sea louse

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several parasitic copepods, especially Caligus elongatus and Lepeophtheirus salmonis, that infect many species of wild and farmed fish.

from the GNU version of the Collaborative International Dictionary of English.

• (Zoöl.) Any one of numerous species of isopod crustaceans of Cymothoa, Livoneca, and allied genera, mostly parasites on fishes.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A copepod within the order Siphonostomatoida; a kind of marine ectoparasite that feeds on the mucus, epidermal tissue and blood of fishes.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun marine isopod crustacean