All species in this family are carnivorous, though while some species are strictly piscivorous, some will feed on other smaller mammals, including other sea monkeys. Phocinus has the most varied diet in this family. It is also the largest and heaviest species in this family, with a total length of about 25 feet long, including the tail. These animals use their large, powerful jaws to crush the head and neck bones of their prey, then they tear chunks out of the flesh using shaking motions underwater. Prey consists of fish, as well as oceanic birds and bats, other sea monkeys, seals, and even occasional deer who wander near the ocean.
The smallest species in the family are in the genus Delphinadapis, with a total length of about 3-4 feet long, including the tail. Like modern dolphins, these animals' preferred method of locomotion is to porpoise through the water, and they are good at it. Porpoising practically doubles their swimming speed, and it shocks fish to congregate into tight groups for easy pickings. These animals are very intelligent, much like modern dolphins. Delphinadapis is also among the fastest swimming pentadactyls, with the ability to reach speeds of up to 45 MPH. Similar in size and lifestyle is Uropinnaps. Though in Uropinnaps, the body is much longer and more slender than in Delphinadapis.
The most unusual member of this family is Leptorca, also known as the spinner sea-monkey. As their name suggests, they spin in the air as they leap out of the water. They also use this spinning motion while swimming. This is somewhat reminiscent of modern sea lions, and like sea lions, this motion helps these animals view their surroundings at all angles. They are the most slender of the sea monkeys, with the longest muzzle. The muzzle is filled with long, sharp teeth, which enables them to grab fish, squid, and even jellyfish. They will also probe in crevices to hunt prey like crabs and shrimp. The flesh at the tip of their muzzle is very sensitive, so much so that they can detect prey in their hiding places just by feeling their vibrations in the water.
Megalobracchium has the most primitive foreflippers, which still resemble the arms of land-dwelling pentadactyls. The flippers are rather short themselves, but powerful. The hands are also still capable of grasping prey, and sometimes, this sea monkey will use their flexible hands to grasp large rocks that they can use to crush open shellfish, such as crabs and lobster, and clams.
Predators of these sea monkeys are basically anything that can capture them; both on land and in the ocean. Giant sea genets are perhaps one of their deadliest enemies. As are larger sea monkeys. Sharks will also prey on these animals. Sometimes even sea-going crocodiles. On land, more often the young are taken by foxes, civets, and even deinognathids. Though rarely are the adults taken by land-based predators. Sometimes there are exceptions even to that rule. A sickened adult may be taken by the larger deinognathids. Swimming is often the best defense for these sea monkeys. Though they can use their sharp teeth and powerful jaws as defensive weapons as well.
To view this family, go to this link.