The seas in which the mermen lived were, in the early years, the favoured habitat of gigantic reptiles and amphibians.
And the mermen were favorite prey because they were neither fast enough, powerful enough, nor venomous enough, but they had enough meat on their bodies to be ignored.
They were constantly attacked.
This led to the mermen always traveling in pairs. More eyes and more strength. But what was most striking was that the mermen had more brains than their predators and devised all sorts of evasive tactics.
Which led to fewer and fewer incidents where the mermen lost their lives.
And they now had enough time to assess their situation. So they began to chase animals out of their shelters and arrange them for their own survival.
But caves that you take over by chasing out the original inhabitants can also be taken over again by other inhabitants.