"The case of the three species of protozoan (I forget the names) which apparently select differently sized grains of sand, etc., is almost the most wonderful fact I ever heard of. One cannot believe that they have mental power enough to do so, and how any structure or kind of viscidity can lead to this result passes all understanding."
Charles Darwin, letter to W.B. Carpenter, 1872
Foraminifera are single-celled organisms (called protists). Their distinguishing features are net-like pseudopods called reticulopodia, and (usually) some sort of organic or shell-like outer protective layer, called a test. They are a very ancient group of organisms, at least 550 million years old.
We are interested in the ecology, evolution, and cell biology of forams. Major projects include figuring out how forams make their tests, what other organisms are related to them, how they organize their massive bodies, and exactly what makes an organism a foram.
Darwin and Carpenter are discussing agglutinating foraminifera in the quote above. These foraminifera make their tests by gluing together particles from their environments, and they are very selective about which particles they choose. Some will select only sponge spicules, others only quartz grains, and yet others only sand grains of a particular size. The foraminiferan Astrammina rara (pictured at top left and below) selects two different sizes of sand grain and uses the smaller ones to fill in the spaces between the large ones. Like Darwin, we think this ability is truly amazing, and part of our research is devoted to figuring out how they do it.
Family Ties: Forams and their relatives
Are forams and cercozoans related?
Forams and dinosaurs: forams show evidence of the end-Cretaceous extinction event, too.
Scanning electron micrograph of the giant Antarctic foraminiferan Astrammina rara
Notice the gigantic size of this single cell! The pseudopodia (food-collecting appendages) of this specimen form an elaborate network extending several millimeters from the cell body. The pseudopodial network (technically called a "reticulopodium") provides the organism with a wide foraging range. The reticulopodium also furnishes a tremendous surface area for the absorption of dissolved nutrients. The species shown here reinforces its pseudopods with tough, sticky, elastic cables that allow it to capture small crustaceans and the juveniles of larger invertebrates such as sea urchins and starfish. It is quite an amazing feat for a single cell to exploit such a wide range of nutrients -- from dissolved organic material to multicellular creatures several times its own size. Such dietary flexibility is undoubtedly an important part of the foraminiferal success strategy.
Given the high density of "giant" foraminifera in Explorers Cove, Antarctica, it is estimated that the sea floor there may be carpeted with the pseudopodia of foraminifera. In our ongoing studies, we seek to determine the role these creatures play in transferring energy within benthic food webs. More importantly, we are examining the role played by these little carnivores in regulating the abundance of larger creatures on the sea floor. To illustrate this latter point, imagine for a moment that you had carefully raised a family of juvenile (~1 cm diameter) starfish in Explorers Cove, and had just released them into the wild. As they crawl along the ocean sediments, they must now compete for food with thousands and thousands of hungry foraminifera. Worse still, the hungry foraminifera are fully capable of eating your family! The juveniles may starve for lack of food, or they may be eaten by foraminifera and other predators. Such is life for invertebrates in Explorers Cove, Antarctica, and also, if we are correct, along the vast expanse of the deep sea floor.
We are also interested in defining the mechanism that foraminifera and related protists use to transport materials along the surfaces of their pseudopodia. These protists use surface transport to collect small food items like bacteria and diatoms, to "glide" along solid substrates, or to gather sediment particles to construct shells. Surface transport is an important way to perform work: for example, it enables foraminifera to rip apart food organisms that are encased by hard shells (e.g., diatoms) so that the contents can be easily digested. Certain foraminifera use the elaborate ornamentations on their shells, which look like teeth in the scanning electron microscope, to accomplish this cellular rasping (which we call skyllocytosis). Without a highly developed surface transport mechanism, such rasping would necessarily damage the foraminifer's pseudopodial membrane, and therefore this feeding mechanism would not be possible. We believe surface transport has been a key factor in the evolution of the seemingly endless variety of shells in this highly successful group of protists.
About the image: This is a scanning electron micrograph of an Astrammina rara cell that was given glass beads of different sizes to build its test with. The foram selected beads of only two particular sizes. In the wild, this species selects particular sizes of sand grains. Other forams will select only certain types of sand.