The Crescent/Frond symbiote is a pretty badass predator. But there’s always a bigger fish. The Deep Biggo (I’m really hoping to come up with a better name before publication) is a filter feeder with a symbiotic trick up its sleeve for getting bigger prey.
The genes of the creatures in this ecosystem are particularly prone to horizontal transfer. In fact, it’s one of the primary forces of evolution in this great ocean: what we would consider interspecies sex, coöperation, or consumption is rarely as distinct as it is on Earth.
The Deep Biggo here consumes fresh cobalt from deeper deep sea vents than the Crescents can descend to. These relatively unpopulated locations allow them to swallow large amounts of the element in relative peace. But that water has much less oxygen than surface water, so it rises into the shallower depths, vacuuming up schools of smaller creatures as it goes, absorbing coboglobin and other nutrients from their bodies.
But when it finds larger prey like the Crescents, it has a second trick: its Interceptors are a separate species that live only in the muscular external ribs of the Deep Biggos. Through an electrical signal, the host organism releases a swarm of Interceptors to tackle larger prey by piercing their flesh with barbed heads and pumping out their bodily fluids. Even if the prey manages to escape, those nutrients become available again to the Deep Biggo. But if the Interceptor has succeeded in weakening a prey individual, it is because it is held in place by its own barbs. Along with the prey animal, the Interceptor is swallowed and absorbed, its coboglobin integrated directly into the bloodstream of the host, and its nutrients fed into the Interceptor eggs protected along the breathing ribs to form a new Interceptor.