Chapter 15 - Mollusks

Abstract

Mollusks are abundant and diverse in marine environments, including some groups such as squid and octopi that are strictly marine. The gastropods comprise almost three-fourths of the roughly 110,000 described species, occurring in the seas, freshwater, and on land. The gastropods and bivalves of river ecosystems are diverse, ecologically important, and perhaps the most seriously imperiled of all animal groups in North America. Their life cycles are interesting and complex; some are colorful, and some, especially bivalves, are economically important for harvest and as nuisances. All freshwater mollusks are limited to waters containing sufficient calcium. Very soft water, defined as having a calcium concentration below 3 mg/L, excludes roughly 95% of all species, and even moderately hard water (< 25 mg/L) excludes roughly half of the freshwater mollusk and fauna. The presence or absence of suitable substrate also can be a factor governing species distribution. Last but not least, predators, including those that crush the snail and those that invade it, likely play a contributing role in snail abundance and distribution.