Threebanded butterflyfish

Chaetodon humeralis is a butterflyfish of the family Chaetodontidae, described by Günther in 1860 ^[1]. In Peru it is called «mariposa» ^[2]. It lives on reefs of the eastern Pacific, from southern California to Peru, the Galápagos Islands included ^[3]. The IUCN Red List assesses it as Least Concern ^[4].

The genus name, Chaetodon, joins two Greek roots: chaite, hair, and odous, tooth ^[3]. The teeth of this species live up to the name: they are tiny and bristle-shaped, grouped in a strip that looks like a comb ^[3].

It is a tall fish, strongly flattened from side to side and almost disc-shaped, with a steeply sloping forehead and a somewhat projecting snout ^[3]. Its creamy-yellow body is crossed top to bottom by 5 or 6 dark bars, from the head back to the tail ^[3]. The dorsal fin has 12 to 13 spines and 18 to 20 soft rays, and the anal fin 3 spines and 15 to 17 rays; it reaches 25.4 cm in total length, although about 12 cm is usual ^[3].

In a DNA barcoding study (COI gene) of rocky reef fishes from northern Peru, specimens of Chaetodon humeralis differed from one another by at most 1.24 %, below the 2 % threshold the authors used, while the closest species in the dataset, Anisotremus interruptus, sat at 21.05 %; in other words, the result agrees with current taxonomy ^[5].

It occurs in the eastern Pacific, from San Diego (California, United States) to Peru, with the Galápagos Islands included ^[3]. It is tied to reefs and has been reported between 3 and 50 m deep, although it is commonest between 3 and 12 m, usually in pairs or small groups over rocky bottoms close to shore ^[3].

It can go deeper: in technical-diving counts made in 2018 and 2019 in the submarine canyon in the south of Banderas Bay (Mexico), it was recorded between 60 and 70 m, with 13 individuals in all, and was the most abundant species at 70 m ^[6]. It also turns up, in low numbers, in samples from the Santa María la Reforma coastal lagoon in the Gulf of California ^[7].

In Peru it lives on the rocky reefs of the north: specimens were collected by diving inside the Mar Tropical de Grau National Reserve for a genetic study ^[5]. Farther south, in diving counts in 1999 and 2000, after the 1997-98 El Niño, it was one of the abundant species in the Bahía Sur of San Bartolo, about 50 km south of Lima and 4 m deep at most; that paper's table, based on a 1998 key, places its southern limit at Pucusana ^[8].

FishBase states that adults eat algae and bottom-dwelling invertebrates ^[3]. A study of the mesophotic fishes of Banderas Bay counted it among the carnivorous species ^[6], and an analysis of the evolution of coral-eating fishes, which sorted butterflyfishes by diet, placed it among the non-corallivores ^[9].

In the stomachs of adult fish speared in Mazatlán and Banderas bays, on the Mexican Pacific coast, no sponge remains were found in Chaetodon humeralis, unlike the angelfishes Pomacanthus zonipectus and Holacanthus passer, which did eat sponges ^[10].

It is oviparous and pairs up to breed; its mating system has been described as monogamous ^[3].

On the reefs of Punta Veleros (Los Órganos, Piura), 9 interactions in which reef fishes cleaned green turtles were documented between October 2022 and March 2023, at 5 to 8 m depth. The turtles stayed close to a rock wall dominated by aggregations of Chaetodon humeralis and Johnrandallia nigrirostris, but Chaetodon humeralis is not on the list of fishes that cleaned them ^[11].

In the Bahía Chamela sanctuary, on the central Mexican Pacific coast, an annotated fish checklist rated it as an abundant species ^[12]. In Banderas Bay it was one of the species that weighed most in the difference between the fish community at 70 m and the one at 50 m ^[6].

The IUCN assessed it in 2009 and lists it as Least Concern ^[4]. In Peru, the specimens of a recent genetic study were collected on reefs of the Mar Tropical de Grau National Reserve, under a permit from the Ministry of Production ^[5].

FishBase lists commercial use for it in both fisheries and the aquarium trade ^[3]. On the Mexican Pacific coast it occasionally turns up in the bycatch of the industrial shrimp fishery, even though it is a fish of rocky and coral bottoms ^[13].

On the reefs of Punta Veleros, at Los Órganos (Piura), scuba observations between October 2022 and March 2023 described a rock wall dominated by aggregations of this species and of Johnrandallia nigrirostris, on a reef 5 to 10 m deep. One of the places observed is known to local divers as the butterflyfish wall ^[11].

At the Lobos de Afuera Islands, some 32 nautical miles offshore and hard to reach because of the distance and the seabed relief, it is among the species recorded by the baseline studies published in 2019, although environmental DNA sampled in November 2018 did not detect it ^[14].

Toward central Peru there are records from after El Niño: in 1999 and 2000 it was abundant in the Bahía Sur of San Bartolo, in Lima ^[8].