Competition amongst organisms for space is a critical limiting factor in coral reefs (see Lang and Chornesky 1990 for review). Scleractinian corals obtain and secure space through using their mesenterial filaments to inflict damage upon surrounding organisms (Lang 1973). New observations reported here suggest the mesenterial filaments of Acropora pulchra at growth margins may act as a mechanism to ‘clean’ substrates prior to colony growth and expansion.
Artificial lesions were induced on multiple colonies of A. pulchra at Heron Island (23.44° S, 151.91° E) to encourage tissue growth onto glass slides. Sedimentation and trapping of coral mucus resulted in a rapid build up of organic detritus on the slides, prior to growth of filamentous algae (Fig. 1a). Peripheral polyp mesenterial filaments at the lesion margins were observed to actively “sweep” and remove detritus within the perimeter surrounding the recovering lesions (Fig. 1b). Our observations of pre-emptive cleaning appeared to be linked with extracoelenteric digestion of detritus by the mesenterial filaments (Fig. 1c), exposing the glass slide to a width of several polyps. The continuous removal of detrital and algal material facilitated tissue expansion by the prevention of competitor settlement and by the preparation of substrates for calcified coral growth (Fig. 1d).
A.pulchra mesenterial filament extrusion appeared to actively and pre-emptively maintain space for growth. We suggest that the high levels of particulate organic carbon and nitrogen present in detrital matter (Wild et al. 2004) represent a rich heterotrophic food source for the polyps bordering regenerating lesions and may in part contribute to the considerable rates of lesion regeneration in this species.