Into my humble opinion, the true rarities should live in environments where the
biomass is low.
The main source of food in deep sea trenches is probably organic debris which has
its origin on land.Trenches are usualy proximal to land or islands and high
quantities of plant material is carried down to the bottom.That should be the
truth especially for trenches in tropic or sub-tropic.
But what about trenches at higher latitudes, like South-Sundvich trench.I don't
believe there is enough organic debris carried down from the islands to support
high biomass..Therefore the biomass
in such environment should be low,as a consequence of lack of food.
As the predators usualy represent just a small part of biomass,  rarities should
belong to predators living in such trenches.
So the predators living in such high latitude trenches should be the true
rarities.
 
Perhaps I'm completely wrong and biomass is high even in such trenches (because
there is enough food available in form of snow of organic particles-dead
foraminifera,fish carcasses...)
Any opinion?
 
Milan