Voaster
((My IRL creative energies are tapped right now, so things may be a bit half-assed for a bit.))
Well, the theoretical practice was good. Let's specify a bit.
Digestive systems:
-One that filters nutrients from the water, targeting the smallest life and the bits of "chum"
-One that is specialized to eat the plant matter
-One that is specialized to eat other living things
-One that eats both of the above
-One that is specialized to eat waste
Respiration systems
-One that uses a filter apparatus to get oxygen from the water (the plants should have made enough by now; if not, brute force it)
-One that has to surface to get the oxygen, and has air-sacs in the torso
Skin systems
-One that has a membranous collection of cells, not unlike the Boxhead
-One that has a specialized cell for skin, that protects anything inside (not unlike expected skin)
-One that uses jointed plates to protect the inside
Circulatory systems
-One that uses a centralized pump hooked to the rest of the body (especially the respiratory and digestive systems) that sends nutrients around to every cell via a cell medium
-One that utilizes more direct connections between organs, and lets the organs handle the nutrient distribution
Reproductive systems (Unless otherwise specified, there's two "genders" in all of these
-One where the two genders mate and the male provides DNA to the female, who gestates the young internally. The mother cares for the young until they can do so themselves
-One where the above happens, except the female lays eggs where the young develop and mature on their own
-One where both genders spray their DNA material into the water, and the successful couplings develop in the water
Locomotion systems
-One with water-adapted fins that allow it to glide through the water, propelled and steered by motion of the fins
-One with a long body and no limbs that indeed "thrashes", but it is specialized for doing so
-One with many tentacles that can be used for both locomotion and feeding
-One with two grasping upper limbs and a lower tail
-One with four grasping upper limbs and a lower tail, that is bifurcated into two tails (one above the other, not two side-by-side, so they're parallel instead of on the same plane)
A one-size fits all excretion system that filters the circulatory system for unwanted waste and pushes it out a hole in the back. (Or for the direct-feed-organ guys, a second "out" tube; each organ filters for itself)
Tall order here, so if this is two turns, no problem.
Pick one of each system and analyze it as a cohesive whole(the example below works): are there any major problems or things that are lacked? If not, proceed:
Finally; mix and match! Make every combination. Seed them throughout the world and see which ones prosper. Go ahead and chum liberally (but not too densely; they have to learn to feed on their own). 900 types of sea life should be a good start. No, I'm not making 900 names for now; they can be numbered for now. Example: A plant eater that surfaces for oxygen, with jointed plates, has a blood-based circulatory system, cares for the young, and has a long "thrash" body would be 223112.