Floating Forams (Taxon of the Week: Globorotaliidae)

The planktic foraminiferan Globorotalia ungulata. Photo by Kenneth Finger.

Foraminifera have previously been covered at Catalogue of Organisms here and here. In those posts, the families covered were benthic forams while the family I'm covering today, Globorotaliidae, contains plaktonic forams.

The benthic vs planktonic division is generally treated as the basic starting point by foram workers. This is not because of any fundamental taxonomic distinction; benthic forams are considerably more diverse than planktonic species and, while past authors have treated planktonic forams as a single order Globigerinida, recent studies are often more consistent with a polyphyletic origin for planktonic lineages (Ujiié et al., 2008). It has even been shown that some individual foram species may alternative between benthic and planktonic stages (Darling et al., 2009). Planktonic forams also have much less history than benthic forams - while the earliest forams deep in the Palaeozoic were benthic, planktonic forms didn't appear until the Triassic (and didn't really become abundant until much later). However, the two groups do have very different ecologies and practical significance. While benthic species may be very localised, planktonic foram species are usually very widespread and abundant. About one-third of the world's ocean floor is covered with "Globigerina ooze", a thick deposit of the shells of dead planktonic forams. This great abundance and distribution, together with a high species turnover rate compared to benthic taxa, has made planktonic forams perhaps the most significant group of organisms of all for marine biostratigraphy.


Live specimen of Globorotalia menardii (Globorotaliidae; left) compared to Globigerinoides sacculifer (Globigerinidae; right). Photos by Colomban de Vargas.

The Globorotaliidae are a family of planktonic forams whose first definite appearance was in the Oligocene (de Vargas et al., 1997) assignations of earlier taxa to the globorotaliids are more contentious). Globorotaliids are distinguished from the other major family of planktonic forams, the Globigerinidae, by their different form (flattened rather than globular) and their smooth shell (globigerinids are spinose and honeycombed). While globigerinids feed on zooplankton as well as phytoplankton, globorotaliids are more specific feeders on phytoplankton. Ochrophyte endosymbionts have also been recorded in globorotaliids though the exact species involved does not appear to have been determined (Gastrich, 1988).

Blow (1979) recognised two subfamilies in the Globorotaliidae, the Globorotaliinae and Pulleniatininae; other authors may recognise them as separate families. In both subfamilies, the initial growth form is trochospiral (chambers arranged like the whorls of a top shell). In Globorotaliinae, it remains so throughout the life span; in Pulleniatininae, the earlier trochospiral stage is followed by a later streptospiral stage (each individual chamber occupies half a whorl and grows over the earlier chambers).


External and X-ray view of Pulleniatina obliquiloculata to show the change in spiral direction during growth. Photos from e-Foram Stock.

REFERENCES

Blow, W. H. 1979. The Cainozoic Globigerinida: A study of the morphology, taxonomy, evolutionary relationships and the stratigraphical distribution of some Globigerinida (mostly Globigerinacea). E. J. Brill: Leiden.

Darling, K. F., E. Thomas, S. A. Kasemann, H. A. Seears, C. W. Smart & C. M. Wade. 2009. Surviving mass extinction by bridging the benthic/planktic divide. Proceedings of the National Academy of Sciences of the USA 106 (31): 12629-12633.

Gastrich, M. D. 1988. Ultrastructure of a new intracellular symbiotic alga found within planktonic foraminifera. Journal of Phycology 23 (4): 623-632.

Ujiié, Y., K. Kimoto & J. Pawlowski. 2008. Molecular evidence for an independent origin of modern triserial planktonic foraminifera from benthic ancestors. Marine Micropaleontology 69 (3-4): 334-340.

Vargas, C. de, L. Zaninetti, H. Hilbrecht & J. Pawlowski. 1997. Phylogeny and rates of molecular evolution of planktonic foraminifera: SSU rDNA Sequences compared to the fossil record. Journal of Molecular Evolution 45 (3): 285-294.