Thank you for the links !
The aquatic aliens had a hidden deep sea base on the human colony before
they retreated from this region of space because some astrophysical (?) de-
saster forced them to give up their home world.
I think the aliens could have captured some humans (not all fishing boats re-
turn to port ...), analyzed their genetics, performed their experiments to crea-
te the amphibious "worker" species, and then cloned as many of them as they
considered useful.
Immortality is a very interesting point. The alien creators of my version of the
Lovecraftian Deep Ones have "disappeared from sight" several centuries ago
(and I have not yet decided where they went, or whether they became ex-
tinct). However, if these Deep Ones would be immortal, they would still have
all the knowledge about the technology of their creators - which would make
their discovery by my colonists far more interesting and dangerous.
A good idea, thank you for it !
Apart from the way they came into existence, I intend to stay as close as
possible to the Lovecraftian Deep Ones, changing only what becomes neces-
sary to make them a potentially spacefaring race with a high technology sui-
table for my setting.