Diversion of fresh water from the Colorado River is threatening the survival of a bivalve mollusk that inhabits the river's delta. Since the 1930's, upstream river management has reduced the input of fresh water and nutrients to the river's delta and estuary. Previous work has shown that (1) Mulinia coloradoensis was once the most abundant species of mollusk in the intertidal zone and (2) M. coloradoensis grew in lower salinity water than found on the delta today. Live specimens are found burrowing in mid- to low intertidal muds. The rarity of this species today makes direct study of its biology difficult. However, abundant dead shells allow the reconstruction of this species' biology from geochemical and hard-part evidence.

I used geochemical and paleoecological methods to better understand the population biology of Mulinia coloradoensis. I examined the growth increments and oxygen isotope variation in the shells of ten pre-dam specimens collected from Isla Montague ( 31 degrees 41.2' N, 114 degrees 43.5'W), in the mouth of the Colorado River. I sectioned the shells along the axis of maximum growth and polished thick (3mm) sections of each shell for analysis.

Size-frequency distribution counts show the average shell to be 30.4 mm in length, 23.6 mm in height. Maximum length reaches 53.4 mm, and minimum length 18.9 mm. The number of external growth rings average 14.2 for the right valve and 16.6 for the left. Analyses of growth increments permit estimation of the age of individual specimens, variation in growth rates during the year, and the season of reproduction. Analyses of variation in oxygen isotopes permit estimation of growth temperatures, salinity tolerances, and the relationship between reproduction, temperature, and salinity. These estimates may prove useful in providing guidelines for the restoration of populations of this species.

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