Much has been written on ascidian larvae and metamorphosis, but little is known of their early juvenile lives.  Biotic and physical conditions for a post-metamorphic tunicate, which may be less than 1mm tall, are likely to be quite distinct from those experienced by the adult.  Fluid viscosities, boundary-layer effects, unique predators, and mechanical constraints on feeding are just some of the challenges facing the tiny juvenile.  This critical post-metamorphic period is investigated for 6 west-coast species at Friday Harbor Laboratories, Washington from the standpoint of ontogenetic scaling of morphological features relating to suspension feeding.  Morphological measurements include overall body shapes, sizes, and areas; siphon heights, diameters, and lengths; and stigmatal characteristics. Because of their small size, early juveniles live deep within velocity boundary layers, and thus may experience poorer quality food and less efficient waste removal.  The researchers predict that feeding capability could therefore be improved by being relatively taller, and having relatively larger siphons and greater area of stigmata in the filtering basket.  Results show ontogenetic differences in body shape for all species, but in differing ways.  An example is shown in the accompanying photographs for Ciona savignyi.  Note that the juvenile is globose, stalked, and has a relatively short, wide incurrent siphon. In the course of development to the adult the body becomes elongated, the stalk is lost, and the siphons become relatively elongated.  Significant positive allometric scaling of incurrent siphon diameter versus linear size is seen in early juveniles/oozooids in most species (slopes of 1.1-2.3, where isometry is predicted to be 1).  Similarly, in Ciona and Styela, and in both colonial species, incurrent siphons increase in length with positive allometry, as predicted.  Height above the substratum also increases with positive allometry in most species.  All of these changes are predicted to lead to relatively improved feeding performance in the juveniles.  The study is a nice one, with clearly defined hypotheses, and interesting and provocative scaling data.  Sherrard & LaBarbera 2005 Mar Ecol Progr Ser 287: 127. Photograph of S. gibbsii courtesy Dave Cowles, Walla Walla University, Washington.

NOTE  the species include 4 solitary species Corella inflata, Ciona savignyi, Boltenia villosa, and Styela gibbsi, and 2 colonial species Botrylloides violaceus and Distaplia occidentalis

NOTE during embryogenesis in a colonial tunicate the fertilised egg hatches to a tadpole larva that settles and metamorphoses into the first unit of the colony, the oozooid.  Subsequent growth is by asexual division to produce many generations of blastozooids

In a companion study done at Friday Harbor Laboratories, Washington, the same authors investigate scaling of pumping rates in 4 species of west-coast tunicates.  Given that the siphon dimensions in earlier developmental stages are relatively smaller than in later stages, the authors predict that there will be an ontogenetic shift from relatively lower volumetric flow rates in early juveniles to relatively higher rates in adults.  Results show, indeed, that the juveniles have relatively lower flow rates than adults.  The allometric increases (see graph), expressed here as pumping power, occur rapidly in the 2 solitary species.  Note in the graph the less efficient pumping power of the juveniles, but an isometric relationship with body size is quickly reached.  Within 2-3wk after settlement size-specific pumping rates in the juveniles are comparable to those in the adults.  The relatively lower volumetric flow rates in the juveniles seems mostly to originate with the higher siphonal resistance of their relatively narrower siphons and not with changes in sigmatal shape (see photograph).  Interestingly, juveniles of one of the colonial species, D. occidentalis, show superior pumping performance in comparison with juveniles of both solitary species.  The authors note that theirs is the first study of feeding-flow rates in early juvenile marine invertebrates. Sherrard & LaBarbera 2005 Mar Ecol Progr Ser 287: 139.

NOTE  measured as the product of average incurrent velocities of particles times siphonal cross-sectional area

NOTE  these include 2 solitary species, Corella inflata and Ciona savignyi, and 2 colonial species, Distaplia occidentalis and Botrylloides violaceus, all included in the earlier study (Research Study 1 above)