A marine vertebrate migrates to fresh water for spawning. After spawning, within a few days, it dies. Its larvae, after metamorphosis, return to the ocean. Which of the following morphological features is characteristic of this animal?

Step-by-step solution

The life cycle described (migration to fresh water for spawning, death after spawning, and larval metamorphosis before returning to the ocean) is a characteristic feature of class Cyclostomata (e.g., Petromyzon). Members of Cyclostomata have an elongated body, a sucking and circular mouth without jaws, and their body is devoid of scales and paired fins. Placoid scales and claspers are features of Chondrichthyes. Ctenoid scales and an air bladder are features of Osteichthyes. A three-chambered heart and moist skin a