Basking sharks are the second largest fish in the ocean, reaching lengths of over 30 feet. With their giant mouths agape as they slowly cruise along the surface, they present an imposing sight. If you’re wondering whether these gentle filter feeders can actually close their mouths, read on for the answer.
If you’re short on time, here’s a quick answer to your question: Yes, basking sharks can fully close their mouths, despite their massive size and feeding method.
Anatomy of the Basking Shark
Mouth and Jaw Structure
The mouth and jaws of the basking shark are uniquely adapted for filter feeding. Their jaws are greatly elongated and the gill slits stretch around nearly the entire head. This allows basking sharks to filter enormous volumes of seawater for the tiny plankton they feed on.
Unlike other sharks, basking sharks cannot powerfully bite down due to lacking strong jaw closing muscles. However, their jaws can still fully close when needed, such as when capturing larger prey. When filter feeding passively, their jaws hang loosely open in a massive gape nearly 1 m wide.
Inside the gaping mouth are long, spiraling gill rakers that filter seawater exiting through the unusually long gill slits lining the head. The rakers extract plankton trapped in mucus while allowing excess water to spill out the slits.
According to the IUCN, the specialized jaws and filter feeding system of basking sharks likely evolved to more efficiently exploit patches of zooplankton such as copepods. This allows them to filter over 1.5 million liters per hour while swimming slowly with their mouths agape.
Gill Slits
As mentioned, basking shark gill slits are remarkably long, encircling nearly the whole head behind the jaws. There are 5 main pairs of gill slit openings, with individual slits nearly 1 m in length.
This expanded gill surface area aids the filtration mechanism. As basking sharks swim with open mouths, seawater enters the front of the jaws and flows across the gills before exiting the slits. Plankton become trapped in mucus on the gill rakers while water spills out the extended slits.
According to Jones et al. 2022 research, the unique cranial morphology of basking sharks evolved to enhance ram filter feeding efficiency. The lengthened gill slits and small branchial chamber provide less flow resistance, increasing filtered volumes.
| Avg. Volume Filtered Per Hour | Over 1.5 million liters! |
Feeding Mechanisms and Behavior
Filter Feeding
As the second largest fish species in the ocean, basking sharks have developed a highly efficient filter feeding mechanism to sustain their immense bodies. They swim with their huge mouths wide open, filtering and trapping plankton and small fish to feed (Ram filter feeding).
Studies show that a basking shark’s gills can filter about 2,000 tons of water per hour. Their wide mouth and specialized gill rakers allow them to extract massive amounts of nutritious plankton from the water very effectively.
Ram Filter Feeding
Basking sharks engage in a type of filter feeding behavior called “ram filter feeding”, where they swim forward with their mouths wide open to channel water in, while their specialized gill rakers trap food particles flowing through. This allows them to feed continuously while swimming.
Some key observations about basking sharks’ ram filter feeding behavior:
- Swim at about 1.5-2 mph during feeding with mouths open up to about 3 feet wide
- Make slow, looping turns while feeding to scan wide areas for plankton concentrations
- Can extract about 50% of available plankton from filtered water
Their ram filter feeding gives basking sharks great feeding efficiency to sustain their huge 20+ ton bodies wandering the oceans.
Threats Facing Basking Sharks
Fishing
Basking sharks have long been targeted by commercial fisheries for their large and valuable fins, liver oil, and meat. They were heavily fished from the 1940s to the late 20th century before protective regulations were established.
According to the International Union for Conservation of Nature (IUCN), nearly a million basking sharks may have been killed globally from historical directed fisheries. Though most targeted fisheries have now ended, they still get caught incidentally in nets meant for other fish.
For example, basking sharks accounted for over 25% of elasmobranch bycatch in a Spanish drift gillnet swordfish fishery in the 1990s as per a 2010 study. Gillnets and trawls pose the greatest threat today, with hotspots in the northeast Atlantic and southeast Pacific.
Further protective policies, fishing gear modifications, and release protocols for incidental catches can aid conservation.
Boat Strikes
These large, slow-moving fish are also vulnerable to boat strikes, especially in coastal habitats. A 2018 study found that over 20% of studied basking sharks around the UK showed signs of previous boat collision injuries. Speed limits in key areas can help prevent such strikes.
For example, the U.S. National Oceanic and Atmospheric Administration (NOAA) implemented a 500-yard protective zone with a “slow operation within 500 yards” rule around basking sharks in 2020. Similar defined buffer zones in other regions may further conservation efforts by allowing these gentle giants to coexist with maritime traffic safely.
Climate Change
As basking sharks rely on plankton blooms triggered by temperature changes and ocean currents, climate change impacts pose a major long-term threat. Studies suggest range shifts and phenology changes have already occurred, with later plankton blooms and delayed shark arrivals at some sites.
A recent study projected severe declines in future basking shark abundance in currently important habitats around Britain and Ireland due to warming seas. Conservation planning must account for such expected distributional shifts from climate change effects going forward.
Conclusion
While basking sharks typically swim with their mouths wide open to filter feed, they are physiologically capable of fully closing their jaws. Their unique anatomy allows them to open extra wide to take in more plankton-filled water.
When not actively feeding or under threat, they often close their mouths partially or fully. Understanding the biology of these endangered giants is key to protecting them.
