Cocoa Damselfish, Stegastes xanthurus
The Cocoa Damselfish, Stegastes xanthurus, whose common Spanish name is jaqueta castaña, is a species in the Damselfish or Pomacentridae Family, known collectively as castañetas and jaquetas in Mexico. This species has historically been referred to as Stegastes variabilis, a separate species found in Brazilian waters. Globally, there are forty species in the genus Stegastes, eleven of which are found in Mexican waters, seven in the Atlantic and four in the Pacific.
The Cocoa Damselfish have oval moderately thick compressed bodies with a depth that is 44 to 48% of standard length, thus similar to freshwater bluegills. Adults vary in color being gray-brown on their head, body, and fins and having a yellowish tint on their belly, anal, caudal, and pectoral fins and dark edges on their scales that form oblique lines on the body; some fish have a large black spot on their last dorsal spines that extends into the upper back. Alternatively, adults can display patterns that are an intermediate between the juvenile and final adult stage. Juveniles are highly colored being dark gray-brown overlaid with dense evenly distributed blue spots on their upper head and the back and front two-thirds of their dorsal fin; the balance of their body and fins is lemon yellow. They also have blue lines that run along their snout, through their eyes, and along their upper back. They have the same dark oblique body lines and large black ocellus spot in the dorsal fin area; most also have a black spot on their upper caudal fin base. Their head has an angular snout with a small protrusible mouth that opens in the front with a single row of long closely set teeth. Their anal fin has two spines and 12 to 15 rays; their caudal fin is bluntly forked with rounded lobes; and their dorsal fin is singular and continuous with 12 spines and 14 to 17 rays. A key to identification are their anal and dorsal fins, which are long and extend past the caudal fin base. They have 8 or 9 gill rakers on their lower arch. Their lateral line is incomplete and ends under the dorsal fin base. Their body is covered with large rough scales.
The Cocoa Damselfish are tropical non-migratory fish found within inshore and offshore coral reefs as solitary individuals at depths up to 190 feet. They reach a maximum length of 12.5 cm (4.9 inches). They are diurnal omnivores that feed primarily on benthic algae, anemones, ascidians, and sponges. Juveniles feed on small invertebrates. Adults are aggressive when breeding, whereas juveniles are aggressive territorially. Reproduction is oviparous with pairing of individuals. Males prepare the nest site and each female releases approximately 1,000 sticky eggs that are distributed demersally and adhere to the substrate. The eggs are fertilized by the males, who then take up watch, provide aeration, and guard them from intruders for seven days. The hatched larvae become pelagic until they settle out in an appropriate environment. They are initially blue in color and transition to brown when they become sexually mature. They use bright colorations as camouflage within the coral reefs to avoid predation. Juveniles are aggressive but become highly social as they mature into adults. They have a lifespan of 12 years in the wild and 18 years in captivity.
In Mexican waters the Cocoa Damselfish are found in all coastal waters of the Atlantic.
The Cocoa Damselfish can be difficult to identify due to its multiple color patterns. It can be confused with the Beaugregory, Stegastes leucostictus (tan yellow base color), the Dusky Damselfish, Stegastes adustus (uniform reddish-brown color; body depth 52 to 56%), the Longfin Damselfish, Stegastes diencaeus (large rounded caudal fin), and the Threespot Damselfish, Stegastes planifrons (body depth 56-60%).
The Cocoa Damselfish are caught and retained by subsistence fishermen using nets and traps. They are classic nibblers and difficult to catch by hook and line. Being small and colorful they are used in the aquarium trade at a minor level. They are abundant and widespread with stable populations and are of limited interest to most which affords them a conservation status of Least Concern. Juveniles are consumed by the invasive Red Lionfish, Pterois volitans, but at present not at a level to affect global populations.
Cocoa Damselfish, Stegastes xanthurus, juvenile transitioning to an adult. Fish caught out off the Dania Pier, Dania Beach, Florida, January 2016. Length: 5.3 cm (2.1 inches). Catch and and photo courtesy of Betty and George Brinkman, Guelph, Ontario, Canada.