5 Super Weird Shark Facts We Bet You Didn’t Know
- Sharks4Kids
- 1 hour ago
- 5 min read
Author: Lindsay Pierce
1. When sharks get a stomachache, they turn inside out
Humans expel what’s in our stomach when we’re sick; sharks skip that step and expel the stomach itself. Sharks rapidly eject their stomach in response to stress or indigestible substances in their digestive tract, such as food particles or mucus. This is a phenomenon termed “gastric eversion” or “stomach eversion”. Stomach eversion has been documented in several shark species, including (but not limited to) the Caribbean reef shark, the tiger shark, and the Atlantic sharpnose shark. Fishing-related stress is a common trigger and accounts for most of our observations of stomach eversion happening, though it has been recorded in wild sharks. Sharks may rapidly eject and subsequently retract their stomach, though retraction has been documented relatively few times. It's a strange and striking behavior, but for sharks, it's just another remarkable adaptation in an already extraordinary group of animals.
Image: Caribbean reef shark (Carcharhinus perezi) exhibiting oral gastric eversion. Black arrows indicate the position of the stomach, while white arrows indicate the position of a horse-eye jack (Caranx latus). Image sourced from Figure 2 of Brunnschweiler et al (2005).
2. Baby sand tiger sharks have their first meal before they even leave the womb
Shark reproductive strategies vary widely depending on the species. Some sharks lay eggs, others give live birth, and still others exhibit a combination of the two where fertilized eggs are nourished by a yolk sac inside the mother. Certain species have remarkably odd reproductive quirks, and some females have even been documented producing offspring without any male contact through parthenogenesis, but sand tiger sharks might just take the cake. Female sand tigers have two uteruses, and in order to maximize the size of newborn pups, offspring is limited to one per uterus. However, it doesn't start out that way. During early gestation, as many as seven embryos can inhabit a single uterus, and the largest embryo preys upon its smaller siblings until only one remains in each uterus, which is scientifically termed intrauterine cannibalism. It is hypothesized that this strange adaptation could result in a competitive advantage for the surviving pups. By the time a sand tiger pup is born, it is already a seasoned predator.
Image: A sand tiger shark (Carcharius taurus), cruising along. Image credit to Paddy Burke.
3. Cookiecutter sharks take bites out of animals (and submarines) many times their size
Cookiecutter sharks (Isistius brasiliensis) are tiny animals, measuring only 16 to 22 inches. You'd think sharks this small would target little prey, but cookiecutter sharks have been documented feeding on dolphins, whales, seals, and even great white sharks. Their feeding mechanism is just as remarkable as their target selection. A cookiecutter shark bites down on its victim and rotates its body to remove a clean, circular plug of flesh, leaving behind a distinctive wound pictured below on a great white shark. Their bites are so powerful that they have even been documented damaging US Navy submarines, gouging chunks out of sonar domes and electrical cables during the 1970s and 80s. The Navy initially suspected a Soviet weapon before identifying the culprit, and eventually had to retrofit their submarines with fiberglass covers to keep the tiny sharks at bay. Not bad for an animal smaller than your forearm.
Image: Great white shark (Carcharhodon carcharias) with a suspected cookiecutter shark (Isistius brasiliensis) bite. Image credit to Mauricio Hoyos-Padilla et al / Pacific Science.
4. Hammerhead sharks can get a tan
Yes, you heard that right. Just like humans, juvenile scalloped hammerhead sharks (Sphyrna lewini) darken in color when exposed to increased solar radiation, documented in a groundbreaking 1996 study by Drs. Chris Lowe and Gwen Goodman-Lowe. Young scalloped hammerheads off the coast of Hawaii spend most of their time in deeper waters, but when researchers held them in shallower water where UV levels are much greater, the sun-exposed sharks were visibly darker than their shaded counterparts. Not only that, but the melanin content and structure of their skin differed between tanned and untanned sharks. Sun-exposed individuals increased their melanin content by 14% over 21 days and 28% over 215 days. Juvenile scalloped hammerheads use deeper, murkier water as a refuge from predators, but as they mature they spend much more time in clearer pelagic waters. While comparable skin color changes have been observed in other sharks and rays after sun exposure, suntanning has not been directly confirmed in any other shark species. For now, the scalloped hammerhead holds the title of the only shark with a verified tan.
