Call me provincial but "fish blood" seems like a pretty restricted ecological resource all by itself. ;-)
Fish flesh too - like most of shark. Or plant leaves - like most of herbivores.
Perhaps by "specialized" they might mean "highly derived"? *shrug* As a layman, I probably don't have the language to articulate what I think I mean. ;-)
Derived from what? We all are a highly derived bacterium. In some sense lamprey is more similar to a vermiform ancestral chordate than a tetrapod.
[]s,
Roberto Takata