Unlike gastropods, to survive, they need a fresh food and oxygen rich water to cover their gills.
(they; need; a fresh food and oxygen rich water; to cover their gills) Context(they need,List([31, 40))):(they; need a fresh food and oxygen rich water to cover; their gills)
they need,List([31, 40))
they they
need need a fresh food and oxygen rich water to cover
a fresh food and oxygen rich water to cover their gills
their gills