The whale shark (Rhincodon typus) is the world’s largest fish, a filter-feeder found across tropical oceans. Despite its size and wide distribution, much of its life cycle remains unknown to science. While migratory patterns and diet are relatively understood, its reproductive biology is a mystery. Scientists rarely observe mating or birthing events, leading to decades of speculation about how these gentle giants reproduce. Fundamental questions, such as the number of babies they have, were unanswered until a single discovery provided clarity.
The Record-Setting Litter Size
The definitive answer to how many babies a whale shark can have came from an unexpected source in 1995. Fishermen off the coast of Taiwan caught a pregnant 11-meter female, providing the only detailed examination of a gravid female to date. This single specimen revealed a reproductive capacity far exceeding any other known shark species.
Inside her two uteri, researchers discovered a record-setting litter of 304 embryos and pups, establishing the whale shark as the most prolific live-bearing shark documented. The developing young ranged widely in size, from small embryos to pups ready for birth. This simultaneous presence of pups at various developmental stages indicated superfetation, where the female carries multiple litters concurrently.
The Mechanism: Ovoviviparity and Staggered Development
The ability of the whale shark to carry over 300 developing young is made possible by a specific reproductive mode called ovoviviparity. This strategy differs significantly from oviparity (egg-laying) and viviparity (placental connection). Whale sharks are ovoviviparous, meaning the embryos develop within egg cases inside the mother’s uterus, hatching internally before being born as live young.
The developing pups initially gain nourishment from the yolk sac of their egg case. Once the yolk is depleted, the pups may receive additional sustenance from uterine secretions until they are ready for birth.
The simultaneous presence of embryos at different sizes and developmental stages within the 1995 female suggests that the female can store sperm from a single mating event. This stored sperm can then be used to fertilize eggs over a prolonged period, allowing the mother to produce a continuous, staggered stream of pups.
This reproductive adaptation, known as superfetation, ensures that the large litter is not released all at once but potentially over many months or even years. This strategy of delayed, sequential births spreads the reproductive risk and increases the chances that some pups will encounter favorable conditions.
Early Life of the Pups
Whale shark pups are born fully developed and independent, typically measuring between 40 and 60 centimeters (about 16 to 24 inches) in total length. They possess the ability to swim and feed on their own immediately, a necessity because the mother provides no parental care after birth. The enormous biological investment required means the pups must be ready for immediate survival upon entering the open ocean.
The high-energy demands placed on the mother are mitigated by releasing the pups in small, successive batches. This staggered release also ensures that the pups are not all exposed to the same environmental threats at once. Despite the large litter size, the first years of a whale shark’s life remain one of the biggest mysteries in marine science. Once born, young whale sharks seem to disappear from common sighting areas, leading to what is often called the “missing juvenile stage.” They are rarely observed until they reach a length of about 2.5 to 4 meters, which suggests they occupy isolated or deep-water habitats. Scientists hypothesize that these small pups seek refuge in the deep sea or in remote, unstudied nursery grounds to avoid predation until they grow large enough to join the more visible adult feeding aggregations.