Silverswimmers are a diverse group of marine crustaceans from the Global Ocean, 200 million AD, that fills in the niches left by most ocean-going fish in The Future is Wild. There are countless different species, coming many different shapes, sizes, and habits. Some live out in the open ocean, others on the seabed. Some are predators, some are scavengers, and others filter feed on waters that are alive with plankton.
The Global Ocean is a complex environment supporting intricate food chains and highly-evolved species, quite unlike anything known from the Quaternary.
Consider the marine arthropods of previous times, namely crustaceans. They were all specialized, preferring a particular food source and habitat. However, their larval forms tended to be generalized, subsisting on diverse food sources and under almost any conditions. Arthropods were incredibly prolific throughout the oceans of the world and in some cases, despite generally being so small as to be invisible to the naked human eye, were effective predators. Versatile and hardy creatures such as these were ideally positioned to adapt and diversify in a time of crisis.