Image: Panel A shows sun-exposed (right) versus shaded (left) juvenile scalloped hammerheads (Sphyrna lewini). B and C show the changes in skin structure between the two sharks (B shaded, C sun-exposed). Image sourced from Lowe and Goodman-Lowe 1996.
5. Nurse sharks have been observed “cuddling”
Sharks have a reputation as lone hunters, but nurse sharks are breaking the stereotype entirely. Multiple species have been observed aggregating in large groups, and it's not always clear why. Female Atlantic nurse sharks (Ginglymostoma cirratum) form piles prior to breeding, and courting males approach the assemblage to find mates. Interestingly, their sister species, the Pacific nurse shark (Ginglymostoma unami), forms groups that appear to be motivated by something else entirely. Both males and females participate in piling, but courtship displays have not been observed during these aggregations. Instead, researchers speculate that these large groups, sometimes 50 to 60 sharks strong, may help the sharks conserve body heat. Staying warm matters because sharks are most metabolically efficient within a narrow temperature range, and huddling together can reduce the rate at which individuals lose heat to the surrounding water. Fittingly, these aggregations grow larger and more frequent during upwelling season, when colder water rises to the surface. Whether they're piling up to mate or just to stay warm, nurse sharks have made it clear that being social has its perks.
Image: Multiple Pacific nurse sharks (Ginglymostoma unami) lying close to one another on the seafloor. Image credit to Andy Murch.
SOURCES:
Brunnschweiler, J. M., Andrews, P. L., Southall, E. J., Pickering, M., & Sims, D. W. (2005). Rapid voluntary stomach eversion in a free-living shark. JMBA-Journal of the Marine Biological Association of the United Kingdom, 85(5), 1141-1144.
Andrzejaczek, S., Gleiss, A., Lear, K., McGregor, F., Chapple, T., & Meekan, M. (2024). Stomach eversion and retraction by a tagged tiger shark at Ningaloo Reef, Western Australia. Fisheries research, 269, 106875.
Christie, B. L. (2012). Intestinal eversion in the Atlantic sharpnose shark, Rhizoprionodon terraenovae, a behavior ubiquitous among elasmobranch fishes?. Gulf of Mexico Science, 30(1), 8.
GILMORE, R. G., DODRILL, J., & LINLEY, P. A. (1983). THE SAND TIGER SHARK, ODONTASPIS TAURUS (RAFINESQUE) ¹. Fishery Bulletin, 81(2), 201.
Wyffels, J., Kneebone, J., Lyons, K., & Christiansen, E. (2023). In Utero Cannibalism: Sand tiger sharks have evolved a unique strategy for producing the biggest progeny possible. Natural History, 131(8), 16-22.
Jones, E. C. (1971). Isistius brasiliensis, a squaloid shark, the probable cause of crater wounds on fishes and cetaceans. Fish. Bull, 69(4), 791-798.
Murakami, C., Yoshida, H., & Yonezaki, S. (2018). Cookie-cutter shark Isistius brasiliensis eats Bryde’s whale Balaenoptera brydei. Ichthyological Research, 65(3), 398-404.
F.G. Wood, "Professional Notes," Proceedings, U.S. Naval Institute, vol. 104/8/906, August 1978.
Lowe, C., & Goodman-Lowe, G. (1996). Suntanning in hammerhead sharks. Nature, 383(6602), 677.
Mora, S.M. (2025, February 10). Pacific nurse sharks like to cuddle! But why? Save our Seas Foundation. https://saveourseas.com/update/pacific-nurse-sharks-like-to-cuddle-but-why/
