Abstract
The key silverside, Menidia conchorum, is studied as an indicator of adaptations to environmental conditions in lagoonal and ponded water habitats of the lower Florida Keys. Evidence is presented to support the current view of ichthyologists that M. conchorum is a valid species. It occurs in lagoons from Grassy Key to Key West, Florida. This lagoonal habitat is shared by a distinct community. Such lagoons are characterized by rapid fluctuations and broad ranges of physico-chemical parameters. M. conchorum feeds on planktonic crustaceans and terrestrial insects. Maximum size for females is about 58 mm (SL) and 50.1 mm for males. Maturation appears to take place at about 40 mm. The species appears to live one year or less in nature. Reproduction continues year-round with peaks in fertility in the spring and fall. Its population size is seasonal, with a low point in late summer and fall. Nearly all populations inhabit lagoons altered by man and the species is judged as threatened with extinction.