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Mass mortality and recolonization of <i>pyura stolonifera</i> (heller) on the south coast of South Africa
Abstract
A mass mortality of the ascidian Pyura stolonifera (red-bait) was recorded along the Tsitsikamma coast of South Africa in May 1991, following the infection of a large proportion of the population with a white microbial
growth. At nine subtidal red-bait beds, reductions in percentage cover of red-bait were greatest on the crests of reefs (42 ± 17%:1SD) and lowest on the landward edges (17 ± 8%). The test of P. stolonifera appeared to be the main site of the microbial infection. Scanning electron and epifluorescent microscopy revealed much higher numbers of bacteria on the tests of infected individuals (64.3 ± 5.5 × 106·mm-2) compared to healthy ones (5.1 ± 0.1 × 106·mm-2). Fat, curved rod bacteria and thick chains, which accounted for more than 45% of the bacteria on infected red-bait tests, were absent from the tests of healthy red-bait. Standard characterization techniques, coupled with API tests, showed that the isolates from infected red-bait tests belonged exclusively to the
genus Vibrio, whereas isolates from healthy tests included the genera Vibrio, Pseudomonas, Aeromonas and Flavobacterium/Cytophaga. The mass mortality therefore appeared to be associated with the proliferation of large curved rod bacteria of the genus Vibrio. Recolonization by P. stolonifera into areas naturally denuded of red-bait was significantly faster than for experimental plots cleared of all organisms during the mid 1980s. A mean cover of 33 ± 4% was recorded for P. stolonifera after 38 months following natural mortality, whereas it took 71 months to achieve a comparable recovery of 35 ± 20% in experimental plots. Possible reasons for this difference (viz. substratum, recruitment and predation) are explored.
growth. At nine subtidal red-bait beds, reductions in percentage cover of red-bait were greatest on the crests of reefs (42 ± 17%:1SD) and lowest on the landward edges (17 ± 8%). The test of P. stolonifera appeared to be the main site of the microbial infection. Scanning electron and epifluorescent microscopy revealed much higher numbers of bacteria on the tests of infected individuals (64.3 ± 5.5 × 106·mm-2) compared to healthy ones (5.1 ± 0.1 × 106·mm-2). Fat, curved rod bacteria and thick chains, which accounted for more than 45% of the bacteria on infected red-bait tests, were absent from the tests of healthy red-bait. Standard characterization techniques, coupled with API tests, showed that the isolates from infected red-bait tests belonged exclusively to the
genus Vibrio, whereas isolates from healthy tests included the genera Vibrio, Pseudomonas, Aeromonas and Flavobacterium/Cytophaga. The mass mortality therefore appeared to be associated with the proliferation of large curved rod bacteria of the genus Vibrio. Recolonization by P. stolonifera into areas naturally denuded of red-bait was significantly faster than for experimental plots cleared of all organisms during the mid 1980s. A mean cover of 33 ± 4% was recorded for P. stolonifera after 38 months following natural mortality, whereas it took 71 months to achieve a comparable recovery of 35 ± 20% in experimental plots. Possible reasons for this difference (viz. substratum, recruitment and predation) are explored.
