Abstract
Large benthic foraminifer (LBF) is a collective term for benthic foraminifers with a relatively large shell size, living in coral reef and associated environments. Current knowledge of taxonomy, phylogeny, physiology and ecology of LBFs is reviewed. Extant LBFs belong to at least 24 genera, six families, and two orders. Fossil records of LBFs demonstrated that all modern genera originated before the Pleistocene (Quaternary).
LBFs have endosymbiotic relationships with different types of microalgae such as dinoflagellates, chrolophytes, pennete diatoms, and rhodophytes. Algal symbiosis is advantageous in nutrient-deficient, clear, well-lit environments. The degree of dependence to algal symbiosis varies with taxa; miliolids are less dependent to algal symbiosis than rotaliids. Most morphological characters possessed in each taxon are considered to be the adaptation to algal symbiosis. Algal symbiosis may be a key driver to the morphological evolution of LBFs.
LBFs grow by adding a new chamber along with protoplasmic growths. The processes of chamber formation and calcification mechanisms differ between rotaliids and miliolids. Life cycles of LBFs are characterized by the alternation of sexual and asexual generations called dimorphism or trimorphism. Gamonts reproduce sexually by releasing free-swimming gametes in the surrounding water. Agamont or schizont reproduces asexually by multiple fission either inside the shell (internal schizogony) or outside the shell (external schizogony).
The biogeographic distribution of LBFs is divided into four regions (West Pacific, Indian to central Pacific, West Indian to the Middle East, Caribbean and Atlantic). Depth distributions of algal symbiont-bearing LBFs are restricted to photic zones (down to 130m in depth) of tropical and subtropical seas. Different genera and species show different preferences along environmental gradients of water depth and reef flats. LBFs are epifauna living on surfaces of macroalgae, seagrass, coral rubble, and sediment.
Population densities and structures of LBFs vary with habitats and seasons; the population density in subtropical environments increases during spring and summer, and decreases during fall and winter. The longevity of LBFs generally ranges from a few months to 1.5 years. Survivorship curves of LBFs may be determined by the size of juveniles and the mode of asexual reproduction. Carbonate production rates of LBFs are generally from 10 to 103g CaCO3 m-2 yr-1.
