Rhizarians
Overview
The Rhizarians are a supergroup of mostly unicellular eukaryotic organisms that are a part of the broad classification of life known as protists. This group is characterized by the presence of long, thin pseudopods, which are extensions of the cytoplasm used for locomotion and feeding. The Rhizarians include various amoeboid and flagellated protists, many of which produce intricate mineral skeletons.
Classification
Rhizarians are classified under the domain Eukaryota, which includes organisms whose cells have a nucleus enclosed within membranes. The Rhizaria supergroup was proposed by Cavalier-Smith in 2002 as a part of a classification system based on molecular phylogenetics. This supergroup is divided into three main groups: Cercozoa, Foraminifera, and Radiolaria.
Morphology
Rhizarians exhibit a wide range of morphologies. However, they are primarily characterized by their long, thin pseudopods. These pseudopods are used for locomotion and feeding, extending out from the cell body to capture prey. Some Rhizarians, like the Foraminifera and Radiolaria, produce intricate mineral skeletons, which can be either external or internal.
Ecology
Rhizarians are found in a variety of environments, from freshwater to marine ecosystems. They play a crucial role in these ecosystems, particularly in the carbon cycle. Foraminifera, for instance, are known for their role in the marine carbon cycle, as they form calcium carbonate shells that sink to the seafloor upon death, effectively sequestering carbon.
Reproduction
Rhizarians reproduce both sexually and asexually. Asexual reproduction typically involves binary fission, where the parent cell divides into two identical daughter cells. Sexual reproduction, on the other hand, involves the fusion of two cells to form a zygote, which then undergoes mitosis to form new individuals.
Evolution
The Rhizaria supergroup is believed to have evolved from a common ancestor with other eukaryotes. Molecular data suggest that Rhizarians are closely related to the archaeplastids and the excavates. The intricate mineral skeletons found in many Rhizarians provide valuable fossils that help in understanding the evolutionary history of this group.