Artifactual effects of caging on the recruitment and survivorship of a subtidal colonial invertebrate

Abstract

As an aid to interpreting previous experiments and designing future experiments, the author tested the hypothesis that the presence of cages has the same effect as the absence of invertebrate grazers on the recruitment of a subtidal colonial ascidian at Goat Island, New Zealand. Recruitment of the ascidian Pseudodistoma novaezelandiae (Brewin) (Polyclinidae: Euherdmaniinae) under cages was compared to recruitment in uncaged quadrats, while both treatments were manually kept clear of 2 invertebrate grazers, the echinoid Evechinus chloroticus (Val.) and the large gastropod Cookia sulcata (Gmelin). Thus, any effect of the cages could not be due to the absence of grazers and was considered artifactual. Although the effects of caging was initially non-significant, in later samples there were significantly more recruits outside than inside cages. Suggested causes of the caging effect include the activities of blennies which sheltered within cages, and the increase of sediment within cages.