An entire family of lice specializes just on carnivorous pinnipeds, (seals, walrus, and sea lions). It’s called Echinophthiriidae, and I had to memorize that name when I was in my first graduate entomology course. And, spell it correctly to get full credit on the exam.
(Why yes, 20 years later, I am still a little bitter about that. On the other hand, at important cocktail parties, I always know how to start a conversation.)
The seal louse has the wonderful species name Echinophthirius horridus; another genus is called Antarctophthirus. Like other sucking lice, they inject a little mouthtube into their host and suck their blood. They only feed on land; in the water they just hang on with their claws.
The lice in this family of insects have several special modifications from regular lice; their cuticle (waxy covering of the exoskeleton) is thicker, it traps seal sebum (body oils), and also forms scales which create a pocket of air under the oil and water for the insect to breathe while the seal is swimming.
How common are they? A 1972 study found about 75% of Northern Fur Seals had lice, and many had more than one species of seal louse. Perhaps not surprisingly, I didn’t find a more recent study in which someone closely examined 75+ seals for lice.
Clearly, there is a publication out there waiting to happen.