A stingray in an isolated tank recently became pregnant, challenging the typical understanding of reproduction. This phenomenon has led many to wonder if these animals can self-impregnate. To understand the possibilities, it is helpful to first establish the standard reproductive process for these cartilaginous relatives of sharks.
Standard Reproduction in Stingrays
Stingrays belong to the class Chondrichthyes and reproduce sexually through internal fertilization. Males use specialized pelvic fin modifications called claspers to deliver seminal fluid into the female’s reproductive tract. This ensures the sperm meets the egg inside the female’s body.
Most stingray species are ovoviviparous, meaning fertilized eggs are retained internally where they develop. The embryo initially relies on its large yolk sac for nutrition. Once the yolk is depleted, the developing young are nourished by histotrophy—a lipid-rich, milky substance secreted by the uterine wall. The female eventually gives birth to fully formed, live young, known as pups.
Parthenogenesis: Asexual Reproduction in Cartilaginous Fish
The most direct interpretation of “self-impregnation” is parthenogenesis, or “virgin birth.” This rare form of asexual reproduction occurs when an embryo develops from an unfertilized egg. In stingrays, this often involves automixis, where the egg fuses with a polar body to restore the full set of chromosomes.
The resulting offspring inherits all its genetic material from the mother. This capability is documented in other cartilaginous fish, including several species of sharks and sawfish, where isolated females in captivity have produced viable young. While parthenogenesis is a biologically plausible explanation for a mysterious birth, it is considered a last-resort strategy when no male is present.
Delayed Fertilization and Sperm Storage
The most common scientific explanation for seemingly miraculous births in isolated stingrays is long-term sperm storage. This biological adaptation allows the female to hold viable sperm within her reproductive tract for extended periods after a single mating event. This phenomenon is also referred to as delayed fertilization.
The sperm is stored in specialized structures within the female’s oviduct, primarily the oviducal gland. The gland’s environment provides the necessary conditions to keep the sperm alive and capable of fertilization for months or even years. This adaptation is highly advantageous in the wild, particularly for nomadic species or those in low population densities.
When a female stingray is moved to an isolated tank, she may possess stored sperm. The sperm remains dormant until the female’s reproductive cycle triggers ovulation. This results in the birth of young long after male contact, leading observers to incorrectly conclude that the animal impregnated itself. Genetic testing confirms the presence of paternal DNA in nearly all documented cases, indicating delayed fertilization was the true mechanism.