Sharks have roamed the world’s oceans for millions of years. Most of it has been swimming up and down our water column, yet a select few have decided to learn to ‘walk.’ Don’t get me wrong – they don’t walk like humans! Instead they move their pectoral and pelvic fins along the seafloor. You might have even seen one: epaulette sharks shot the fame when nature documentaries showed them scampering atop coral reefs in the Great Barrier Reef and outside the warm Australian water during low tide allowing them to prey upon anything they can find while the tide is out. Until 2008, scientists believed there were only five species of walking sharks, which changed when another four species were described earlier this year.
While not a true walking shark, it seems these small little animals aren’t the only one who use their pectoral fins in an unusual way. A new publication led by Kristian J. Parton of the Centre for Ecology and Conservation at the University of Exeter recorded the first video evidence of nurse sharks displaying a walking behavior to feed. In collaboration with NGO Beneath The Waves, the Big Blue Collective and the University of Exeter, the team has highlighted a unique feeding strategy we seldom see elasmobranchs (sharks, skates and rays) exhibit.
Nurse sharks (Ginglymostoma cirratum) are a common shark to see in tropical and subtropical reefs worldwide. Their brown-mottled skin is hard to miss amongst color coral, spending their days lounging on the seafloor and slowly moving around to find prey; it is thought their common name comes from a sucking sound they make when feeding. “To advance our understanding of nurse shark behaviour and ecology, we used opportunistic video observations gathered throughout the islands of Turks and Caicos from September 2020 to April 2021,” the scientists explain. “We made 233 observations from 78 camera deployments and identified five behaviours, four of which were attributed to foraging.”
The first behavior observed was vertical feeding, where the shark positioned its body above the bait cage, maintaining a head-down vertical posture; the camera captured the shark suction feeding on the cannister. The second behavior was pectoral positioning, where the shark significantly bent or arched one or both pectoral fins and placed the tips on the ground where it used the “resistance of the seabed to manoeuvre its body into a more favourable position above/around the bait cannister, where suction feeding was performed.” This behavior was usually a precursor to stationary horizontal feeding, which saw the shark stay in a motional position on the seafloor with the head close to the bait cage where it slowly suction fed. Sometimes the shark would exhibit ‘ventral feeding’ which was when the shark rotated on its back to feed. The fifth behavior that wasn’t related to foraging was when the shark would pass by, uninterested in the camera set up or food.
“We observed that the display of stationary horizontal feeding (SHF) behaviour was almost three times as frequent on bank habitats relative to reefs [and] speculate that this particular behaviour may expend low amounts of energy, given that individuals lay flat on the seabed, almost motionless,” the authors state in their new paper. “While we did not find strong evidence for the influence of depth or habitat type on the remaining feeding behaviours, we cannot discount the role of other environmental factors in driving the display of specific feeding behaviours.” The authors agree that the diversity of feeding behaviors displayed by nurse sharks could contribute to their long-term ecological persistence and widespread distribution across a diversity of tropical and subtropical habitat types.
And while the fin movement and positioning of these nurse sharks isn’t on the same level of mobility as in other elasmobranch families – namely bamboo sharks (family: Hemiscyllidae), sleeper rays (Narkidae) and smooth skates (Anacanthobatidae) – it’s likely this behavior has evolved to support active foraging in dynamic environments. Will it be something we see in other shark species? Only time will tell.