Macroinvertebrate communities associated with intertidal and subtidal beds of Pyura stolonifera (Heller) (Tunicata: Ascidiacea) on the Natal coast

Abstract

The solitary ascidian Pyura stolonifera occurs in dense beds on the low intertidal and shallow subtidal rocky shore along the entire South African coastline. The organism is used as bait by fishermen and is also heavily exploited for food in certain areas. The crevices and interstices between individuals in dense beds of P. stoloniferaprovide a safe and stable habitat for a wide variety of benthic macroinvertebrates. Sixty-four and 61 taxa representing 10 phyla, of associated organisms were recorded respectively in an intertidal and a subtidal P. stolonifera bed. Forty-two taxa were common to both, but communities of macroinvertebrates associated with intertidal and subtidal P. stolonifera beds were different. Numerically, polychaetes (30%) were the dominant group intertidally, and crustaceans (40%) subtidally. Porifera formed 71% of the biomass of associated intertidal organisms, while subtidal biomass was dominated by the bivalves Striostrea margaritacea and Perna perna (87%). Mean dry biomass of macroinvertebrates was 366 g.m ⁻² ( SD = 196) intertidally and 670 g.m ⁻² (SD = 119) subtidally. These values are between four and eight times higher than those recorded on the southern Cape coast. Recolonization of cleared areas is slow, so considerable secondary production is lost when harvesting practices result in bare patches in P. stolonifera beds.