[1][2] Branchiurans were once thought to be copepods but are now recognised as a separate subclass in the superclass Oligostraca due to their distinct morphological characteristics.

[6][7][8] They are compressed dorsoventrally and can vary in size from just a few millimetres to over 30 mm (1.2 in) long, with females usually somewhat larger than the males.

The maxillules provide sucking capability, and in the genera Argulus, Chonopeltis, and Dipteropeltis, the adults have a pair of suction cups that are from modified first maxillae.

[6] Their compound eyes are prominent, and the mouthparts and the first pair of antennae are modified to form a hooked, spiny proboscis armed with suckers, as an adaptation to parasitic life.

Branchiurans are obligate ectoparasites that are found primarily on marine and freshwater fish (only the genus Argulus occurs in marine environments),[13] but can also be found on other aquatic organisms such as invertebrates, salamanders, tadpoles and alligators.

Branchiura are able to attach to hosts through two mechanisms, hooked maxillae (as seen in Dolops) or suction disks.

[15] From this study, it is thought that Tobacco leaf dust (containing nicotine) can safely and effectively eliminate adult Branchiurans from fish, although this may be specific to only Argulus bengalensis.

[16] Branchiurans are not permanently attached to their hosts, and leave them for up to three weeks to mate and lay eggs, and reattach behind the fish's operculum, where they feed on mucus and sloughed-off scales, or pierce the skin and feed on the internal fluids.

They are opportunistic in selecting host species of fish, and females are motile in their pursuit of locality of egg-laying.