Ecological structuring and evolution of deep sea agglutinated foraminifera - A review

Recent agglutinated foraminiferal communities exhibit both simple and complex ecological structuring within habitats. Simple ecological structuring (i.e. a limitation of the entire fauna at or close to the sediment-water interface and absence of erect suspension feeding forms) is mainly observed under low oxygen conditions and high organic flux or after severe substrate disturbance. Oligotrophic deep sea conditions with low vertical particle flux and little substrate disturbance as in the central areas of the modern oceans lead to the development of complex ecological structuring with various highly adapted "specialists" (i.e. mobile infaunal forms, a variety of epifaunal suspension feeders, and epifaunal deposit feeders with special mechanisms to avoid energy loss and use food resources not exploitable by other organisms). The paleontologic record of species diversity and habitat preferences since the late Jurassic exhibits two trends : (1) a general trend from simple to complex ecological structuring within habitats leads to an overall increase in diversity ; (2) fluctuations in this trend coinciding with major paleoceanographic events, i.e. changes in deep water oxygenation and paleoproductivity. From a comparison of Recent and fossil communities from various trophic situations we conclude, that the ecological structuring of agglutinated foraminifera within habitats may have been one of the most important driving forces (selection mechanisms) in the evolution of this special group of protists.