Plain Cardinalfish , Apogon atricaudus

Plain Cardinalfish, Apogon atricaudus. Underwater photographs taken in Zihuantanejo Bay, Guerrero, March 2018. Photographs courtesy of Ron Woheau, Zihuantanejo.

The Plain Cardinalfish, Apogon atricaudus, is a member of the Cardinalfish or Apogonidae Family, and is known in Mexico as cardenal sencillo. Globally, there are one hundred ninety species in the Apogon Genus, of which seventeen are found in Mexican waters, twelve in the Atlantic and five in the Pacific Ocean.

The Plain Cardinalfish has a small, laterally compressed, and oblong body. They are reddish-pink overall, without markings, tending to be darker with the bluish tinge anteriorly. Their head is large with a short, pointed snout, large eyes, and a large oblique mouth equipped with bands of small teeth. Their anal fin has 2 spines and 8 rays; their caudal fin is concave; their first dorsal fin has 6 spines; their second dorsal fin has 1 spine and 9 rays; and, their pectoral fins have 11 or 12 rays. They have 18 (5 + 13) gill rakers. They are covered with rough scales. Their lateral line is complete and extends into the tail base.

The Plain Cardinalfish is found within rocky reefs at depths up to 30 m (100 feet). They reach a maximum of 9.0 cm (3.5 inches) in length. They are nocturnal predators emerging at night forming small schools while remaining secluded within shelter during the day. They consume small fish and small crustaceans, mollusks and other invertebrates including crab and shrimp. They are one of the rare marine species exhibiting oral brooding; males incubate the fertilized eggs in their mouth for several days before releasing hundreds of larvae measuring 2 to 4 mm into the ocean. Larvae remain in planktonic form for several weeks before developing into juveniles. The Plain Cardinalfish is poorly studied with very limited information available about their lifestyle and behavioral patterns including specific details on age, diet, growth, habitat, longevity, movement patterns, and reproduction.

The Plain Cardinalfish is ENDEMIC to Mexican waters of the Pacific Ocean but  has a limited distribution being found sporaicially with isolated populations  around the islands west of Guerrero Negro, Baja California around the southern tip of the Baja Peninsula, Baja California Sur, in coastal waters of Guyamus, Sonora, and from Mazatlan, Sinola south to Zihuantanejo, Guerrero.

The Plain Cardinalfish is a straightforward identification and the only cardinalfish found in  Mexican waters of the Pacific Ocean that is unmarked lacking bars and stripes.

The Plain Cardinalfish from a conservation perspective are current considered to be of Least Concern, with a wide distribution and stable populations. As they are very rare and small in stature they are of limited interest to most.