which would have created a reproductive barrier, allowing speciation to occur. Although the low number of species in the genus and high levels of homoplasy have frustrated attempts to determine whether the northern Pacific H. olidus or H. nipponensis are the basal species of Hypomesus, the most recent speciation event in Hypomesus is known to have been between the two native east Pacific species, H. pretiosus and H. transpacificus. This is plausibly due to a geographic isolation of a widespread eastern Pacific ancestor, of which some members were isolated in a freshwater basin in western California, possibly in the lakes that would have been located in the southern San Joaquin Valley during the Pleistocene epoch. Habitat The delta smelt is endemic to the Sacramento–San Joaquin River Delta in California, where it is distributed from the Suisun Bay upstream through the delta in Contra Costa, Sacramento, San Joaquin, and Solano Counties. The delta smelt is a pelagic (lives in the open water column away from the bottom) and euryhaline species (tolerant of a wide salinity range). It has been collected from estuarine waters with salinities up to 14 parts per thousand. Historically, delta smelt were distributed from San Pablo Bay upstream to Sacramento on the Sacramento River and Mossdale on the San Joaquin River, which varied seasonally and with freshwater outflow. Today, large areas of historic delta smelt habitat and designated critical habitat have become unsuitable for some life history stages of the species, though key environmental characteristics (e.g. temperature, salinity, water depth) of these areas have not changed. Delta smelt disappeared from the southern portion of their historic habitat in the late 1970s, which coincides with substantial increases in the amounts of water exported from the delta. Water export operations likely have a great effect on the distribution, abundance, and genetic diversity of delta smelt. Lifecycle The delta smelt is semelparous, living one year and dying after its first spawning. Their spawning occurs in spring in river channels and tidally influenced backwater sloughs upstream of the mixing zone where salt water meets fresh water. The Sacramento and San Joaquin Rivers then transport the delta smelt larvae downstream to the mixing zone, normally located in the Suisun Bay. Young delta smelt then feed and grow in the mixing zone before starting their upstream spawning migration in late fall or early winter. The delta smelt is preyed upon by larger fish, especially