Hagfish (eptatretus cirrhauts) are a jawless marine vertebrate that are isotonic with their environment and are considered to be osmoconformers. how might this interesting adaptation limit the habitat that the hagfish can tolerate

I think the Hagfish habitat is limited by the salinity of the environment. Hagfish are the most pleisiomorphic extant craniates, and based on the similarity of ionic concentrations between their internal milieu and sea water, they have long referred as a model for osmo- and ionoconformation.