Il est proposé une relation de cause à affet entre les flambées de ciguatera consécutives à une agression subie par une zone récifale et les sorties d'eaux interstitielles par endo upwelling riches en nutriments. Ceux-ci n'étant plus utilisés par un écosystème algo corallien stressé ou éliminé, le sont par des planctontes épibenthiques comme #Gambierdiscus toxicus$ dont la population augmente brutalement. (Résumé d'auteur

Bagnis, R.

Rougerie, Francis

Horizon / Pleins textes

Bull. Soc. Path. Ex., Key-words: Nutrients, Interstitial water, Gambierdiscus 
toxicus, Ciguatera. 
toxicus, Ciguatera. 
Mots-cZés : Nutriments, Eau interstitielle, Gambierdiscus 
BURSTS OF CIGUATERA 
AND ENDO-UPWELLING PROCESS ON CORAL REEF 
By Francis ROUGERIE (*) & Raymond BAGNIS (**) 
Flambées de ciguatera et endo-upwelling sur récif corralien. 
Résumé : II est proposé une relation de cause à effet entre les flambées de ciguatera consécutives à une agression subie 
par une zone récifale et les sorties d’eaux interstitielles par endo-upwelling riches en nutriments. Ceux-ci n’étant plus 
utilisés par un écosystème algo corallien stressé OU éliminé, le sont par des planctontes épibenthiques comme Gambierdiscus 
toxicus dont la population augmente brutalement. 
Summary: We propose a significant relationship between bursts of ciguatera following disturbance on a reef area and 
seepages of endo-upwelled interstitial waters, rich in nutrients. Such waters cannot continue to be used by a stressed 
or eliminated algal-coral ecosystem and are taken up by epibenthic planktons i.e. Gambierdiscus toxicus, of which popula- 
tion increases sharply. - 
I. INTRODUCTION 
The epidemiology of ciguatera bursts was carefully 
studied in French Polynesia since 1967. In many cir- 
cumstances the occurence of poisonous fish is directly 
linked to patent changes on reef. New high risk zones 
are very often areas disturbed by human activities (2). 
In these zones the populations of toxic Gambierdiscus 
toxicus (ADACHI & FUKUYO, 1979) (1) poor in normal 
environmental conditions may increase suddenly 
among macroalgae covering dead coral beds or newly 
denuded surfaces (4). There, the harmful fish ase first 
herbivores detritus feeders, followed within a few 
months by carnivores. The link between ciguatera and 
natural disturbance, less easily experienced by islan- 
ders, was questioned (3). But the intimate mechanism 
of the phenomenon is still to be elucidate. The pos- 
sible role of endo-upwelling process is hereafter dis- 
cussed. 
II. REEF FUNCTIONING AND 
THE ENDO-UPWELLING PROCESS 
A dynamic model of reef functioning by G geo- 
thermal endo-upwelling >> was proposed to bridge the 
paradox between the enormouslhuge primary pro- 
ductivity of coral reef ecosystems bathed by a tro- 
pical ocean permanently barren of nutrients. The 
(*) ORSTOM Tahiti, BP 529 Papeete, Polynésie française. 
(**) Université française du Pacifique, BP 4635, Papeete, Tahiti. 
f M  7-9 
model is based on a geothermally driven upward cir- 
culation of deep oceanic water that enters the porous 
barrier or atoll reef foundation of underlying vol- 
canic basement (6). 
’ Algo-coral communities are settled where these 
endo-upwelled waters seep out and provide the dis- 
solved nutrients (chiefly P, N and Si), CO, and 
other elements (as iron) to the symbiotic micro-algae 
dinoflagellates (zooxantgellae) living in the cells and 
tissues of the coral polyps which in turn furnish 
organic carbon to polyp cells by, direct translocation. 
Because of the steady supply of nutrients provided 
by endo-upwelling (7), the zooxanthellae grow at 
optimal rates (population doubling time is just a few 
days) and reach densities in the polyps of 1-10 million 
cells/cm3. The population growth rates of the 
zooxanthellae exceed the tissue growth rate of .the 
polyps so that it is necessary to expel the excess 
zooxanthellae at rates of about 1,000 cells/hour/cm2 
of polyps surface (5) .  These expelled zooxanthellae 
