Pomacentrids (damselfishes) are one of the most common and diverse group of marine fishes foundon coral reefs. However, their digenean fauna and cleaning interactions with the bluestreak cleanerwrasse, Labroides dimidiatus, are poorly studied. This thesis explores the digenean trematode faunain damselfishes from Lizard Island, Great Barrier Reef (GBR), Australia and examines severalaspects of the role of L. dimidiatus in the recruitment of young damselfishes.My first study aimed to expand our current knowledge of the digenean trematode fauna ofdamselfishes by examining this group of fishes from Lizard Island on the northern GBR. In acomprehensive study of the digenean trematodes of damselfishes, 358 individuals from 32 speciesof damselfishes were examined. I found 19 species of digeneans, 54 host/parasite combinations, 18were new host records, and three were new species (Fellodistomidae n. sp., Gyliauchenidae n. sp.and Pseudobacciger cheneyae). Combined molecular and morphological analyses show thatHysterolecitha nahaensis, the single most common trematode, comprises a complex of crypticspecies rather than just one species. This work highlights the importance of using both techniques inconjunction in order to identify digenean species. The host-specificity of digeneans within thisgroup of fishes is relatively low. Most of the species possess either euryxenic (infecting multiplerelated species) or stenoxenic (infecting a diverse range of hosts) specificity, with only a handful ofspecies being convincingly oioxenic (only found in one host species). Overall, the trematoderichness in pomacentrid fishes is relatively low compared with that of many other coral reef fishfamilies. The richness is probably best explained by the small size of most pomacentrids. In termsof beta diversity, of the total 23 trematode species having now been reported for the GBR inpomacentrids, just 14 are shared between Heron and Lizard Island. Five species have been recordedonly from Lizard Island and four only from Heron Island. A total of 14 species reported from thetwo sites can be classified as core pomacentrid-specific parasites.I then examined whether the abundance and diversity of damselfish recruits differed on reefsrelative to reefs from which cleaner wrasse had been removed for 12 years. Past studies have shownthat cleaner wrasse enhance abundance and diversity of adult resident damselfishes (species that aresite-attached), however, it is not known whether this effect occurs at recruitment or is driven bypost-settlement events such as migration or differential mortality. I characterised the abundance ofdamselfish recruits on reefs with and without cleaner wrasse for five days in each of three lunarcycles, beginning four days after the new moon (November, December and January). The totalabundance of damselfish recruits and specifically two species, Chrysiptera rollandi and P.amboinensis, was higher on reefs with cleaner wrasse than on reefs without. However, overall species diversity of damselfish recruits did not differ. This study shows that the effects of thepresence of cleaner wrasse begin at recruitment.Although the presence of cleaner wrasse increases the abundance of recruiting damselfish, itis unclear what mechanism(s) are involved in this process. I conducted aquarium experiments toexamine whether young P. amboinensis preferentially select a microhabitat that is adjacent to acleaner wrasse. The study showed that both settlement-stage and juvenile P. amboinensis spend agreater proportion of time in a microhabitat adjacent to a cleaner wrasse than in one adjacent to acontrol wrasse (a non-cleaner wrasse, Halichoeres melanurus) or where no fish was present. Moretime was spent next to juvenile than adult cleaner wrasse. I then conducted field observations todetermine if recruits that are in a 1 m radius of a juvenile cleaner wrasse receive any direct benefits(cleaning) and whether juvenile cleaner wrasse preferentially cleaned larger individuals. The sizeand identity of all fish that were cleaned within a 1 m radius of a cleaner were recorded, along withnearby non-cleaned conspecifics. I showed that only 2 % of the clients that a juvenile cleanerwrasse clean per 20 min observation were small recruits (< 20 mm, TL) whereas largerdamselfishes comprised 58 % of clients. Juvenile cleaner wrasse also preferred to clean largerindividuals than the median size of the surrounding nearby non-cleaned conspecifics. These resultsshow that cleaner wrasse serve as a positive cue during microhabitat selection but that directbenefits to a small recruit are not obvious. The possibilities of indirect rather than direct (cleaning)benefits are discussed.I then used sentinel traps to investigate the cumulative number of gnathiid isopods infectingthe ambon damselfish, Pomacentrus amboinensis, on patch reefs with and without cleaner wrasse.Previous studies may have under surveyed the number of gnathiids on a host related to the presenceor absence of cleaner fish, due to the sampling techniques used (wired cages or handnets). Asgnathiids are temporary parasites, they can drop off a host and return to the benthos before the fishis sampled; sentinel traps are able to retain such individuals. Nocturnal collections produced almostthree times more gnathiids than diurnal collections. These results are consistent with previousstudies. There was no difference in the number of gnathiids found in sentinel traps placed on reefswith and without a cleaner wrasse. These results appear initially to be at variance with the manyadvantages to fishes reported as being associated with the presence of L. dimidiatus. Theseobservations lead to the novel hypothesis that the advantage to clients from cleaners is in thereduction in the length of parasitism rather than the number of infections being reduced.This research has provided important information regarding the parasites of damselfishes,from the starting point of the characterisation of the parasite fauna to how cleaning behaviouraffects the ecology of recruiting fish, thus increasing our understanding of recruitment processes oncoral reefs. My study has demonstrated that the richness of damselfishes exceeds that of their trematode parasites on the GBR although the parasite diversity (11 families) is remarkable. Thework has contributed insight into both host range and species richness, having uncovered both noveltrematode species, and novel hosts for known species. Regarding cleaning interactions, I haveshown that 1) the abundance of damselfish recruits is higher on reefs with cleaner wrasse than onthose without; 2) damselfish recruits preferentially select a microhabitat that is adjacent to a cleanerwrasse; and 3) that the benefits for young fish to be near cleaners are likely to be indirect rather thandirect. These aspects of my study demonstrates the importance of cleaning mutualism not only foradult fishes, but for juvenile fishes during the recruitment period.