The white-headed hagfish may grow up to long. It is a seven-gilled hagfish; it can be distinguished from related species by its large number of toothcusps: between 44 and 51. The Irish M. ios population is distinguished from the southern variety by its white head and whitish List of [anatomical lines|middorsal or midventral line].
Behaviour
M. ios is a scavenger of dead or disabled fish, which it bores into. Its eggs are large,.