Krill are tiny crustaceans which make up an important part of several marine food chains. They feed on phytoplankton and zooplankton and in turn are eaten in colossal numbers by animals higher in the food chain. This allows these larger animals to access nutrients from microscopic plankton, maximising their intake by feeding on lots of smaller animals, rather than fewer larger ones.
Filter feeding is generally thought to be the most efficient way to capture and eat large amounts of small prey. However this requires an enormous amount of specialisation. The vast majority of filter feeding animals, such as the toothless baleen whales have evolved so that they cannot feed in any other way.
Leopard seals are exceptionally unusual in that not only do they feed on larger prey such as penguins and other seals but also large amounts of krill. This behaviour has only recently been directly observed by scientists working in Taronga zoo. Their theories exact mechanisms of the behaviour are backed up with examinations of the dentistry of the skulls found in Museum Victoria.
The seals possess “lateral lips” which allows them to suck seawater containing small prey into their mouths. They then expel this seawater. However, as the seawater leaves their mouths it is filtered through a row of teeth behind their canines. These teeth lock together to create a sieve which traps any prey which the seal managed to trap in its mouth.
These teeth lock together so closely that upon examination of museum specimens of leopard skulls, it was discovered that the teeth on one jaw wear down the surfaces of teeth on the opposing jaw. This leads to a polished look at the points of contact, which is completely different to the damage observed on the canine teeth. These abrasions are a result of biting into larger prey or occasionally ice!
This indicates a completely different use for each type of tooth. The canines are used for biting larger prey items, while the post-canine teeth are used to form the sieve to filter krill. The more delicate post canine teeth likely force the seals into biting and shaking their prey in order to rip off chunks. This method of attack is slower and takes more energy than other strategies, but allows them to protect their sieving teeth.
Looking after their teeth like this allows the leopard seal to prey upon large numbers of arctic krill, a huge resource which a predator more dedicated to killing large prey wouldn’t be able to exploit. Their adaptations are highly specialised towards this two strategy approach, as opposed to simply being generalist feeders. This flexibility helps leopard seals enormously when larger prey is scarce allowing them to function as a predator and a planktivore. Making them pretty unique in the animal world!
Reference: Leopard seals (Hydrurga leptonyx) use suction and filter feeding when hunting small prey underwater www.springerlink.com/content/942w33570v54l511/
Image Credits: Wikimedia commons