Dolphins are nektonic. An animal in the benthic category would be something that lived on the ocean floor such as an octopus, coral or small fish.
Yes
No it is toxic
It seems that not many people know about the Bloodybelly Comb Jelly, and we are still in the dark ages with the amount of knowledge we about them.
They can have 10,000
Ctenophora
200
they eat fish
Adult horseshoe crabs are benthic, in that they live on the bottom in sandy or muddy ocean areas, occasionally coming ashore; and don't swim around in nektonic fashion like most fish.
Common Northern Comb Jelly
The deepest part of the "benthic zone" for aqueous ecology is the "hadal" zone (6000 meters or more beneath the surface). It is also known as the hadopelagic or "trench zone" of deep-sea organisms.
A shrimp is a benthic animal, falling in the benthos category. They feed on plankton.