Frilled Shark

The frilled shark, also known as the lizard shark, is one of the two extant species of shark in the family Chlamydoselachidae. The frilled shark is considered a living fossil, because of its primitive, anguilliform (eel-like) physical traits, such as a dark-brown color, amphistyly, and a 2.0 m (6.6 ft)–long body, which has dorsal, pelvic, and anal fins located towards the tail. The common name, frilled shark, derives from the fringed appearance of the six pairs of gill slits at the shark's throat.

Frilled sharks are primarily solitary and are believed to be slow-moving, ambush predators. They exhibit anguilliform (eel-like) locomotion, undulating their bodies to propel themselves through the water. Their hunting strategy involves lurking near the seafloor or in the water column, using their dark coloration as camouflage in the low-light environment. When prey approaches, the frilled shark can lunge forward and open its highly distensible jaws to engulf prey whole. Diet analyses indicate a preference for cephalopods, particularly squid, but they also consume bony fishes and other small sharks. Their teeth are uniquely adapted to snag soft-bodied prey, preventing escape. There is little evidence of social behavior; individuals are rarely observed together, and there are no known aggregations or cooperative behaviors. Activity patterns are not well understood, but some evidence suggests vertical migrations may occur, possibly following prey or environmental gradients.

Frilled sharks are ovoviviparous, meaning embryos develop inside eggs that remain within the mother's body until hatching. Females possess two functional ovaries and a pair of uteri. The gestation period is exceptionally long, estimated at 3.5 years (up to 42 months), which is among the longest known for any vertebrate. Litter sizes range from 2 to 15 pups, with an average of 6. Newborns measure 40–60 cm in length and are fully formed at birth, receiving no further parental care. There is no evidence of a defined breeding season; reproduction may occur year-round, likely due to the stable environmental conditions of the deep sea. Mating behavior has not been directly observed, but it is presumed to involve internal fertilization typical of elasmobranchs.

The frilled shark exhibits several adaptations for deep-sea survival. Its elongated, flexible body allows for efficient anguilliform swimming and maneuverability in tight spaces. The large, oil-rich liver provides neutral buoyancy, compensating for the lack of a swim bladder. The backward-curved, tricuspid teeth are specialized for gripping soft, slippery prey such as squid. Reduced calcification of the skeleton and persistent notochord confer flexibility. The dark coloration offers camouflage in the deep-sea environment, and the large eyes enhance sensitivity to low light. The slow metabolic rate and long gestation period are typical of deep-sea species, reflecting adaptation to low food availability and stable conditions. The amphistylic jaw suspension, a primitive trait, allows for wide gape and rapid prey capture.

The frilled shark has little direct significance in human culture due to its rarity and deep-sea habitat. However, its unusual appearance and 'living fossil' status have made it a subject of fascination in popular media, often depicted as a prehistoric or 'sea monster' figure. Occasional strandings or captures have inspired local legends and curiosity, particularly in Japan, where it is sometimes referred to as 'tsurushi same' (hanging shark). There are no known traditional uses or significant roles in folklore, but the species is increasingly recognized as an icon of deep-sea biodiversity and evolutionary history.

Recent research on the frilled shark has focused on its reproductive biology, genetic diversity, and deep-sea adaptations. Molecular studies have clarified its phylogenetic position as one of the most basal extant sharks, providing insights into early elasmobranch evolution. Advances in deep-sea exploration technology, such as remotely operated vehicles (ROVs), have enabled rare in situ observations of behavior and habitat use. Studies of stomach contents and stable isotopes have improved understanding of its diet and ecological role. Ongoing research aims to assess population structure, connectivity between Atlantic and Pacific populations, and responses to environmental change. The species remains poorly understood due to the logistical challenges of deep-sea research.

Sources

The Frilled Shark is currently classified as Near Threatened on the IUCN Red List.

Although the frilled shark is currently listed as 'Least Concern' by the IUCN, it faces several threats. Bycatch in deep-sea fisheries, particularly trawl and longline operations, is a significant source of mortality. The species' slow reproductive rate and long gestation make populations vulnerable to overexploitation. Habitat degradation from deep-sea mining and climate change-induced shifts in ocean chemistry and temperature may also pose future risks. There is limited data on population size and trends, but the species is considered naturally rare, with patchy distribution and low encounter rates. Conservation challenges include the lack of targeted management measures and the difficulty of studying deep-sea organisms.