Post-settlement predation on ascidian recruits: predator responses to changing prey density

Abstract

To understand factors contributing to sessile marine species mortality, we assessed the role of two species of small predaceous neogastropods (Mitrella lunata and Anachis lafresnayi) on mortality patterns of different species of ascidian recruits. By using the functional response approach (i.e., predator response to variations in prey density) we obtained comparative information on the survival and potential local persistence patterns of several species of ascidians common to southern New England, USA, coastal waters. In addition, we were able to compare the influence of the predators on ascidian species which were relatively recent invaders (e.g., last ~30 yrs or less) into southern New England from those species which have been resident in the region for more than a century. Ascidian recruits examined included both solitary (Ciona intestinalis, Styela clava, Molgula manhattensis, Ascidiella aspersa) and colonial (Botryllus schlosseri, Botrylloides violaceus and Diplosoma listerianum) forms. Collectively, our studies indicated that the predators displayed fairly generalized predation patterns and neither species readily consumed Botrylloides recruits. When ascidian recruit densities were manipulated over a 1 to 3 order of magnitude range, predator consumption rates differed between prey species. Proportional mortality was prey density independent (Type I functional response) when Anachis was preying on Styela, Ciona, Botrylloides and Diplosoma and when Mitrella was foraging on Diplosoma. Mortality was inversely prey density dependent (Type II functional response) when Anachis was feeding on Molgula and Mitrella was feeding on Styela, Ciona and Botryllus recruits. Low prey density mortality rates, suggestive of a Type III functional response, were observed when Anachis was feeding on Ascidiella and Botryllus and when Mitrella was feeding on Molgula recruits. Collectively, results indicate that, for the most part, the predators are capable of effectively controlling the prey regardless of naturally occurring variations in ascidian recruitment densities.