The photograph at left is of a sediment core retrieved by divers from Explorers Cove, Antarctica. Extending from the sediment surface are tree-like foraminifera called Notodendrodes antarctikos. This species, like certain other larger agglutinated foraminifera from Antarctica, may represent a "missing link" in the evolution of shelled protists. Molecular phylogenetic studies indicate that the ancestral foraminifera evolved during the Proterozoic (ca. 850 million years ago), before cells began building skeletonized structures. The modern analog to these ancestral forms are the allogromiids, i.e., foraminifera with an organic wall covering the cell body rather than a hard shell. Notodendrodes has such an allogromiid body that is free to move within the cavities of its larger agglutinated shell (see diagram, left). In essence, it is possible that Notodendrodes represents a primitive allogromiid that has "learned" to build a hard shell made of cemented sand grains. The emergence of agglutinated foraminifera is recorded in the fossil record from Early- to Mid-Cambrian strata. The Explorers Cove foraminifera are an important living resource for understanding the physiological processes used by their ancient ancestors to establish hard skeletons. In learning this, we will better define the cellular basis for the evolution of skeletonization - the key event in the emergence of "higher" organisms, including man.
Notodendrodes antarctikos is also interesting to ecologists because it beautifully illustrates the principle of "trophic plasticity" that makes foraminifera such a successful group of protists. We believe that Notodendrodes evolved from ancestors that had a more conventional shape; the other foram in this genus, Notodendrodes hyalinosphaira (left), is found both in a "tree" morphology and as a simple sphere. Genetic studies have also identified other, non-tree-forming relatives, such as Rhabdammina cornuta (see phylogenetic tree to left.)
The complex shape of the "tree" helps Notodendrodes to exploit food sources ranging from dissolved nutrients in the sediment (taken up by pseudopodia in the root-like structures) to the algae raining down from the underside of the ice cover (captured by pseudopodia suspended from the branch-like tree structures). The interplay between pseudopodia - the organelles that do the work of food collection - and the shell - the nonliving platforms upon which the pseudopodia are deployed - is a neglected aspect of foraminiferal biology. We believe that understanding this interplay is crucial for explaining the evolutionary success of these important marine organisms, and current work in our lab is designed to help fill this gap in knowledge.
Bowser, S.S., Gooday, A.J., Alexander, S.P., Bernhard, J.M. (1995) Larger agglutinated foraminifera of McMurdo Sound, Antarctica: Are Astrammina rara and Notodendrodes antarctikos allogromiids incognito? Mar. Micropal. 26:75-88.
DeLaca, T.E., Lipps, J.H., Hessler, R.R. (1980) The morphology and ecology of a new large agglutinated antarctic foraminifer (Textulariina: Notodendrodidae nov). Zool. J. Lin. Soc. 69:205-224.
DeLaca, T.E. (1982) The use of dissolved amino acids by the foraminifer Notodendrodes antarctikos. Am. Zool. 22:683-690.