N. Hanekom et al., Mass mortality and recolonization of Pyura stolonifera (Heller) on the South coast of South Africa, S AFR J MAR, 21, 1999, pp. 117-133
Citations number
71
Categorie Soggetti
Aquatic Sciences
Journal title
SOUTH AFRICAN JOURNAL OF MARINE SCIENCE-SUID-AFRIKAANSE TYDSKRIF VIR SEEWETENSKAP
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.