9,000 years before the arrival of domestic cats, humans attempted to cultivate a healthy biosphere in the oceans. However the majority of species were a failure to establish. The ones that survive struggle with an unbalanced ecology.

There are only two sessile animals. The California Mussel (Mytilus californianus) hangs on up at the intertidal zone, having suffered greatly from the climate swings. Similarly hit hard by movement of their habitable zones, but bouncing back with more vigour is the  feral descendant of the commercial Bath Sponge (Spongia officinalis). It blankets rocky warm shallow seas uncontested. They serve as a place for prawns and young small fish to hide.

Humboldt squid are the largest surviving animals of the oceans and they have not changed much since their introduction. They experience severe boom and bust cycles due to the lack of suitable prey for them to hunt, which causes their populations to crash when their prey populations crash, and then explode when their prey populations recover. Most of them are cannibalistic, and some retain a small size into adulthood and subsist on scavenged scraps. Often they experience short lives due to the hostility of other squid, even when they are biologically capable of living for over five years, one or two years is more typical for their life expectancy in this less than ideal situation.

The pressure was high for common prawns to avoid becoming squid food, as there were few alternatives for squid to eat so the prawns became a frequent target. In colder Northern shallow seas some prawns became more transparent all but the green and brown colours inside their digestive tract. In environments where algae grow in abundance it helps to resemble it to avoid being seen by a predator. Where red or brown species of algae are more prevalent, these colours show through the prawn's bodies from their insides, so it is not always green. Many prawns are still eaten as the squid don't just rely on sight to find them. However it ensures they aren't all picked off so easily and some will definitely survive to carry on the future generations.

Only two original Earth species of fish ever fully established and survived to bear descendants. These were the Peruvian Anchoveta (Engraulis ringens) and the Rainbow and Steelhead Trout (Oncorhynchus mykiss irideus). The trout sourvived in both the North and the Sourth of the planet in the cooler oceans, and the North and South diverged in their evolution. There are entirely freshwater species of trout now in both the North and South, with the South river genus Rivertrap Trouts (Laqueum sp.)  growing larger due to consuming mammals and birds. The Steelhead Trout exists much in it's original dualistic form in the North, having access to many rivers with no large predators to give their offspring a head start, but spending most of their life in the cool seas. There are also entirely marine trout including the South hemisphere Oceanus genus who never go far into the rivers at all. There are more threats to young fish in the South rivers in the form of both cats and Rivertrap Trout, so they smolt as soon as they hatch.

Planktonic Life:

There are many kinds of planktonic lifeforms in the oceans. They range from small crustaceans, with microscopic animals and unicellular eukaryotes in-between, all the way down to cyanobacteria. While there are generalist plankton feeders, most are more specialised to feeding on planton of a particular size, kingdom and population density. The largest planktonivores, inflated descendants of peruvian anchovetta, are carnivorous and go for shoals of krill (functionally plankton-like due to their size and high numbers, but many species are strong-swimming against the current unlike actual plankton) in a team effort because if the return on their investment is too small and lacking in animal protein they will starve. For tinier animals and phytoplankton especially in lower population densities, sessile filter feeders may be more specialised, and some larger plankton such as krill and copepods will feed on the smaller plankton, unicellular and bacterial plankton.

Presently larger animals such as fish, large decapods and squid have kept most phytoplankton from entering the competition of larger sizes, instead maintaining small body size life strategies. That said, the distribution of larger animals is uneven across the planet's oceans meaning there are pockets of ocean where nutrients are high but competition from larger animals is low. Some krill have grown to larger sizes to out-grow their filter-feeder fish predators, so when they do encounter them the adult krill get "filtered" out by their size as too large to consume. Some of these larger krill, such as backwards krill, no longer travel the oceans using their swimmerets but instead use a more graceful and efficient evolution of the "lobstering" motion. 

They effectively travel backwards through the oceans. Their eyes stick out more to the sides to allow a fuller range of vision, including backward. Being larger also helps the krill cross larger distances with less energy, and allows their bodies to store more energy, which allows them to have more control over where the ocean currents take them and allows them to gravitate towards places with higher nutrient density. It's no surprise that this ends up with some krill-eat-krill situations.

Although mussels, sponges and cockles do help filter much of the algal abundance that bloom and choke out the oceans (leading to localised anoxic events) their evolutionary progress is slowed due to outbreaks of parasitic copepods, which do not have as many predators here to control them as they do back on earth. By 100,000 the evolution of new species of filter feeders and planktonic predators started to control these numbers more effectively, but they are at times still a problem (exept for the sponges, which were spreading fast in spite of being occasionally parasitized). Mussels were (as mentioned prior) already suffering from the climate change, and this only further damaged their populations, reducing their genetic diversity even further. They are part of the reason why mussels barely hang on in a few places now.