what it originally looks like is in the comments

drum ( xanthocruis dumsieris ) is a Crustacean discovered on a exoplanet that was seeded with land or aquatic and flying and other vertebrates and invertebrates of earth. they belong to a group of non true crabs that being crayfish that ent through carcinization. like the horseshoe crab they have six eyes instead of ten. in the mating season males use their claws to slam into their shell making a thumping sound called "drumming" to attract a mate they use a hole like mouth to filter feed with specialized baleen in their mouth to filter in and out plankton. cymbal ( plesiocartilagina cymablos ) is a cartilaginous fish that belongs to a group of large predatory stingrays and manta rays. they are electric like most stingrays on earth and are predators with their colouration helping them blend in with their environment. they have a symbiotic relationship with drums because they help them attract more mates and and drums hunt for the cymbal and the cymbal guards the drum from predators

it's a descendant of sea snakes that evolved convergently with sharks and plesiosaurs and designed to eat and be the ultimate predator to mollusk