Mass mortality and recolonization of Pyura stolonifera (Heller) on the South coast of South Africa

A mass mortality of the ascidian Pyura stolonifera (red-bait) was recorded along the Tsitsikamma coast of South Africa in May 1991, following the infe ction of a large proportion of the population with a white microbial growth . At nine subtidal red-bait beds, reductions in percentage cover of red-bai t were greatest on the crests of reefs (42 +/- 17%:1SD) and lowest on the l andward edges (17 +/- 8%). The test of P. stolonifera appeared to be the ma in site of the microbial infection. Scanning electron and epifluorescent mi croscopy revealed much higher numbers of bacteria on the tests of infected individuals (64.3 +/- 5.5 x 10(6).mm(-2)) compared to healthy ones (5.1 +/- 0.1 x 10(6).mm(-2)). Fat, curved rod bacteria and thick chains, which acco unted for more than 45% of the bacteria on infected red-bait rests, were ab sent from the tests of healthy red-bait. Standard characterization techniqu es, coupled with API tests, showed that the isolates from infected red-bait tests belonged exclusively to the genus Vibrio, whereas isolates from heal thy tests included the genera Vibrio, Pseudomonas, Aeromonas and Flavobacte rium/Cytophaga. The mass mortality therefore appeared to be associated with the proliferation of large curved rod bacteria of the genus Vibrio. Recolo nization by P. stolonifera into areas naturally denuded of red-bait was sig nificantly faster than for experimental plots cleared of all organisms duri ng the mid 1980s. A mean cover of 33 +/- 4% was recorded for P. stolonifera after 38 months following natural mortality, whereas it look 71 months to achieve a comparable recovery of 35 +/- 20% in experimental plots. Possible reasons for this difference (viz. substratum, recruitment and predation) a re explored.