Propagule transport as a key method of dispersal in benthic foraminifera (Protista)

The distributional patterns of benthic foraminifera provide key information for paleoclimatic, paleoecologic, and paleoceanographic studies. Yet the dispersal mechanisms that provide a crucial link between local populations and large-scale biogeographic patterns are not well documented. We experimentally demonstrate the dispersal of propagules, which include both sexually and asexually produced young (perhaps only the proloculus), though the sexually produced young appear to have a greater potential for dispersal. The propagules can rest in a cryptic state for months, constitute a substantial bank of individuals in sediments from water depths beyond the natural distribution of conspecific adults, and grow in these sediments when exposed to favorable conditions. Propagule dispersal probably provides an effective mechanism for colonization of widely separated habitats in a fashion broadly similar to other eukaryotic microbial groups and many groups of marine invertebrates. Abundant and diverse in most marine and brackish settings, benthic foraminifera are sensitive indicators of environmental conditions. Their distribution in modern and ancient sediments therefore provides the basis for numerous studies documenting environmental change over a range of temporal and spatial scales. Benthic foraminifera disperse and effectively colonize new or remediated settings (e.g., Buzas and Culver 1994; Alve 1995a; McGann et al. in press), yet their dispersal mechanisms are poorly understood (Alve 1999). An understanding of these mechanisms is required to address some key issues in foraminiferal ecology and biodiversity. What controls large-scale patterns of foraminiferal biogeography? How do benthic foraminifera colonize habitats so effectively following major disturbances such as prolonged anoxia (e.g., Jorissen 1999), emplacement of turbidites (e.g., Kaminski et al. 1996), ash falls (e.g., Hess et al. 2001), or pollution and other human impacts (e.g., Alve 1995b)? Are local communities randomly assembled from the local species pool, or do similar assemblages arise in similar habitats? Knowledge of dispersal mechanisms is fundamental to approach these questions and to interpret modern and fossil colonization and distribution patterns. This study was stimulated by two observations: (1) individuals of the shallow-water foraminifer Ammonia beccarii (Linné) grew in sealed bulk sediments collected in Oslofjord, Norway, from water depths down to 84 m, far deeper than where it normally grows and reproduces, following exposure to ‘‘shallow-water’’ conditions (light, pressure, and temperature); and (2) after several months, individuals of another 1 Corresponding author ([email protected]).