The bonnethead shark ( Sphyrna tiburo ) is the smallest member of the hammerhead shark family, Sphyrnidae. Characterized by its unique shovel-shaped head, the bonnethead shark presents a fascinating case study in dietary adaptation. This article delves into the dietary habits of the bonnethead shark, exploring its classification as an omnivore and the implications of its diverse diet.
Physical Characteristics and Distribution
Bonnethead sharks have gray, tan, or greenish upper bodies and paler bellies. Their backs and sides are often scattered with small dark spots. The species can reach an average of 3 to 4 feet, with a maximum length of about 5 feet. The heaviest recorded weight for a bonnethead shark is 24 pounds. Female bonnethead sharks are typically larger than males. Adult females have an evenly rounded head, while adult males have a distinct bulge in the center of their heads.
In the Atlantic Ocean, bonnethead sharks are primarily found off the coast of North Carolina to southern Brazil, including the Gulf of America and Caribbean Sea. Although it’s rare, they have been seen as far north as Rhode Island. In the Pacific Ocean, the species’ range spans from southern California to Ecuador. Bonnethead sharks live in shallow coastal waters over sandy or muddy bottoms, coral reefs, and grass flats.
General Dietary Habits
Bonnethead sharks feed during the day with foraging activity peaking in the late afternoon. The shark feeds primarily on crustaceans, consisting mostly of blue crabs, but also shrimp, mollusks, and small fish. Bonnethead sharks are known to swim in small groups of five to 15 individuals. They move constantly following changes in water temperature and to maintain respiration. Its feeding behavior involves swimming across the seafloor, moving its head in arc patterns like a metal detector, looking for minute electromagnetic disturbances produced by crabs and other creatures hiding in the sediment. Upon discovery, it sharply turns around and bites into the sediment where the disturbance was detected. The tooth structure and jaws of bonnetheads has been adapted for feeding on both hard-shelled and soft prey. They have small sharp teeth at the front of their jaw for grasping soft-bodied prey, and broad, flat molar-like teeth at the back for crushing hard-shelled invertebrates.
The Discovery of Omnivory
Bonnetheads also ingest large amounts of seagrass, which has been found to make up around 62.1% of gut content mass. For one shark subspecies, this fictional scenario isn’t actually too far from the truth, according to a new study published in Proceedings of the Royal Society B. Gizmodo’s George Dvorsky reports that the bonnethead is the first shark species to receive the official designation of omnivore rather than carnivore.
Read also: Comprehensive Bala Shark Guide
Previously, it was assumed that the consumption of seagrass by bonnethead sharks was incidental and provided no nutritional value. However, recent research has challenged this assumption, revealing that seagrass plays a significant role in the shark's diet.
The Seagrass Digestion Study
To test the bonnethead’s response to seagrass, Leigh and her colleagues fed five sharks a diet of 90 percent seagrass-grown in water sprinkled with sodium bicarbonate powder to create a unique carbon isotope signature-and 10 percent squid. According to Dvorsky, the bonnethead shark possesses special digestive enzymes that enable it to break down seagrass. The animals tested digested more than half of the organic material found in the seagrass, Sample adds, and even used these nutrients to build up and maintain their overall health, as evidenced by traces of the seagrass carbon isotope found in the sharks’ blood and liver tissue.
Implications of Omnivory
The scientists’ findings point to the need for further study of marine ecosystems. The revelation that bonnethead sharks are omnivorous has significant implications for our understanding of marine ecosystems and food webs. It challenges the traditional view of sharks as purely carnivorous predators and highlights the complex and interconnected nature of marine life.
Criticisms of the Study
Still, the study has its critics: Gavin Naylor, director of the Florida Program for Shark Research at the University of Florida, tells Newsweek’s Osborne that the scale of the experiment was “insufficient to be compelling. “Predators control the populations of the herbivores upon which they prey, which, in turn, control the vegetation upon which they feed,” he continued.
