In the same way as the tropical rainforest, the coral reefs of warm seas are among the richest ecosystems of the world in terms of their biodiversity. In fact the best conserved areas harbour over 700 species of coral, 600 species of mollusc and nearly 4000 species of fish. These fish have been well studied by reef biodiversity specialists over the past few years, yet still little is known about their parasites. Two studies conducted by IRD researchers of Noumea have brought out evidence of this parasite species richness in two grouper species of the New Caledonian coral reef.

The new species described by the IRD taxonomists are microscopic animals less than 0.5 mm long. These tiny parasitic worms all belong to the class of monogeneans (Monogenea). They live on gills of marine fish where they find both refuge and food. Identification of each taxon is facilitated by the morphology of the genital apparatus which is characteristic for each species.

In 2006, researchers from the research unit "Systématique, Adaptation, Évolution" (UMR 148) at Noumea first studied Epinephelus maculatus, a grouper species commonly called "loche grisette" and quite common in the New Caledonia lagoon. Microscopic observation of nearly 800 specimens of monogeneans collected from gills of 10 individuals of this fish allowed biologists to identify 12 different species. By comparison, the Mediterranean grouper (Epinephelus marginatus) are host to just four species belonging to this class of gill parasite. At least 10 of these monogeneans found in the New Caledonian "loche grisette" are strictly specific: they live exclusively on this fish.

A second investigation focused on the malabar grouper (Epinephelus malabaricus, "mère loche"). It confirmed the diversity of monogenean gill parasites of the New Caledonian coral reef. For this second grouper species, 11 species of associated monogeneans were identified among more than 300 collected from the gills of two malabar specimens The existence of a rich fauna of monogeneans in the New Caledonia grouper was therefore confirmed. The ensuing article gives a detailed list of 44 parasite species already recorded in the malabar grouper in the whole of the Pacific Ocean.

The malabar grouper has a high initial growth rate and the adult reaches 50 kg, making it a species highly prized by aquaculture operators in South-East Asia. Young malabar groupers are taken directly from their natural habitat in order to supply the fish farms. They are subsequently fattened up quickly, as are red tuna in the Mediterranean. As an example, the Thai production of this fish thus grew from 15 000 individuals in 1991 to 265 000 in 1995. Parasite control in such breeding conditions, where high density of fish populations favours high rates of parasite infection, is crucially important, as the parasitized specimens usually have a lower than average growth rate. Better knowledge of the parasite species present in wild malabar grouper populations could help improve rearing management.

The IRD Noumea research team also substantiated the hypothesis that reef fish in general have an especially rich parasite biodiversity. The New Caledonian coral reef harbours nearly 2000 species of fish. Extrapolation of these results to the whole of this Pacific island's coral reef yielded an estimated fish parasite biodiversity of about 10 000 species. Extrapolating again, to such an aquatic ecosystem at the global scale can certainly give a figure two or three times as large.

Biologists currently have very little knowledge about the host-parasite relation that links the monogeneans to the fish. Like all parasites, they must be closely tied with their host. The specificity the newly described monogenean species have to particular groupers shows this. The disappearance of one species of fish would threfore likely lead to that of the parasites associated with it.

Hosts and parasites form a system that reaches a certain equilibrium as evolution proceeds. Destruction of this equilibrium could influence the regulation of fish populations, in letting the less strongly parasitized species become more invasive, and modify the structure of coral reef communities. Maintenance of this ecosystem in a good state is already seriously threatened by global warming, pollution and the development of tourism. Therefore it is more necessary than ever to conserve the entirety of this natural habitat.

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