legio1

@sphennings How is the possibility of multiple plausible answers somehow inherently subjective? Maybe five different answers give five different plausible modes of locomotion, how is that a problem? It's rare for there to be only one way of doing something in the real world, why should there only be one way in a created world?

It took me a couple of reads before I noticed the "xenogenesis" bit at the end. You're starting with Anaximander's 'theory of evolution'. In Anaximander's theory, the adults are humans ready to go off and make normal human babies. The fish was just a launch stage, never needed again. Are you looking for the origin of Anaximander's fish? And how much of Anaximander's story are you set on keeping?

ignoring the pheromone issue, this species reproduces by: (1) airborne broadcast spawning; (2) only using partial "sub-gametes"; (3) the sub-gametes accumulate on a suitable host brain until they can combine to form a gamete proper; (4) this happens twice (both sex gametes) in the same host; (5) once the gametes join, the parasite begins eating the host's brain; (6) eventually the host brain is entirely consumed; (7) the rest of the host body does not die but is taken over by the parasite; and (8) these parasites are sapient. That seems an implausible chain to me. Others may disagree.