euharpacticoids

Euharpacticoids are a diverse and ecologically significant group of small crustaceans belonging to the subclass Copepoda, within the class Maxillopoda. They are commonly referred to as harpacticoid copepods and are among the most abundant and widespread marine and freshwater microfauna. The name "euharpacticoids" is derived from the Greek *eu-* (true) and *harpacticoid*, referring to their close relationship with the traditional harpacticoid copepods, which were historically classified under the order Harpacticoida.

Euharpacticoids are typically less than 1 millimeter in length, with a segmented body divided into a cephalosome (head and first thoracic segment fused) and five free thoracic segments, followed by a urosome (abdominal region). Their appendages are adapted for swimming, feeding, and sensory perception, with some species exhibiting specialized structures for burrowing or attachment to substrates. They possess a single median eye (ocellus) and antennae used for navigation and detecting prey.

These copepods play crucial roles in aquatic ecosystems as primary consumers, feeding on bacteria, algae, detritus, and smaller protozoans. In turn, they serve as a vital food source for fish larvae, benthic invertebrates, and other higher trophic levels. Euharpacticoids are found in a variety of habitats, including intertidal zones, deep-sea sediments, freshwater lakes, and even hypersaline environments. Some species exhibit remarkable adaptations, such as tolerance to extreme salinity or temperature fluctuations.

Taxonomically, euharpacticoids are classified within the infraorder Harpacticoida, which includes over 3,000 described species. Their classification has evolved with advancements in molecular phylogenetics, revealing greater diversity and relationships within the group. Studies of euharpacticoids contribute to understanding aquatic food webs, biogeochemical cycling, and the impacts of environmental changes on microbial communities. Their small size and ecological importance make them key indicators of water quality and ecosystem health.