Megalotetraodon

Megalotetraodon art done by Tortoiseman on reddit.

The epipelagic zone out in the open ocean was hit hard and practically left barren by the end of the Holocene due to overfishing,  acidification and a number of other different factors. But that won't stop life from recovering.

The open ocean is now teeming with life and at the top is Megalotetraodon (genus Megalotetraodon), a descendant of the northern puffer.

Biology

Pufferfish aren't really built for being fast apex predators, but the Megalotetradon finds ways around that. They have larger fins and stronger fin muscles to unjulate their fins and tail at rapid speeds to propel themselves. Megalotetraodon can swim 30 mph in short bursts, faster than humans can run. Their normal swimming speed is 2 mph.

Megalotetraodon measures at 17 feet in length, making it the largest fish of the calidocene. The fish have a countershaded blue-gray and white color scheme.

 They have a diet consisting of large fish, marine mammals and sometimes each other. These fish have a powerful set of jaws with a bite force of 4500 psi. The jaws open up very quickly and as they open, the puffer sucks in its prey. Four teeth tear up prey and sharpen themselves as they bite down.

Despite how unstoppable they seem, they have one weakness, their stomachs. A pod of porphins on the defensive can ram themselves into the stomach of the megalotetradon to escape, only a few groups have figured this out though. They also face a threat from other megalotetraodons. To defend its weak spot, it has small spikes only on its underside to hurt anything standing up to it when it puffs up, its not always foolproof though.

Behavior

Megalotetradons are pretty intelligent creatures, being socially intelligent enough to make friends and smart enough to come up with strategies for hunting. They sometimes team up during hunts as well. This partnership can turn sour as one fish will eat another if they get the short end of the deal.

They mate by the female laying a floating lump of eggs with the male fertilizing them shortly after. The larvae live in a planktonic stage for a few weeks, most don't make it past this stage. The juveniles feed on fish until they hit maturity at 2 and get big enough to feed on marine mammals and bigger fish. The young grow up under stormrafts where they stay until maturity where they go off to hunt on their own in the open ocean.