Early ontogenetic development of Cynodon gibbus (Characiformes: Cynodontidae) in the Amazon River basin

Abstract

Knowledge regarding the early life history of neotropical fish, although incipient, is crucial for the accurate identification of larvae collected from the natural environment. Such details are important for defining spawning, drift, and nursery areas as well as the reproductive periods of species. Herein, the larval development of Cynodon gibbus was described using morphological, meristic, and morphometric characteristics. Specimens were collected from open water limnetic zones and under macrophyte stands in the Amazon basin from 2010 to 2023. Forty-eight larvae with standard lengths ranging from 5.73 to 21.57 mm were studied. The larvae have a long to very long body, small eyes, and a small head, which progresses to a moderate size in the preflexion stage. Furthermore, the larvae had ample mouths with numerous conical teeth, an oval-triangular swim bladder, overlapping dorsal and anal fins, and the tip of the pectoral fin not reaching the swim bladder. Pigmentation pattern comprised two punctate pigments above the fontanelle, the rectum, and at the lower and upper ends of the caudal peduncle. Various body parts exhibited predominant allometric relationships, reflecting differential developmental rates among them in response to fundamental ecological requirements for survival. The C. gibbus larvae can be identified by a combination of the position of dorsal, anal and pectoral fins, pigmentation pattern, swim bladder shape, and the total number of myomeres (51 to 53).