Negative influence of Gammarinema gammari (Nematoda) on the fecundity of Microphallus papillorobustus (Trematoda): field and experimental evidence

Autonomy and integration in complex parasite life cycles

Complex life cycles are common in free-living and parasitic organisms alike. The adaptive decoupling hypothesis postulates that separate life cycle stages have a degree of developmental and genetic autonomy, allowing them to be independently optimized for dissimilar, competing tasks. That is, complex life cycles evolved to facilitate functional specialization. Here, I review the connections between the different stages in parasite life cycles. I first examine evolutionary connections between life stages, such as the genetic coupling of parasite performance in consecutive hosts, the interspecific correlations between traits expressed in different hosts, and the developmental and functional obstacles to stage loss. Then, I evaluate how environmental factors link life stages through carryover effects, where stressful larval conditions impact parasites even after transmission to a new host. There is evidence for both autonomy and integration across stages, so the relevant question becomes how integrated are parasite life cycles and through what mechanisms? By highlighting how genetics, development, selection and the environment can lead to interdependencies among successive life stages, I wish to promote a holistic approach to studying complex life cycle parasites and emphasize that what happens in one stage is potentially highly relevant for later stages.

Is a vertebrate a better host for a parasite than an invertebrate host? Fecundity of Proctoeces cf lintoni (Digenea: Fellodistomidae), a parasite of fish and gastropods in northern Chile

The fecundity, the relationship between fecundity and worm size, as well as abundance of the adult worm Proctoeces cf. lintoni (Digenea: Fellodistomidae) in four gastropod hosts (keyhole limpets of the genus Fissurella) and one vertebrate host, the clingfish Sicyases sanguineus from northern Chile, were compared. Worms obtained from Fissurella latimarginata were smaller than those from Fissurella maxima, Fissurella limbata, and S. sanguineus. The mean fecundity of parasites from Fissurella crassa was significantly lower than those from F. limbata, F. maxima and S. sanguineus. For all invertebrate hosts, there were significant, positive relationships between worm size and fecundity, and the slopes of the regression did not differ significantly for the invertebrate hosts. Parasites of the keyhole limpet F. limbata and the clingfish showed the highest intensity, prevalence, and mean fecundity, and these species must be considered to be the hosts in which P. cf lintoni shows the best fitness.

Interactions between parasites of the cockle Austrovenus stutchburyi: hitch-hikers, resident-cleaners, and habitat-facilitators

The patterns of association between parasites within a particular host are determined by a number of factors. One of these factors is whether or not infection by one parasite influences the probability of acquiring other parasite species. This study investigates the pattern of association between various parasites of the New Zealand cockle Austrovenus stutchburyi. Hundreds of cockles were collected from one locality within Otago Harbour, New Zealand and examined for trematode metacercariae and other symbionts. Two interspecific associations emerged from the study. First, the presence of the myicolid copepod Pseudomyicola spinosus was positively associated with higher infection intensity by echinostomes. The side-effect of the copepod's activities within the cockle is suggested as the proximate mechanism that facilitates infection by echinostome cercariae, leading to a greater rate of accumulation of metacercariae in cockles harbouring the copepod. Second, a positive association was also found between infection intensity of the metacercariae of foot-encysting echinostomes and that of gymnophallid metacercariae. This supports earlier findings and suggests that the gymnophallid is a hitch-hiker parasite because, in addition to the pattern of positive association, it (a) shares the same transmission route as the echinostomes, and (b) unlike the echinostomes, it is not capable of increasing the host's susceptibility to avian predation. Thus, both active hitch-hiking and incidental facilitation lead to non-random infection patterns in this parasite community.

Larval helminths in intermediate hosts: Does competition early in life determine the fitness of adult parasites?

Density-dependent effects on parasite fitness have been documented from adult helminths in their definitive hosts. There have, however, been no studies on the cost of sharing an intermediate host with other parasites in terms of reduced adult parasite fecundity. Even if larval parasites suffer a reduction in size, caused by crowding, virtually nothing is known about longer-lasting effects after transmission to the definitive host. This study is the first to use in vitro cultivation with feeding of adult trematodes to investigate how numbers of parasites in the intermediate host affect the size and fecundity of adult parasites. For this purpose, we examined two different infracommunities of parasites in crustacean hosts. Firstly, we used experimental infections of Maritrema novaezealandensis in the amphipod, Paracalliope novizealandiae, to investigate potential density-dependent effects in single-species infections. Secondly, we used the crab, Macrophthalmus hirtipes (Ocypodidae), naturally infected by the trematodes, M. novaezealandensis and Levinseniella sp., the acanthocephalan, Profilicollis spp., and an acuariid nematode. These four helminths all develop and grow in their crustacean host before transmission to their bird definitive host by predation. In experimental infections, we found an intensity-dependent establishment success, with a decrease in the success rate of cercariae developing into infective metacercariae with an increasing dose of cercariae applied to each amphipod. In natural infections, we found that M. novaezealandensis-metacercariae achieved a smaller volume, on average, when infrapopulations of this parasite were large. Small metacercariae produced small in vitro-adult worms, which in turn produced fewer eggs. Crowding effects in the intermediate host thus were expressed at the adult stage in spite of the worms being cultured in a nutrient-rich medium. Furthermore, excystment success and egg-production in M. novaezealandensis in naturally infected crabs were influenced by the number of co-occurring Profilicollis cystacanths, indicating interspecific interactions between the two species. Our results thus indicate that the infracommunity of larval helminths in their intermediate host is interactive and that any density-dependent effect in the intermediate host may have lasting effects on individual parasite fitness.

In vitro cultivation of Maritrema novaezealandensis (Microphallidae): the effect of culture medium on excystation, survival and egg production

In this study, the effect of the composition of culture medium on excystation, growth, survival and egg production was investigated for the recently discovered microphallid trematode Maritrema novaezealandensis. Metacercariae from the two second intermediate crab hosts, Macrophthalmus hirtipes and Halicarcinus whitei, were incubated in either: (1) 0.85% saline solution, (2) the commercial cell culture medium, NCTC-109, (3) NCTC-109 supplemented with 20%, or (4) NCTC-109 supplemented with 40% chicken serum. Furthermore, excysted metacercariae were cultured for 5 days in each of the three media: NCTC-109 and NCTC-109 supplemented with 20% or 40% chicken serum. Excystment was rapid for all media used in the experiment. However, metacercariae in 0.85% saline solution had a significantly lower excystment rate over the first 4 h of incubation compared to the other media. Egg production was highest in specimens cultured in media with a supplement of chicken serum and reached a maximum after 2 days of cultivation. Growth, however, did not occur after the first day of cultivation in any of the three media.