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Selection Mechanisms Associated with Intraspecific Shell Variation in Littorina picta (Prosobranchia: Mesogastropoda)

Jeannette Whipple Struhsaker
Evolution
Vol. 22, No. 3 (Sep., 1968), pp. 459-480
DOI: 10.2307/2406874
Stable URL: http://www.jstor.org/stable/2406874
Page Count: 22
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Selection Mechanisms Associated with Intraspecific Shell Variation in Littorina picta (Prosobranchia: Mesogastropoda)
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Abstract

The intraspecific variation in shell sculpture of Littorina picta Philippi was studied. The morphological variation in external sculpture appears genetically associated with certain physiological variations. Both types of variation are correlated with particular supratidal environments. The smooth-form populations predominate on low angle benches with heavy horizontal wave force, while the sculptured-form populations predominate on high angle benches receiving spray and slight horizontal wave force. Populations with intergrades in shell sculpture occur on intermediate substrata. Larvae from different sculpture populations of L. picta were reared under the same laboratory environmental conditions. The larvae vary in shell morphology, shell size, growth rate and mortality rate. Progeny of larvae from parents with sculptured shells have a higher growth rate and lower mortality rate than larvae from smooth parents. The progeny of sculptured parents have heavily sculptured, larger shells; the progeny of smooth parents, almost no sculpture and small shells; progeny of intermediate parents, intermediate shell sculpture and size. The rearing experiments indicate that the shell variations have a genetic basis independent of any environmental effect. The major environmental factor selecting for the shell variations are probably: wave action, prolonged submersion, desiccation, high temperature and extreme salinity. These factors appear to act most strongly against settling larvae, post-veligers and juvenile stages within the supratidal environment. The shell sculpture variation may be an example of balanced, adaptive polymorphism maintained by differential selection and non-random mating within two major types of supratidal habitats in the Hawaiian islands. The variation between supratidal environments from winter to summer may be one of the most important balancing mechanisms of the polymorphism.

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