Eumalacostraca

Eumalacostraca is the largest and most diverse infraclass of crustaceans, encompassing about 75% of all described crustacean species. This group includes familiar animals such as crabs, lobsters, shrimp, krill, and woodlice. The defining characteristic of Eumalacostraca is a specific body plan that includes a set number of segments: eight thoracic segments, which bear biramous appendages, and six abdominal segments, the last of which is fused with a terminal telson. This fundamental body segmentation is a key evolutionary trait that unites this vast assemblage of arthropods. Within Eumalacostraca, several major orders exist, each with its own distinct adaptations and ecological roles. For instance, the Decapoda order is characterized by having ten legs, including the prominent claws found on many shrimp and lobsters. The Isopoda, on the other hand, are typically flattened dorsoventrally and include terrestrial forms like pill bugs. The Amphipoda are laterally compressed and often found in marine and freshwater environments, known for their leaping locomotion. The distribution of Eumalacostraca is nearly global, found in all oceans, freshwater habitats, and even terrestrial environments. Their ecological importance is significant, as they occupy a wide range of trophic levels, acting as both predators and prey, and playing crucial roles in nutrient cycling.