Cortez Triplefin, Axoclinus nigricaudus

Cortez Triplefin, Axoclinus nigricaudus. Underwater photograph taken in the greater Los Cabos area, Baja California Sur, October 2019. Photograph courtesy of Bob Hillis, Ivins, Utah.

The Cortez Triplefin, Axoclinus nigricaudus, is a member of the Triplefin or Tripterygiidae Family, that are known in Mexico as tres aletas colinegra. Its common name stems from the division of its dorsal fin into three parts, two of which have spines. Globally, there are one hundred forty-five members in the Tripterygiidae Family placed in thirty genera with five known species in the genus Axoclinus, of which three are found in Mexican waters of the Pacific Ocean.

The Cortez Triplefin has a short robust body with a large head that has 1 pair of small simple cirri over the nostrils. They green to brownish dorsally, whitish ventrally and have five oblique brown to blackish bars on the sides. They have 5 black bars and the caudal fin is clear with a thin white bar at its base. Breeding males had red throats and bellies. They are dimorphic with the front 4 bars reduced to black blotches along the mid-line. Their anal fin has 2 spines and 18 rays; they have three dorsal fins, the first with 3 spines, the second with 12 or 13 spines, and the third with 10 or 11 rays; and, their pelvic fins are completely separate. They are covered with small scales except on their head, breast, belly, and pectoral fin base.

The Cortez Triplefin is common within the shall rocky intertidal zone to depths up to 5 m (16 feet). They are the most common triplefin within the Sea of Cortez. They reach a maximum of 4.8 cm (1.9 inches) in length. They consume small invertebrates. Reproduction is oviparous with hemispherical eggs that attach themselves to the substrate via sticky threads. The larvae are planktonic and are found in shallow inshore waters. The Cottez Triplefin is poorly studied and very little is known about their behavioral patterns.

The Cortez Triplefin is a resident of all Mexican waters of the Pacific Ocean but has a limited distribution being found only throughout the Sea of Cortez.

The Cortez Triplefin is most likely confused with the Carmine Triplefin, Enneanectes carminalis (red color, has orbital cirri, and 4 bars mid-flank).

From a conservation perspective the Cortez Triplefin is currently considered to be of Least Concern with stable, widely distributed populations. They are small in stature, of limited interest to most, and seldom seen by humans.