Mexican Stoneroller, Campostoma ornatum

Mexican Stoneroller, Campostoma ornatum, Breeding Female. Fish caught from a creek in Rucker Canyon, Arizona, April 2019. Length: 8.9 cm (3.5 inches) each. Catch, photograph, and identification courtesy of Chris Moore, Peoria, Arizona.

The Mexican Stoneroller, Campostoma ornatum, is a member of the Carp and Minnow or Cyprinidae Family, and is known in Mexico as rodapiedras mexicano. They obtained their common name from the males’ habit of rolling stones along the bottom with their noses while nest building. Globally, there are six species in the Campostom Genus, of which two are found in Mexico’s freshwater systems.

The Mexican Stoneroller is small in stature and has a very stout cylindrical body. They are highly variable in color with various shades of mottling with transparent scales. They have purple-blackish regions dorsally that transition to golden-brown and yellow ventrally with black spots on the flanks and a black patch at the base of all fins. Breeding males develop milky white anal and dorsal fins, orange pectoral fins, and velvet-black pelvic fins and the black caudal spot becomes longer turning into a band. Normally females develop small tubercles on the snout and above the eyes and a black band on the dorsal fin develops in females several months before breeding season. The juveniles lack mottling but have a dark mid-lateral stripe that extends from the snout to the caudal peduncle and has a small caudal spot. Their head is very large with a very large snout and a very small sub-terminal mouth equipped with pharyngeal teeth. Their anal fin has 7 or 8 rays; their caudal fin has 18 to 20 rays; their dorsal fin has 8 rays; their pectoral fins have 16 to 18 rays; and, their pelvic fins have 8 rays. They have 14 to 20 moderately long, well separated gill rakers. They are covered with scales along the lateral line.

Mexican Stonerollers are found in the shallow waters of small to medium sized streams near gravel and stone substrates with larger individuals found in sandy-bottomed streams. They utilize vegetation for cover to avoid predation and humans. They reach a maximum of 16 cm (6.3 inches) in length with males being longer than females. The Mexican Stoneroller is primarily a herbivore that feeds on algae, bacteria and diatoms. They rely on their cryptic coloration for camouflage to avoid predation but are known to be heavily preyed upon by the non-native highly invasive Green Sunfish, Lepomis cyanellus and Rainbow Trout, Oncorhynchus mykiss. Reproduction is polygynandrous (promiscuous) and oviparous with external fertilization that involves annual breeding cycles. It involves an elaborate process which starts with the adult males constructing spawning pits by digging into gravel with their snouts and mouths. The males possess tubercles that they can use as tubercles as weapons to fend off intruders and stimulate and attract females. The females deposit eggs in these pits which are sticky and adhere to the substrate, which are fertilized by the males, and then abandoned and left to fend for themselves. Males are known to take over nests of other fish and can become very aggressive and highly territorial during this period.

The Mexican Stoneroller is a resident of the freshwater systems of northern Mexico at elevations between 800 m (2,600 feet) and 2,000 m (6,600 feet) being wide-spread in the Big Bend region of Chihuahua west to the Rio Yaqui and Rio Sonora in Sonora and southward thru Durango to the Nazas-Aguanaval basins in Zacatecas.

The Mexican Stoneroller is similar in stature and can be confused with the Central Stoneroller, Campostoma anomalum (fewer lateral scales and a shorter lower jaw).

 From a conservation perspective the Mexican Stoneroller is currently considered to be of Least Concern with stable, widely distributed populations that are estimated to be in excess of 100,000 individuals. Their populations have been adversely affected in some area by habitat destruction due to human development that alters water flow regimens, pollution and debris, and increased siltation and aquatic vegetation. They are small in stature and of limited interest to most.