Form and function in juvenile ascidians. II. Ontogenetic scaling of volumetric flow rates

Very little is known of the challenges to suspension feeding performance facing early juvenile marine invertebrates, although scaling considerations suggest juveniles are often at a disadvantage. For example, early juvenile ascidians have relatively, as well as absolutely, narrower siphons than later stages, generating high resistance to flow (Sherrard & LaBarbera 2005: Mar Ecol Prog Ser 287:127–138, this issue). To test whether feeding flow rates are correspondingly decremented in early juveniles, we measured volumetric flow rates during the ontogeny of 4 species of ascidians, 2 solitary and 2 colonial. Early juveniles of all species had relatively lower flow rates than adults. They exhibited rapid, positively allometric increases in volumetric flow rates with respect to body size, followed in the solitary species by slightly positively allometric increases thereafter. Positively allometric scaling occurred within different size ranges depending on species and appears to be driven by reduced resistance as siphons expand rather than a shift in hydrodynamic regime. By 2 to 3 wk after settlement, juveniles of solitary species generated size-specific volumetric flow rates nearly as high as those found in adults. Juveniles of the colonial species Distaplia occidentalis had superior feeding performance at small size compared with solitary juveniles, but juveniles of another colonial species Botrylloides violaceus did not. Because ascidian larvae are non-feeding, early juveniles probably benefit from feeding as soon as possible, even at lower efficiency.