Just how lamprey sex is determined is unclear. From Erin Ross at Nature:
A team led by biologist Nick Johnson, at the US Geological Survey in Millersburg, Michigan, identified lamprey habitats in and near streams leading to the Great Lakes. Some areas were productive, with lots of food, whereas others were unproductive sites with little food. After taking measures to ensure no wild lamprey were present, they released between 1,500 and 3,000 wire-tagged larval lamprey into each of the study sites.
The researchers recaptured the tagged lamprey and checked their sex after the larvae had metamorphosed into adults and migrated upstream. They found that lamprey in productive streams with lots of food were larger, reached maturity earlier and were more likely to be female. But in unproductive sites, smaller, male lamprey dominated, Johnson’s team reports in a paper published on 29 March in Proceedings of the Royal Society B More.
Sex is a bit of a puzzle because it is a goal on which life forms seem to converge by many different ways. This one is different but logical.
See also: Evolution appears to converge on goals—but in Darwinian terms, is that possible?
and
Can lampreys offer insight into the evolution of gut neurons?
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