Assembly and persistence of foraminifera in introduced mangroves on Moorea, French Polynesia

Abstract

Foraminifera inhabiting the muddy substrata within the coastal mangrove forests (Rhizophora stylosa Griff.) on Moorea, French Polynesia, are distinct from other near shore foraminiferal assemblages around the island. Yet the mangroves were introduced from New Caledonia only in 1937 at Vaianahe Bay; they have since spread to many other sites around Moorea. They invade and occupy mud substrata below Paspalum marshes, the marshes themselves and mud under overhanging Hibiscus stands. The mangrove foraminifera cluster uniquely among themselves when compared to other lagoonal habitats, including those invaded by the mangroves. Thus, the introduction of mangroves has apparently allowed the development of a new community or assemblage of foraminifera.Mangrove foraminifera include Ammoastuta salsa, Ammotium tepida, Paratrochammina stoeni, Reophax cf. R. bacillaris, Trichohylus aguayoi, Trochammina inflata, and other subsidiary species of Ammonia, Elphidium, Quinqueloculina, Cornuspira, and Rosalina. Arenoparrellla mexicana, a characteristic species in mangroves elsewhere, is absent from Moorea, perhaps because it was never there like the mangroves themselves. The mangrove species cluster midway between those of shallow-water lagoonal mud, salt grass Paspalum marshes, and the muddy habitats below Hibiscus trees, even when they occur at similar elevations above or below sea level.The foraminifera in the unique cluster in the mangroves were not introduced with the initial mangrove trees, as the trees were established from propagules carried to the site by humans. The cluster is also found in other newer stands on the island that were colonized by floating propagules. The foraminiferal mangrove assemblage at Moorea is a more recent amalgamation of taxa recruited from the species pools in the associated salt grass Paspalum, Hibiscus and shallow-water lagoonal mud habitats. This recruitment happened quickly, most likely as soon as the mangroves grew to sufficient size to trap fine sediment and organic material, as these species are constantly transported about the lagoon chiefly by birds, rafts of algae or foam, and turbidity created by winds and storms. Some but not all foraminiferal species from these environments were able to occupy the new mangrove habitat. This indicates that the unique clusters of foraminifera have no biologic unity, but are simply associations of species that tolerate slightly different but similar environments at different elevations and with increased organic content, and with decreased wave action caused by mangrove roots. Previous studies of foraminiferal and other associations also indicate that in many situations marine organisms are not necessarily dependent on other specific species. Even in seemingly structured reef and rocky-shore communities with geologic histories of similar communities over thousands of years, the rapidity with which dispersal of larvae, eggs, or adult organisms takes place, and similar environmental conditions maintain this similarity of community, not principally any biolo...

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