Feeding and growth rates of the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

Abstract

The goal of this research is to enhance our knowledge of the contributions of doliolids to the planktonic community as consumers and secondary producers. The objectives are to quantify feeding and growth rates of Dolioletta gegenbauri gonozooids at four food concentrations and four temperatures in order to determine their impact as grazers throughout the water column. Although doliolids are abundant in numerous regions of the coastal ocean, and are considered to be major planktonic grazers, data on rates of feeding and growth are scarce. Laboratory experiments were conducted at 16.5, 20, 23.5 and 26.5°C to quantify removal of a 50:50 volumetric concentration of Thalassiosira weissflogii and Rhodomonas sp. at four different food concentrations of 20, 60, 160 and 390 μg C l-1. Results from these experiments suggest that clearance rates are similar at concentrations from 20 to 60 μg C l-1, and decrease as the food concentrations increase to 160 and 390 μg C l-1. The ingestion rates increase over a range of phytoplankton concentrations from 20 to 160 μg C l-1, then decrease when abnormally high concentrations of 390 μg C l-1 are offered. Clearance and ingestion rates increase as temperature increases from 16.5 to 26.5°C. The exponential growth rates range from k = 0.2-0.7, with the lowest rates occurring at the highest food concentration. Growth rates increase with increasing temperature from K = 0.1-0.3 at 16.5°C to 0.45-0.7 at 26.5°C. In each case, the small- and medium-sized zooids had higher growth rates than the larger gonozooids. These results suggest that doliolid feeding and growth rates are a function of environmental food concentrations and temperatures, and imply that they can be important consumers in a changing neritic environment.