gulf of California to Peru, and at the oceanic islands of Clarion and the Revillagigedos, Clipperton, Cocos and Galápagos; there is also an old record from northern Chile, between Arica and Iquique ^[2]. The material Gomon examined includes a Peruvian specimen from the Lobos de Afuera islands ^[2].

It lives in seagrass beds, on coral reefs and over rocky bottom, and is often abundant near islands; most specimens have been taken between 1 and 15 m deep, though it reaches about 60 m ^[2]. It has been recorded between 5 and 76 m deep, usually between 5 and 18 m ^[5].

It begins its reproductive life as a female and may later turn into a functional male ^[5]. In the Gulf of California, males hold breeding territories that are not permanent, and females mate every day, although courtship and social life take up little of their time ^[6].

In that study, males spent 45.9 % of the time eating and females 76.5 %; the author explains this by the breeding success of these males hinging more on time given to courtship and social life than on the energy they gain by eating ^[6]. It lives alone or in groups of a few fish, and at night several gather in rock crevices and caves to sleep ^[5].

It feeds on crabs, brittle stars, molluscs and sea urchins, and its trophic level estimated from diet is 3.4 ^[5].

In experiments in Galápagos at 10 to 12 m depth, adults preyed on small pencil urchins (Eucidaris galapagensis) tethered to the bottom but failed to eat the large ones ^[7]. It was the most frequent predator around the trials, present 32.1 % of the time in the pencil-urchin trial and 21.7 % in the green-urchin (Lytechinus semituberculatus) trial ^[7]. It often circled triggerfish closely while they ate an urchin, tried to bite the prey and slowed them down; in 11 of 18 such interferences (61.1 %) it ended up eating the remains the triggerfish left, and once it was seen stealing an urchin ^[7].

During the 2015-2016 El Niño it was one of 18 Galápagos fish species in which an ulcerative skin disease was seen; it was first seen affected on 13 January 2016 at Cuatro Hermanos East island ^[8]. Its gills in Tumbes host 2 species of parasitic monogenean worms described from this fish, Haliotrema diplotaenia and Haliotrema saezae ^[9].

Its global category on the IUCN Red List is Least Concern, according to a 2007 assessment ^[4]. FishBase rates its resilience as low, with a minimum population doubling time of 4.5 to 14 years, and its vulnerability to fishing as high, 58 out of 100 ^[5].

It is taken in commercial fisheries, sold fresh and also traded as an aquarium fish ^[5].

In fish markets and restaurants of La Paz, Mexico, a genetic study found it being sold under the trade names of more prized species; the authors count it among the less popular local species used to meet demand for the valued ones when these run short ^[10].

A DNA barcoding study, under a 2020 collecting permit from the Ministry of Production, sampled the rocky reefs of 3 areas of the Mar Tropical de Grau National Reserve by scuba diving: Isla Foca, the coast between Cabo Blanco and Los Órganos, and Arrecifes de Punta Sal in Tumbes; Bodianus diplotaenia is among the fishes it sequenced, with a maximum genetic distance between its specimens of 0.62 % ^[11]. The authors note that these reefs, the only ones known in Peru, are hard to sample and to reach ^[11]. The fish in which its gill parasites were described came from the coast of Tumbes ^[9].