Sea-born twins

Secluded (at the beginning of your upkeep you
may exile this creature under an island or return
this creature from exile if it is secluded. If this
creature is the only creature exiled under an
island it is "secluded").
When this creature enters the battlefield create a
2/2 blue merfolk creature token with "If a
creature named Sea-born twins is secluded this
creature gets +1/+1.