The Habits of the Bonnethead Shark
The shallow waters beside a mangrove forest near Apollo Beach may look like an unremarkable stretch of the eastern shore of Tampa Bay, but they are a field laboratory for Dr. Bryan Franks, assistant professor of marine biology, and his students. These coastal flats are the home to an amazing ecosystem of small marine life, including the target of Dr. Franks’s research, bonnethead sharks.
Dr. Franks is a specialist in sharks and regularly takes his students on field trips, including occasional visits to a noted marine biology laboratory in Bimini, The Bahamas. The research project in Eastern Tampa Bay is examining the diet and internal parasites of the local bonnetheads, the smallest species of hammerhead shark, which only grow to be 2 to 3 feet long.
“They’re fairly common, but this area has not had a lot of research about bonnetheads. We’re trying to learn about their ecology and biology,” he said.
Over the past year, Dr. Franks has been assisted by four students, including Jenna Karr and Samantha Rucker. Last summer, the team set gill nets and caught a number of sharks, some of which were kept for later dissection while others had their stomach contents extracted. How does one remove the stomach contents of a shark?
Franks explained that sharks regurgitate by turning their stomachs inside out. He and his students anesthetize the sharks and use forceps to duplicate that process. The sharks are then released unharmed, if a bit famished, and the stomach contents are bagged for examination.
Rucker, a senior marine biology major, assisted with both the diet and parasite phases of the research, dissecting the specimens and preparing slides for microscopic examination. She found two species of parasites in the sharks’ digestive tracts, a roundworm and a nematode.
“Parasites in sharks have been poorly studied,” said Dr. Franks. “These species of parasites can be specific to particular species of sharks, suggesting they evolved together. That led us to wonder what they were eating.”
In the samples so far, Rucker has found the bonnetheads mostly feed on blue, horseshoe, and spider crabs. More samples will be needed before the research will produce a scientific paper.
“I learned a lot about the protocols of gathering samples. I got a lot of experience on how to conduct research,” said Rucker, who intends to pursue graduate work in marine biology at Nova Southeastern University and someday work at a shark laboratory. “I also learned about the marine food web. It was a great experience.”
As with any research, answers only lead to more questions. Dr. Franks is now interested in how the sharks’ diet may affect its reproductive cycle.
“The bonnetheads in Tampa Bay give birth about two months later than other bonnetheads,” he said. “We don’t know why that is, but diet may be a big factor.”