A Mutually Beneficial Arrangement... and a Cheater

In the seas of Thermocene Serina, millions of species of fishes now thrive in warm, productive shallow seas. As on Earth, these fishes interact in myriad ways. Predators hunt prey, and prey try to avoid their predators. But not all relationships are antagonistic... some are symbiotic, benefiting both parties, with harm to neither.

Such is the case with a very unusual species of derived guppy known as the polkadot flipjaw. This fish, about as long as a man's arm, is a commensal fish which has a neutral to symbiotic relationship with much larger fishes, particularly giant swordwhales such as the gulpy. The mouth of the flipjaw has moved to the very top of its head and developed a suction-producing apparatus which allows the flipjaw to cling beneath much larger fishes. With a free ride through the open sea, the flipjaw then can wait and detach when the host feeds, taking some of the scraps. But the flipjaw is not only a moocher, it also performs a beneficial service in scraping its host's skin free of algae and invertebrate parasites that would otherwise reduce its fitness, and so the flipjaw is not only tolerated but encouraged to follow along even by normally very dangerous predator fishes such as the gigadon, which often allows several of these small fish to ride along its underside and partake in its leftovers in exchange for being kept free of parasites. But not every fish that clings to the belly of these larger fish is so beneficial to their hosts. Hiding in plain sight, albeit at lower population density than the flipjaw, is a sinister imposter.

A very close look at a shoal of flipjaws hiding under the bellies of any large swordwhale will often reveal one very different sort of fish that, though superficially marked with a similar polkadot pattern, is otherwise pulling an incredible ruse. Imitating the flipjaw remarkably well, it is very hard to tell that the deceptive sniplips does so by swimming upside down, with its normally downturned mouth held up against the belly of its host and used not to scrub off bugs, but to cut and devour small slices of flesh! The sniplips is in fact a small species of swordwhale itself, and not closely related to the flipjaw. Its imitation of flipjaw anatomy is in fact entirely convergent and flipped over; as it mimics the flipjaw by turning on its back, its dorsal fin has two spikes which mime the pectorals of the flipjaw, while it holds its own pectorals together to resemble the flipjaw's small dorsal fin. Even the gonopodium of a male flipjaw is imitated with a small pointed spine on the back of the sniplips' dorsal fin.

Sniplips have evolved to so closely resemble flipjaws that even though they swim upside down, they copy their model's behavior so closely that larger fishes cannot distinguish them. While flipjaws clean their hosts and improve their fitness, sniplips attack them, using two scissor-like teeth that overlap when the mouth closes to shear chunks of skin and meat, causing open wounds and reducing their health. This ruse only works if flipjaws outnumber their carnivorous copycats substantially, or the swordwhales would not tolerate either fish around them, and so the sniplips can never become highly abundant. But by remaining relatively rare, and hiding among larger schools of benevolent lookalikes, it has found a secure and successful niche for itself... as a liar, and a thief.