JACOBS, M. W.; SHERRARD, K. M.: Shape Matters: The Effect of Nutritional Stress on Juvenile Growth Rates of Four Species of Ascidians
Ascidians are common invasive species, and often competitively dominant in fouling communities. Ascidian juveniles exhibit a range of early growth strategies, from encrusting (flat, low in the boundary layer) to erect (tall, high in the boundary layer). We examine the effect of nutritional stress on four species of ascidian: Distaplia occidentalis (colonial/erect), Botrylloides violacea (colonial/encrusting), Corella inflata (solitary/erect), and Boltenia villosa (solitary/encrusting). Newly settled juveniles were placed in one of four food treatments: starved (.45µm filtered seawater), low food (.45µm fsw+1×105 cells/liter Rhodomonas+4×105 cells/liter Isochrysis), high food (.45µm fsw+2.5×106 cells/liter Rhodomonas+1×107 cells/liter Isochrysis), or dock (35µm fsw from FHL dock). Juveniles were photographed from the top and side 5 times over 8 weeks, and ash free dry weights of survivors were measured. Mortality was lowest in starved treatments for all species. Encrusting juveniles, whether solitary or colonial, showed little difference between treatments through week 5, but by week 8 we observed a significant increase in growth rate in response to high food and dock water treatments. Erect species showed more complex responses to nutritional stress. Starved Corella grew as much as or more than those in the high food treatment. Distaplia in high food and dock water treatments appeared to outgrow those in starve and low food treatments, but the trends were not significant. Erect species may compensate in low food environments by increasing branchial basket size or height, or specializing in capture of smaller particles such as bacteria.
