<< Agnathans are very successful in certain ecological
niches even with their so-called primitive characteristics. >>
I agree. My knowledge of all the occupiers of every "piscine" ecological
niche is incomplete but as far as I know there aren't any other fish that are
parasitic in the same manner as the parasitic members of the lamprey order.
I'm sure that the ammocoete phase for all the lamprey species is a unique
adaption that hasn't been duplicated or successfully out-competed by other
"fish" species, which is evidenced bythe lampreys' continued presence today.
In a changing world with periodic cataclysms throughout history it seems
like a lot of continued survival for an organism is all about finding that
niche. Myxiniformes and Petromyzontiformes seem to have done particularly
well in that area.You could say that filter-feeding lampreys occupy one of
the best niches for an aquatic vertebrate; there's always going to be a place
for relatively small filter-feeders.
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