The reproductive strategy of Acanthochondria cornuta, a parasitic copepod with dwarf, hyperparasitic males, is described in detail. The potential impact of male parasitism on the reproductive output was evaluated by determining the size of each sex and the female's fecundity, egg size and total reproductive effort for each pair/trio throughout the four seasons of the year. Marked seasonal differences were observed in female size and reproductive output, suggesting two distinct generations, but no differences were observed for male size. No statistically significant correlations were observed between male size and any measure of reproductive effort, but females with larger males had lower fecundity. A trade-off between egg number and egg size was recorded. Overall, the reproductive effort in A. cornuta seems to be determined mostly by female size, and larger females do not hold smaller males.

During a parasitological survey of the brown mussel Perna perna from highly productive culture fields in the southern sector of Brazil, monstrilloid copepods were discovered in the mantle of this mytilid bivalve. Numerous specimens of endoparasitic copepods were found within nodules in the mantle of the host; they belong to an undetermined species of Monstrilla. Monstrilloid copepods were known to be endoparasitic in polychaetes and in one species of prosobranch mollusc only; their occurrence as parasites of bivalve molluscs has not been previously documented. This is also the first record of these crustaceans infecting commercial molluscs. The prevalence of Monstrilla sp. infecting the brown mussel was 25.6%. At the histological level, it was observed that the larvae induced a strong haemocytic infiltration resulting from the complete larval encapsulation within the host mantle. The effect of this monstrilloid in the cultured populations of P. perna deserves further study.

As a potential ultimate cause of microhabitat restriction for ectoparasites of fish, it has been postulated that selection has favoured narrow microhabitats to facilitate mating. The evidence for this hypothesis is evaluated with males and females of the parasitic copepod Lernanthropus cynoscicola, by using methods commonly used in community ecology, and considering the influence of host ontogeny on site selection. No evidence supporting the mating hypothesis was found after comparison of niche measurements between both sexes. Furthermore, aggregation among individuals of the same sex was stronger than among males and females, and the co-occurrence of both sexes did not depart from that expected by chance; also, negative correlations between the intensity of infection of both sexes were observed. Restriction of the microhabitats cannot therefore be due to facilitation of mating. Moreover, both sexes displayed a sequential displacement over the gill arches and differential preferences by specific gill sections in response to an increasing host size, probably as a consequence of a host-size related cline of optimal conditions where fitness is improved. Thus, reproductive benefits other than increased chances of mating may have played a role in the evolution of restricted niches in parasites, and selection may have favoured a narrowing of the niche around sites where fitness is maximized.