Hi! For many species it doesn’t matter whether males or females are used, but in some species (like humans) one gender has slightly different chromosomes. For naval shipworms, they begin life as male, and then turn into females, with no change of chromosomes, so we can use either!
Both would be interesting, but to study paternal care and the male pregnancy displayed by snake pipefish and all members of its family (all seahorses, pipefish and seadragons), it would make more sense to sequence a male.
It probably doesn’t matter, except for the fact that male voles have a Y chromosome that female voles don’t, whereas there are no bits of DNA in female voles that are not found in male voles. So given the choice we’d probably sequence a male. In reality we’d actually sequence lots of voles, both male and female.