Deep sea sharks get less attention than their surface-loving counterparts. Take for example the bluntnose sixgill shark (Hexanchus griseus). At 16 feet in length, this shark is almost large as a white shark or a tiger shark, and the species has a wide global distribution. H. griseus might be a top predator, but because he tends to be found up to a mile below the surface, you don’t really hear much about him.
He’s also really, really slow.
H. griseus’ sluggish and deliberate nature has earned him his common name – the cow shark. He is known to be a scavenger that feeds opportunistically on carrion.
UH Mānoa undergraduate researcher Danielle Garcia focused her Honors project on metabolic rate of H. griseus.
“A species’ metabolic rate can be used to elucidate life history characteristics, such as locomotion, growth rate, sexual maturity, and ecological role,” said Garcia, a southern California native who graduated in December 2013.
Garcia used a non-lethal sampling technique to collect pieces of the shark’s white muscle tissue – the tissues responsible for burst locomotion in high-speed hunting. The tissue samples were collected from sharks caught on deep-set fishing lines off the coast of O‘ahu. She used these samples to conduct assays of four key metabolic enzymes considered as proxies for metabolic rate.
“We found that these sharks have some of the lowest enzyme activity levels that we’ve ever discovered in sharks. This indicates a low metabolism or metabolic rate” Garcia said.
“These animals are not exceptionally athletic,” she said. “They’re not running down their prey.”
Garcia’s metabolism results suggest that H. griseus may perhaps be most comparable to the Pacific sleeper shark, Somniosus pacificus. Future tests with red muscle tissues – those used for routine swimming – would give a more inclusive whole metabolic picture of the bluntnose sixgill shark, Garcia said.
Garcia conducted her research with mentors Kevin Weng and Jeff Drazen in the Department of Oceanography.
“In our laboratory, we’re using satellite and acoustic telemetry, as well as biochemical and stable isotope techniques, to understand biology of these poorly understood species,” said Weng, who is the manager of the Pelagic Fisheries Research Program at UH Mānoa.
“Danielle’s work fits into a larger effort to understand how these unusual animals are adapted to the deep ocean environment,” Weng said. “The low oxygen levels where sixgill sharks live are a harbinger of things to come with global climate change. If we can understand how the sixgill shark is adapted to low oxygen, we may have a better idea of how other species may react to the coming decrease in oxygen in the world’s oceans.”
Garcia credits her positive experience in the UH Mānoa Honors program with helping her to focus her career aspirations in marine biology.
“Kevin had me build all of the gear that we used for the study,” said Garcia. “I got more and more comfortable in the field over the course of the project.” She plans to use the coming year after graduation to volunteer or intern at a marine biology research center before applying to graduate programs.
In November 2013, Garcia presented her metabolic rate results at her first-ever professional conference, The Western Society of Naturalists’ Annual Meeting in Oxnard, CA. It was an opportunity to help educate people about the little-known H. griseus species that has come to fascinate her.
“They are sharks, and they do deserve respect,” Garcia said. “But they’re so slow moving that they’re kind of adorable.”