Three species of rhizocephalan barnacle were found to be infesting 293 specimens of Munida sarsi Huus from the Porcupine Sea-bight (49–52°N, 11–14°W): Tortugaster boschmai, Lernaeodiscus ingolfi and, by far the most common, Triangulus munidae. Each species was found to have its own preferred abdominal segment site of emergence and particular effect on both male and female host pleopod structure. Triangulus munidae, however, caused a series of pleopod structure forms which led to the definition of a range of pleopod ‘Types’ for male and female hosts, including a peculiar masculinization of the female hosts' pleopods. The hormonal mechanisms behind these modifications are discussed.
Depth was found to have an influence upon the host-parasite relationship, there being both an increase in the percentage infestation of M. sarsi and a decrease in the effects of T. munidae on the host's pleopod structure with an increase in depth. At extremes of their bathymetric ranges, the host becomes more susceptible to infestation, while the effect of the parasite on its host breaks down.