ARENDT, Jeff; Univ. of California, Riverside: The Proximate Determinates of Body Size in Spadefoot Tadpoles: cell size, cell number and functional consequences

Body size is an important correlate in most areas of biology including predator-prey interactions, competition, allometry, and life-history. Despite being one of the most commonly measured traits in biology, we know remarkably little about the proximate mechanisms that determine body size. Here, I examine the cellular components of body size in two species of spadefoot toad tadpoles (additional species will be added as data are collected). The Great Basin spadefoot (Spea intermontanus) breeds in permanent pools, has the longest larval period and largest tadpole of all spadefoot species. Western spadefoot toads (Spea hammondi) have a shorter larval period and metamorphose at a smaller size. I examine cellular characters of the tail muscle in these species because this muscle makes up a relatively large proportion of the total mass, is homogenous in cell type, and the data can later be correlated with function (i.e., swimming speed) in an ecological context. By comparing multiple ages, but matching species for size, I found that S. intermontanus maintain cell recruitment longer than do the faster developing S. hammondi. However, at a given size or age S. intermontanus have fewer but larger cells than S. hammondi. Continued recruitment allows S. intermontanus to reach a larger ultimate size but it may pay a cost in terms of having larger muscle cells early in ontogeny, limiting swimming efficiency. However, this growth pattern should allow S. intermontanus more flexibility in growth giving it greater phenotypic plasticity in response to variation in food and temperature.