Microcrustaceans

The crustaceans are anatomically a very diverse group of organisms with an ubiquitous character and the ability to adapt to a diversity of habitats. Body shape varies by fusing body segments or developing specialised body segments and appendages which makes it difficult to generalize a body shape. The meiobenthic microcrustaceans are comprised of the Copepoda, 'Cladocera' and Ostracoda, which are abundant in streams and rivers1-3.

• The segmented Copepoda body is elongated cylindrical that is divided into: head, thorax and abdomen. The thorax is composed of seven segments, but the first two are fused forming a cephalothorax, covered with a carapace. Sometimes the fourth and fifth segment are also fused. The abdomen consists of three to five segments depending on the species. The two major groups of Copepoda in the streambed sediments are the Cyclopoida and the Harpacticoida; these two groups can be distinguished from each other by the shape of the cephalothorax. The Cyclopoida are carnivores while the Harpacticoida are detritivores or omnivorous. Both groups have a sexual reproduction.
• The 'Cladocera' body is not clearly segmented and in most species the thoracic and abdominal regions are covered by a carapace (shell), with a bivalve appearance. The shell has various forms: oval, circular or elongated. The benthic species are detritivorous and feed by scrapping particle surfaces coated with biofilm. Reproduction is mostly parthenogenetic.
• The body parts of the Ostracoda, including their head and appendages, are completely enclosed in a laterally compressed bivalve carapace. They are algivorous/detritivorous and feed by ingesting particulate organic matter with its associated microflora. Reproduction is sexual although occasionally parthenogenesis can occur in this group.
• Species identification is not difficult but requires dissection of body parts that are small. There are good keys available for all three meiobenthic groups1,2.