Long-term growth and male sexual maturation of captive-born Japanese wobbegongs (Orectolobus japonicus): implications for life-history research and conservation

To understand how sharks live, grow, and reproduce, scientists must know their ages and how fast they get bigger. For many shark species, this information is missing because these animals are hard to catch, hard to keep track of in the wild, and often die before researchers can study them. Without knowing when a shark becomes an adult or how many babies it has, it is difficult to protect the species from overfishing or habitat loss. This is especially true for bottom-dwelling sharks, which spend their lives hidden on the ocean floor. While scientists have long studied sharks in aquariums to learn about their growth without harming them, most of these studies only lasted a few years. To truly understand a shark's life, researchers need to watch it from the moment it is born until it is fully grown and ready to have its own offspring.

A team of researchers at the Osaka Aquarium KAIYUKAN in Japan has finally provided this long-term view for the Japanese wobbegong, a small, camouflaged shark that lives in the waters around Japan. By keeping a group of these sharks in captivity and recording their growth every month for several years, the scientists were able to map out exactly how this species develops. They found that these sharks grow in a very specific way, getting heavier faster than they get longer, and they identified the precise moment when male sharks become capable of reproducing. This work offers a rare, clear picture of the life history of a species that is otherwise difficult to study, giving conservationists a solid baseline to compare against wild populations.

The story begins with the birth of twenty-two baby sharks. In 2018, five pups were discovered in a large tank, and in 2020, a pregnant female was moved to a separate, isolated tank where she gave birth to seventeen more. Because the researchers watched the tanks every single day, they knew the exact birth date for every single pup, with an uncertainty of less than twenty-four hours. This level of precision is almost impossible to achieve in the wild. The scientists then measured the length and weight of these young sharks about once a month. They also kept a close eye on the male sharks, measuring the length of their claspers, which are the paired reproductive organs found on the underside of male sharks. In male sharks, these organs start out small and soft but grow longer and harder as the shark matures, eventually becoming the tool used for reproduction.

The data revealed a clear pattern in how these sharks grow. As the sharks got bigger, they gained weight much faster than they gained length. This is known as positive allometric growth, meaning the sharks become stockier and heavier as they age rather than just stretching out. This pattern was the same for both males and females. The researchers tested several mathematical models to see which one best described the growth curve of the Japanese wobbegong. They found that a logistic model, which describes growth that starts fast and then slows down as the animal reaches its maximum size, fit the data better than other common models. According to this model, the sharks stop growing when they reach a total length of just over 100 centimeters. Males were estimated to grow slightly larger, reaching about 107 centimeters, while females reached about 102 centimeters.

One of the most significant findings concerned when the male sharks became sexually mature. The researchers watched the claspers of four male sharks closely. They saw that rapid growth of these organs began when the sharks were about 1,000 days old, or roughly 2.7 years, and measured about 75 centimeters in length. By the time the sharks were around 1,200 days old, or just over three years, the claspers had stopped growing relative to the shark's body size and had reached a length of about 84 centimeters. The ultimate proof of maturity came from a single male shark. At 1,997 days old, or nearly 5.5 years, and measuring 97.8 centimeters in length, this shark was observed releasing seminal fluid from its clasper. This confirmed that the shark had reached functional sexual maturity, meaning it was physically capable of reproducing.

The study also provided a reliable count for the number of babies a Japanese wobbegong can have at once. While previous reports had suggested a litter size of 32 based on two females, those numbers were not confirmed. In this study, the isolated female in 2020 gave birth to exactly 17 pups. This number fits well with what is known about related shark species, which typically have litters ranging from a few to several dozen. The researchers noted that the 2018 litter might have been larger, but some pups could have been eaten by other fish in the tank before they were found, which is why the 2020 count from the isolated tank is considered more accurate.

These long-term records are valuable because they provide a known-age baseline for the Japanese wobbegong. In the wild, it is very difficult to know exactly how old a shark is or when it becomes an adult. By having this detailed data from sharks born in captivity, scientists can now compare wild sharks to see if they grow at the same rate or reach maturity at the same size. The researchers caution that sharks in aquariums might grow differently than those in the ocean because they have a steady supply of food and stable temperatures. However, this study establishes a fundamental understanding of the species' life cycle. It confirms that the Japanese wobbegong grows slowly, matures late, and has a moderate number of offspring, all traits that make the species vulnerable to human activities. With this new information, conservation efforts can be better informed to ensure these bottom-dwelling sharks continue to thrive in the Pacific Ocean.