ONTOGENY OF FUSIFORM FORAMINIFERA

Fusiform foraminifera (Fusulinids and Alveolinids) dominated shallow carbonate environments from the mid-Paleozoic to the early Tertiary and still occupy similar habitats today. In the Paleozoic through early Tertiary, they were important rock builders, and are widely used as biostratigraphic tools because of their abundance and rapid evolutionary change. Obviously a successful group, little is known about their biology. Alveolinella, the only living large fusiform foraminifera, occurs throughout the western Indo-Pacific in reef systems. They occupy two distinct habitats—coral rubble and sandy substrata covered by loose microbial mats. In these places they live on the surface where their photoendosymbiotic diatoms are active. They reproduce largely by asexual cell division producing a large number of tiny juvenile individuals. They form from a nucleus formed by mitotic cell divisions and some adult cytoplasm that contains photoendosymbionts. When they grow to a two or three-chambered stage, they are released from the adult test, which is then empty, by the dissolution of the apertural face. The young are spherical in shape initially, but begin to add additional chambers and chamberlets rapidly. In a short time, they have the characteristic fusiform shape marked by chambers parallel to the long axis and chamberlets dividing each chamber into numerous small boxes. As the foraminifera grow, the symbionts become more numerous occupying the chamberlets. Without the symbionts, Alveolinella is not capable of secreting large amounts of CaCO3. The ontogenetic process in Alveolinella, and most likely in extinct forms, is highly coordinated with the possession of symbionts. This association, most likely not tightly evolved, appeared multiple times among fusiform foraminifera, as they responded to take advantage of sunlight more directly as an energy source.