Halichoeres sanchezi

Labridae is a very large group that is broken into smaller subfamilies. H. sanchezi falls into the Julidinae subfamily, which contains the saltwater wrasse. Some of the other subfamilies in Labridae include Scarinae, the parrotfish, previously thought to be their own family, and Cheilininae, which holds the largest wrasse species, the humphead wrasse, which can grow to 180 kg.^[2] Most of the diversity in Halichoeres is located in the tropics of the Indian Ocean and Central America. Within its family and subfamily, Halichoeres is one of the most species-rich genera, with over 30 species in the Central American tropics alone.^[3] H. sanchezi is most closely related to H. melanotis, which is found in Panama, with its next closest relative being H. salmofasciatus near Cocos Island. Ichthyologists call this group the H. melanotis species complex, as most wrasse in the area are separated by very small genetic differences. The easiest way to differentiate H. sanchezi from these other species is by its namesake spot on the caudal fin.

H. sanchezi is an endemic species found only in the Revillagigedo Archipelago, four volcanic islands off the coast of Mexico in the tropical eastern Pacific Ocean.^[4] The four islands are Roca Partida, Socorro, Clarion, and San Benedicto, with the latter being where samples were collected to identify H. sanchezi as a new species.^[5] However, pictures taken by Kreg Martin on Socorro Island in 2013 were retroactively identified as H. sanchezi, so they may not be exclusive to San Benedicto.

H. sanchezi was only seen at one site on San Benedicto Island during its discovery, a relatively flat, rocky plain south of the island. The site is around 21-22 meters deep with volcanic rock of varying sizes on the floor. Most of the rock is dark in color, with some patches of white rock. Juvenile H. sanchezi stayed close to the bottom, but bigger IP and terminal phase (TP) individuals were found higher in the water. Both groups were observed near boulders of the volcanic rock, which could provide protection if a predator approached.

H. sanchezi has a fusiform body shape, with large TP males being around 120 mm in size, while juveniles and IP fish range from 30-50 mm. The dorsal fin runs from just behind the head to right before the caudal peduncle with 9 spines and 12 rays. Pectoral fins are fan-shaped with 13 rays, and the pelvic fins are thoracic, 1 spine and 5 rays. The anal fin begins at the midsection of the fish; the 3 spines and 12 rays are of similar size to those of the dorsal fin. Finally, the caudal fin has 12 rays with the central 4 being segmented. Fin rays are an important way to distinguish one species from another, especially in species-rich families such as Labridae, where it can be difficult to tell close relatives from one another.

Their mouths are small and terminal, with two enlarged pairs of canines on the upper and lower jaws, followed by a row of 10-15 caniniform/conical teeth. Eyes are medium-sized, 5.5 mm. Their gill rakers are short, as they do not need the longer gill rakers found in filter-feeding fish. They have thin, cycloid scales, but not on the head, save for some irregular patches of scales where the head meets the body.

Juveniles and IP fish tend to be more brightly colored and patterned, becoming muted if they transition to TP males. Initial phase fish are a bright orange color dorsally with a gray ventral side. The orange is broken up by one lateral reddish band, and when disturbed, forms vertical bands, giving their dorsum a checkered appearance. Their irises are white and orange, with the orange part lining up with the orange on their dorsum. IP fins are mostly translucent with some brown to purplish color in the rays. They have three distinct black spots, the smallest of which is right above the pectoral fin, behind the gills. The next largest is at the base of the caudal fin, right above the midline of the fish. Finally, the largest of the three is about midway through the dorsal fin, with a yellow border around the black spot. Juveniles look very similar to IP fish, but their dorsal and caudal spots tend to be larger. Research suggests that this black spot on the caudal fin mimics the eye of a larger predator, scaring off would-be predators of H. sanchezi, which explains why it is larger on the smaller juveniles, as they need additional protection from predation.^[6]

Terminal phase males have a reddish-orange head that transitions into a brownish-orange dorsum with a yellow ventral side. The opercular flap is darker than the rest of the head, with purple and black patches. Their scales are lighter, more yellow, on the anterior side and darker brown on the posterior side, creating a crosshatch pattern on the body. Where their caudal peduncle transitions into caudal fin is black, covering the whole peduncle and the proximal half of the dorsal fin. Fins range from yellow to orange to brown, becoming less saturated the more distal they are. The iris is a pronounced shade of reddish-orange. TP males have a small black spot on their dorsal fin, on the membrane between the first and second rays.