A manta ray is not a mammal, despite its oceanic habitat and massive size. It belongs to a completely different biological class, which dictates its anatomy and physiology. Understanding the manta ray’s true identity clarifies why it shares the water with marine mammals like whales and dolphins but remains fundamentally distinct. The confusion stems from certain life history characteristics that mimic traits associated with mammals. Examining the defining features of both classes makes the differences clear.
The Manta Ray’s True Identity
The manta ray is correctly classified as a fish, belonging to the Class Chondrichthyes, or cartilaginous fishes. This class includes all sharks and rays, collectively known as elasmobranchs. Their defining feature is a skeletal structure composed entirely of cartilage instead of hardened bone, separating them from bony fish and mammals.
Manta rays draw oxygen from the water using five pairs of gill slits located on their flattened underside. This aquatic respiration method is a hallmark of fish and differs fundamentally from the air-breathing required by mammals. Manta rays also lack the smooth skin associated with mammals, instead possessing a covering of microscopic, tooth-like structures called dermal denticles.
The Essential Criteria for Mammals
Membership in the Class Mammalia is determined by a specific suite of biological characteristics. The presence of mammary glands, specialized organs that produce milk to nourish their young after birth, is a unique and non-negotiable feature.
All mammals also possess hair or fur at some point during their development, even if sparse or limited, as seen in some marine species. Mammals are endothermic, meaning they are “warm-blooded” and can internally regulate a constant, high body temperature regardless of the surrounding environment. They also breathe atmospheric air using lungs and possess a muscular diaphragm to aid in respiration.
Addressing the Confusion: Viviparity and Size
The primary reasons people mistake manta rays for mammals are their capacity for live birth and their size. Giant oceanic manta rays can reach a wingspan of up to 7 meters (23 feet) and weigh more than 2,000 kilograms, rivaling the scale of many marine mammals. However, physical size alone is not a factor in biological classification.
While giving birth to live young, known as viviparity, is a common mammalian trait, it is also a reproductive strategy found across the animal kingdom. Manta rays are ovoviviparous; the developing pup hatches from an egg inside the mother’s uterus and is born live after a long gestation period. The growing pup is nourished by a yolk sac and a protein- and lipid-rich uterine “milk,” or histotrophe, secreted by the mother. This developmental process is distinct from the placental connection seen in most mammals, confirming that live birth in elasmobranchs is a case of convergent evolution, not mammalian heritage.