Hydray

This one could easily be an Atlantean or otherwise GMO biomachine, but I was thinking of it more in terms of a panspermic chimerical organism with an alternate life cycle analogous to



Almost the exact opposite of the above, in fact, because--

In this case the species exhibits sexual dimorphism, with the females being bottom-dwelling substrate sifters and the males active pelagic hunters. [Go ahead and blowtorch me for my sexist assumptions] The major lifestyle transition occurs in mating season, when prolonged ventrally aligned coitus results in the fusion of male and female adults into a single conjoined individual that then becomes a furtive, coral-dwelling ambush predator during the "broody" nesting period (note budding natal tentacles depending as barbels from the lower jaw). Eventually the gravid gestalt form adopts a filter-feeding lifestyle in which the sprouted dependent young "tow" the parent organism as it trawls the upper near-shore strata for plankton to nurture its offsprings' continued growth and development. (Opportunistic predatory and scavenging behavior on the part of the juveniles is also very likely at this point.)

Eventually the young detach from their parent(s) and the mature adult resorbs the remaining umbiluci/tentacles, reverting to a relatively sedentary "Stage 3" coral-dwelling lifestyle in it senescence.

Initial mythology-minded human observers of the adult form were captivated by its unique feeding and locomotion style, dubbing the animal "Poseidon's Chariot"--or, more poetically, "Poseidon's Steed."