Nicothoë astaci or the 'lobster louse' is an ectoparasitic copepod that parasitises the gills of the European lobster species Homarus gammarus.

[2] N. astaci has been found on lobsters inhabiting locations including Scotland,[3] Lundy Island in the Bristol Channel[4] and as far south as France and Portugal.

The parasite occurs in groups, particularly near the base of the gills, and study has gone into its effects on the lobsters,[7] which are considerably important, commercially.

The adult female consists of three parts, the cephalothorax, a thorax bearing two large lateral expansions or wings, and the abdomen which is responsible for carrying two oval egg-sacs.

[8] The adult female has a complete set of segments and limbs, and is considered to be a modification of, rather than a degeneration from its usual copepod form.

It is unknown if this species requires any intermediate hosts; the earliest stage of Nicothoe seen on lobster gills was a last-stage copepodid.

The parasite feeds upon the haemolymph (blood) of the lobster by attaching to the gill filament using its circular ('oral disc') mouthparts and limbs.

[4] Recent cryo-scanning electron microscopy has revealed structural adaptations that facilitate attachment of these parasites to the gill filaments of their lobster host.

The data suggest that lobsters with gills damaged by the feeding activities of N. astaci respond by producing higher levels of haemocyanin, which is both a key defence response and may compensate for their decreased respiratory functioning (i.e. to bolster the oxygen transport capability of haemolymph).