DISPERSAL, DORMANCY, AND THE EVOLUTION OF THE TEST AND LIFE CYCLE IN FORAMINIFERA

The foraminifera are diverse rhizarian protists with a fossil record that reportedly extends from the Neoproterozoic. The major foraminiferal clades diverged largely in the Paleozoic to mid-Mesozoic and have long, independent evolutionary histories. Several features (reticulopodia, test, life cycle) are broadly shared across these clades. Reticulopodia, which perform multiple functions, occur in foraminifera and probably several other groups of rhizarian protists. The test, though secondarily lost in some species, is highly varied in composition, size, and morphological complexity and architecture. Foraminifera also share a fundamental pattern of reproduction that includes an alternation of generations. Further, most extant foraminifera are gametogamous, releasing numerous flagellated gametes directly into the water column where syngamy produces small juveniles (propagules) that undergo dispersal. This mode of dispersal is broadly analogous to spawning in marine invertebrates. These broadly shared features (presence of a test and gametogamous reproduction) are common among foraminifera and occur in both basal and highly evolved clades. To what extent are these features interrelated, and did they evolve in tandem? I propose that the foraminiferal life cycle and the test share an evolutionary history united by dispersal and dormancy in propagules or juveniles.

Propagules of modern benthic species can be abundant in fine-grained sediment where they accumulate and form a “propagule bank.” These small juveniles, covered by the proloculus or juvenile test, can remain dormant for months to years and subsequently grow once exposed to favorable environmental conditions. The covering over the propagule then functions as a cyst, and this then may have been the initial adaptive function of the test in the foraminifera. Rather than shedding the cyst, as so many other protists and other organisms do, foraminifera retain it and subsequently grow by enlarging the small initial chamber, elongating it into a tube, or adding a series of chambers or some other structure, thus using the “cyst” as the initial chamber. This view expands our understanding of the function of the test and may explain differences in structure between the proloculus (microspheric generation) and later growth stages.