DIATOMS AND DINOFLAGELLATES

NATURAL DIVERSITY 

 

Diatoms are a favorite tool among ecological and evolutionary researchers because their hard silica shells are often well preserved—and they have been found in virtually every type of water body on Earth, both fresh and salt, from oceans to farm ponds, from the Great Lakes to the West African coast. Some diatoms can even be found in moist rocks, soils, and bark. Many diatoms like Biddulphia laevis, Actinocyclus, and Skeletonema punctatum are called centric diatoms, meaning they are symmetrical around a single point. Dinoflagellates like Peridinium furca are best known for two transparent whiplike flagella—one that encircles the body, the other arising from between the two points. Dinoflagellates are also known for their diversity. They come in many forms. Some exist by photosynthesis, some by eating other organisms. Some produce their own light, like fireflies. Some become parasites in fish and other aquatic organisms. Some emit toxins that can paralyze or kill such shellfish as clams, mussels, and oysters and can adversely affect the food chain all the way up to humans.

COCCOLITHOPHORIDS


SEM scans courtesy of Markus Geisen

LITTLE SOLDIERS

Pull a bucket of water from anywhere in any ocean and several million specimens of Emiliania huxleyi (top left) could very well be floating inside. They are the most common of coccolithophorids, single-celled plants with overlapping plates called coccoliths, which are apparently used for protection. Other coccolithophorids, like Calcidiscus quadriperforatus (top center), also have discuslike plates, but coccoliths can take many forms. The Discosphaera tubifera (top right), for example, surrounds itself with what appear to be trumpets, making it seem much bigger than it actually is. The specific forms of each coccolith most likely evolved under pressure from zooplankton, the microscopic animal group a notch up the food chain that developed mandibles, claws, and other accoutrements for getting past the hard calcite coccoliths. Some coccolithophorids may develop symbiotic relationships with diatoms. For example, plates from the unusual Reticulofenestra sessilis (bottom) form a phytoplankton association with Thalassiosira, a diatom. Just what benefit the two confer on each other—if any—is unknown.

More images of plankton can be found in the August 2004 print issue of Discover