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Harrington, R.J., 1985

Growth patterns within the genus Protothaca (Bivalvia: Veneridae) from the Gulf of Alaska to Panama: Paleotemperatures, paleobiogeography and paleolatitudes

Bibliographic Reference

Harrington, R.J., 1985, Growth patterns within the genus Protothaca (Bivalvia: Veneridae) from the Gulf of Alaska to Panama: Paleotemperatures, paleobiogeography and paleolatitudes: University of California, Santa Barbara, Ph.D. dissertation, 235 p., illust.


The dissertation is a comprehensive examination of growth patterns of species belonging to the genus Protothaca at 35 localities from the Recent, Pleistocene, and Pliocene, ranging from the Gulf of Alaska to the eastern equatorial Pacific. Data are based on over 4000 measurements of externally expressed growth annuli, lengths, heights, and diameters of shells, plus measurements of shell volumes, and counts of microgrowth increments observed in shell cross-sections. Based on growth equations such as Ford-Walford plots, generalizations are made concerning the interrelationships between latitudinal trends in somatic growth rate, longevity, maximum sizes attained, and estimates of lifetime reproductive output of individuals. Coupled with present knowledge of reproductive patterns and age-specific survivorship of populations in modern habitats, the data indicates: (1) High initial growth rates are always coupled with rapid growth rate decelerations later in life, and substantially reduced longevities. Low initial growth rates are coupled with a more gradual growth rate decline later in life, and greater longevity. (2) Growth rate deceleration is constant and highly correlated to longevity and temperature. Hence, average longevities of individuals from a given habitat, and the mean temperature at which they lived, may be closely estimated. (3) The range end-points of species are constrained by growth rate histories. At the northern range end-point, growth exhibits only minor deceleration, but does not achieve isometry. Progressing toward the tropical end of the range, the pattern of deceleration intensifies (as a function of temperature) until annual increases are minimized to a value of approximately 40 to 50% of each previous year's growth increase. Here the species can no longer expand its range equatorward. These biogeographic patterns may be explained in terms of trade-off between volumetric increases and individual longevities. A required assumption is that fecundity is a function of individual size once sexual maturity is attained. Since individuals at the northern and southern range end-points produce far fewer, but approximately equal numbers of young during their lifetimes then individuals from the mid-point of the species range, this suggests that geographic ranges of many species may be limited by population level effects involving recruitment levels, and that range end-points are not necessarily determined by tolerance effects. (Abstract shortened with permission of author.)

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