constitute an important potential food resource for 
such planktonic consumers as copepods, oysters and 
other filter-feeders. 
III. DISTURBANCE SCARS 
ON THE REEF SURFACE 
AND THE BLOOM OF TOXIC 
EPIBENTHIC DINOFLAGELLATES 
The endo-upwelling model may provide an expla- 
. nation for ciguatera events: any physical disturbance 
8. R.S.T.0.M. Fonds Decumen€aíie 
BULLETIN DE LA SOCIÉTÉ DE PATHOLOGIE EXOTIQUE 465 
I - INli lAL SIATE 
- - -p 11 - LOCAL DESTRUCTION I DISBALANCE 
copipodr I 
4 1  I Zooxantellae - 
1 
I 
b fishes & - predators 
toxic fishes 
y+  . . 
intoxicated predators 
NUTRIENTS * 
[CIGUATER A] 
- -I- 
JI - ALGO CORAL RECOVERING 
(healmg of the reel) 
4 
Fig. I .  - Scenario linking coral destruction and/or removal, dinoflagellate (G. toxicus) blooms and bursts of ciguatera. 
I. Initial step. The normal functioning of the reef ecosystem, including expulsion of symbiotic zooxanthellae which are 
consumed by zooplankton (copepods, larvae) and secondary herbivores (fishes) and predators. 
II. Local destruction of the algo-coral cover. Seepage of nutrient-rich water is facilitated in areas where the normal consu- 
mers (micro- and macro-algae) are elimited. Opportunistic epibenthic dinoflagellates (e.g. G. toxicus) bloom, their cigua- 
tera toxins enter the reef food chain and inflicthnfect fishes and their predators, including humans. 
III. Recovery of the surficial corals. G. toxicus population declines as the algo-coral cover returns to the reef (cicatrization). 
toward- or removal of the algo-coral veneer, corres- 
ponds for the reef to a loss of its live skin, similar 
to a scar on the flesh. Seepage of interstitial water 
is facilitated. In places where there are few or no 
more zooxanthellae or benthic calcareous algae to 
absorb the upcoming nutrients, these will be used 
by other opportunist macroalgae, planktonic and epi- 
benthic plants, including the dinoflagellate Gambier- 
discus toxicus which is of similar size and close meta- 
bolism to the zooxanthellae. Both microalgae use for 
their in vitro growth seawater enriched with silicates, 
vitamins and metallic salts. More color changes in 
the Plasts suggest an absorption capacity for organic 
substances present in the culture medium, and a mixo- 
trophic diet. A bloom of G. toxicus may then rapidly 
follow the local death or loss of corals, leading to 
the abnormally high production of toxins and their 
uptake by epibenthic feeders. In this way an atypical 
food chain is established, based on toxic dinoflagel- 
lates, instead of zooxanthellae, which extends up the 
toxicity to benthic fish and their human consumers. 
When recolonization of corals begins, which is equi- 
valent to cicatrization of a wound, the dinoflagellate 
bloom collapses and the ciguatera occurrence dimi- 
nishes. This scenario (fig. 1) can also account for 
ciguatera events that follow natural reef stresses such 
as cyclonic devastation or coral bleaching. 
CONCLUSIONS 
We propose that bursts of ciguatera are an ecolo- 
gical consequence of a concomitant increase in the 
seepage of interstitial nutrients where normal sessile 
users are stressed or eliminated. In well-balanced algo- 
coral reefs, these nutrients are consumed by symbiotic 
zooxanthellae whereas in disturbed or stressed reefs 
they promote bursts in populations of Gambierdiscus 
toxicus because of the reduced nutrient uptake by 
the coral-zooxanthellae cover. During periods of dis- 
turbance or stress, an atypical food chain is created 
with toxicity transmitted throughout the food web. 
This scenario that implies the endo-upwelling pro- 
cess in the incidence of ciguatera, needs to be checked 
by appropriate studies and simulation both in vitro 
and in situ. 
466 BULLETIN DE L A  SOCIÉTÉ DE PATHOLOGIE EXOTIQUE - 3  
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Bursts of ciguatera and endo-upwelling process on coral reef

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