ARENDT, Jeff; Univ. of California, Riverside: Cell size vs. cell number. Does it matter?

Many environmental factors influence realized body size and, all else being equal, organismal size is determined by a combination of cell size and cell number. In some species, temperature is thought to affect body size primarily through cell size while food is thought to act primarily through cell number. It is, however, not known whether or not the cellular mechanism underlying variation in body size matters for whole organism performance. Here, I test this hypothesis by rearing tadpoles of the Western spadefoot toad at different temperatures and food levels. Performance is measured in terms of burst swimming speed and these correlated with cell size and number in the tail muscle. Tadpoles reared at warmer temperatures are slower swimmers, a pattern seen in other anuran species. Food level, however, does not influence swimming speed independent of its affect on overall body size. Differences in swimming speed are correlated with differences in muscle composition. Warm-reared tadpoles had many small muscle fibres in comparison to cold-reared tadpoles. Patterns of muscle development in these tadpoles and several teleosts suggest this is due to accelerated recruitment of new cells. As a result, warm-reared tadpoles have many, small, immature muscle fibres which may explain their slower swimming speeds. It appears that composition (cell size vs. cell number) matters for performance, at least in some tissues.