Conservation Status and Management
The bonnethead was formerly classified as a least-concern species by the IUCN. It is heavily targeted by commercial and recreational fisheries and constitutes up to 50% of all small shark landings in the eastern US, but is still reasonably abundant there as well as in the Atlantic coasts of the Bahamas and Mexico. NOAA Fisheries manages bonnethead sharks in federal waters of the Atlantic Ocean, including the Gulf of America and Caribbean Sea, under the Consolidated Atlantic Highly Migratory Species (HMS) Management Plan. Individual states have their own rules and regulations for fishing in state waters, which extend to 3 miles from shore in the Atlantic and 9 miles from the shore in the Gulf of America.
Read also: What do reef sharks eat?
Commercial and Recreational Fishing Regulations
These regulations apply to anglers targeting bonnethead sharks in federal waters off the coast of the eastern United States, in the Gulf of America (formerly Gulf of Mexico), and in the Caribbean Sea. Vessels must have a valid Atlantic Highly Migratory Species (HMS) Angling or HMS Charter/Headboat permit to fish for bonnethead sharks. The permit covers everyone fishing on that vessel. Tournament vessels may also fish for bonnethead sharks with an Atlantic Tunas General category or Swordfish General Commercial permit. All HMS permit holders must have a shark endorsement on their permit before they can fish recreationally for bonnethead sharks.
Anglers targeting bonnethead sharks must use handline or rod and reel gear with non-offset, non-stainless steel circle hooks, except when fishing with flies or artificial lures. Permitted vessels can land one bonnethead shark per person on the fishing trip. There is no minimum size requirement for bonnethead sharks. Bonnethead sharks must be landed with their fins naturally attached. If an angler catches but doesn’t keep a shark, they must release it immediately, with minimal injury, and without removing it from the water.
These regulations apply to commercial fishermen targeting bonnethead sharks in federal waters off the coast of the eastern United States, in the Gulf of America* (formerly Gulf of Mexico), and in the Caribbean Sea. To commercially harvest or retain bonnethead sharks, vessel owners must have one of the following permits: Shark Directed permit, Shark Incidental permit, HMS Commercial Caribbean Small Boat permit (only for vessels measuring up to 45 feet long). Charter and headboat vessels that want to sell their bonnethead shark catch must have a valid HMS Charter/Headboat permit with a commercial sale endorsement in addition to the appropriate commercial shark permit.
Fishermen with a directed or incidental shark permit are authorized to use these fishing gears: Bottom longline, Pelagic longline, Rod and reel, Handline, Bandit gear, Gillnet. There is no minimum size requirement or species-specific trip limit for bonnethead sharks. Bonnethead sharks must be landed with their fins naturally attached. If a fisherman catches but doesn’t keep a shark, they must release it immediately, with minimal injury, and without removing it from the water.
Management and Research Efforts
We manage bonnethead sharks as part of the small coastal shark complex. For commercial fishing, we’ve established combined annual quotas and catch limits for bonnethead, Atlantic sharpnose, and Atlantic finetooth sharks. The commercial annual non-blacknose small coastal shark quota in the Atlantic region is 264.1 metric tons, dressed weight. In the Gulf of America region, the quota is 112.6 mt dw. NOAA Fisheries conducts shark research from New England and the Mid-Atlantic to the South Atlantic and Gulf of America (formerly Gulf of Mexico), studying shark life history and performing long-term monitoring activities essential for stock assessment and management.
Read also: Keto Gummies: A Deep Dive
NOAA Fisheries runs the Cooperative Shark Tagging Program to study the life history of Atlantic sharks. We manage several surveys along the East Coast and in the Gulf of America focused on shark nursery habitats in estuaries and nearshore water. The goals of these programs are to identify shark nursery habitat and determine and monitor species composition, habitat use, abundance, and distribution of sharks while they are present in these critical habitats.
tags: #bonnethead #shark #diet #facts