Lobose testate amoebae
Testate amoebae build a test or shell either from proteinaceous, calcareous, or siliceous material. Some of them form agglutinated tests by gluing together organic or mineral particles (sand grains, diatom shells) from their surrounding environment. These tests range from 10 µm up to 800 µm in size with a high diversity of morphologies, which allows identification to species level and are usually well preserved in peat and lake sediments.
When the building units are collected from the vicinity, they are called xenosomes. Usually these units are minute sand grains and diatoms. Some amoebae produce building units themselves by secreting them. These endogenous building units are called idiosomes. They are made of silicium or calcium. Some species secrete organic hollow globules to form an areolate shell (Arcella, Microchlamys).
Shelled amoebae can be divided in testate or thecate amoebae. A theca is normally closely adherent to the protoplast whereas a test is typically looser.
To the right a drawing of a shelled amoeba, Difflugia capreolata. It shows the most important parts and terms.
The shell (theca, test) is composed of building material, here mineral grains, glued together by an organic material which forms a cement matrix. The shell protects the amoeba against predators. The amoeba can retract itself completely into the shell.
The aperture is also called pseudostome. Species that live in drier habitats, as mosses, have evolved a slit-like aperture to prevent dehydration.
Within the shell the protoplasm is fixed to the inner wall by tiny strands of plasm, called epipodia.
The shape and structure of the nucleus are important characters for species identification.
The protoplasm is usually filled with food particles like diatoms and green algae. They are digested in food vacuoles.
Some species have zoochlorellae, symbiotic algae inside the cytoplasm.
Three basic groups are recognized, based on pseudopod morphology:
- Arcellinida: with lobose pseudopodia.
- Euglyphida: with filose pseudopodia.
- Granuloreticulosa: with anastomosing networks of reticulopodia. This group has recently been placed among testate amoebae with filose pseudopodia (Meisterfeld 2002b).
The phylogenetic position of Arcellinida and Euglyphida, among Amoebozoa and Cercozoa, respectively, has been established based on ribosomal DNA sequences.