It is the region inhabited by plankton, which are minute organisms that drift or float at various depths in the water, and by nekton , which are free-swimming organisms.
This may have made it the most easily accessible prey for predators in the nekton , such as fishes.
Plants and animals arrive as plankton, nekton (free-swimming), fouling organisms (attached inside and on the hulls, propellers, and intake systems of vessels), and benthos (bottom dwellers).
On average, collections were made every 3-4 d for phytoplankton and zoo-plankton, 7 d for benthos, and 10-14 d for nekton (fish and swimming benthic invertebrates).
The subsequent evolution of the nekton is largely determined by a parallel development of the diversity of its components, which together reflect the pattern of sea level change.
In fact, cephalopods themselves were the only Ordovician nektonic durophagous predators.
Sponges, corals, bryozoa, gastropods, bivalves, and ammonoid and belemnite cephalopods all flourished, the latter two groups becoming the dominant nektonic invertebrates for the rest of the Mesozoic.
Such is the case with the great majority of marine crustacean species, which almost always pass through a planktonic larval existence before metamorphosing to benthic or nektonic adult forms.
This shows on one hand a strong correspondence of nektonic life with sea level changes and, on the other, the strong interrelationship (competitive and/or predatory) of the nektonic biota.
Both nektonic and benthic forms are known, although both were motile to varying degrees.