Dinoflagellés et ciguatera

Are mesophotic seamounts reservoirs for potentially toxic dinoflagellates associated with Ciguatera poisoning? A case study from the SW Indian ocean (expédition La Pérouse, 2019)

Fig. 1. Location map of La Pérouse seamount off La Réunion.

This study will provide a significant contribution for better knowledge of habitat preferences and vertical dis- tribution of CFP sources. While CFP is traditionally associated with coral reefs near the ocean surface where sunlight conditions are optimal for growth and toxin production, in this case only a low concentration of light penetrates to the depths encountered at La Pérouse sea- mount, considered in the mesophotic zone. Only blue light reaches the ben- thic community here and literature about the influence of these conditions on toxin production by primary progen- itors involved in seafood poisonings is scarce.

In the western Indian Ocean, a Ciguatera Fish Poisoning (CFP) hotspot is located in the coastal waters of the Mascarene Islands (Mauritius, Rodrigues, Réunion) and the offshore fishing banks of the Mauritius-Seychelles ridge, seamounts near Réunion and Mauritius (Fig. 1).

The size of the La Pérouse seamount, 160 km northwest of Réunion Island on the oceanic floor, is comparable to that of Mont Blanc in the Alps. La Pérouse  is one of the rare shallow (-60m) sea- mounts located in the region. The pla- teau comprises mainly fossil limestone plaques covered with macrophytes, sponges, corals (few), and sand and rubble zones (see back page photo).

In 2005, an expedition organized  by ARVAM at La Pérouse seamount (also known by local fishermen as 90 miles bank) collected carnivorous fish containing I-CTXs for analysis, as well as samples of sediment and macroal- gae which subsequently facilitated the identification of Gambierdiscus spp in the area.

During   October-November   2019, a diving and scientific expedition (La Pérouse 2019) was organized to gen- erate a first description of the habi- tats and associated fauna and flora on the seamount based on a High Quality (HQ) photographic inventory. Although macroalgal biodiversity and sediments meant it was possible to isolate the microalgae assemblage from these samples.

A preliminary inventory from these samples collected at depths between -60 & -120m, revealed the presence of the main genera observed in typical CFP habitats: Gambierdiscus, Prorocen- trum, Coolia but also athecate taxa such as Amphidinium and gymnodinoid dino- flagellates (Fig. 2), and thecate taxa such as Cabra, Sinophysis, Bysmatrum and scrippsielloids. Interestingly, among potentially toxic taxa, Gambierdiscus spp were clearly predominant in the samples (approx. 95% of isolated cells) while Ostreopsis spp and Fukuyoa spp were very rare. In order to give a pre- cise identification and better evaluation of the diversity, preserved cells from ap- prox. 35-40 morphotaxa have been iso- lated individually in order to allow SEM observations and molecular sequenc- ing of LSU rDNA from single-cells in the coming months.

This study will also inform results of “source & sink” mechanisms that allow harmful species to spread at regional scales from these seamounts to coastal ecosystems.

 Fig. 2. Benthic dinoflagellates from La Pérouse seamount. (A) Gambierdiscus sp. (B) Amphidinium
sp. (C) Prorocentrum sp. (D) Prorocentrum sp. (E) Coolia sp. Scale bars, 10 μm.

Acknowledgements

To Sophie Durville and Thierry Mu- lochau for their enthusiasm and to Lau- rent Ballesta, Florian Holon and Thiba- ult Rauby for diving deep and bringing samples to the surface.

 Authors

Jean-Pascal Quod, Arvam-Pareto, Technopole de la Réunion, 97438 Réunion (France).

Nicolas Chomérat & Gwenael Bilien, Ifremer, LER BO, Station de Biologie Marine, Place de la Croix, F-29900 Concarneau (France)

Patrick Durville, SAS Galaxea, Saline les Bains, 97434 Réunion (France)

Email corresponding author: jpascal.quod@arvam.com