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Ecological studies have demonstrated that parasites are capable of influencing various aspects of host life history and can play an important role in the structure of animal populations. We investigated the influence of infection by castrating trematodes on the reproduction, survival and population density of the intertidal snail Zeacumantus subcarinatus, using both laboratory and field studies. The results demonstrate a highly significant reduction in the reproductive output in heavily infected populations compared to populations with low trematode prevalence. A long-term laboratory study showed reduced survival of infected snails compared to uninfected specimens, for snails held at 18 and 25°C. Furthermore, parasite-induced mortality in the field was inferred from a reduction in prevalence of infection among larger size classes, indicating that infected individuals disappear from the population, although the effect of parasites varied between localities. A field survey from 13 localities including 2897 snails demonstrated that prevalence of castrating trematodes had a significant negative effect on both population density and biomass of Z. subcarinatus. This study provides one of the first demonstrations of population-level effects of parasites on their hosts in the field. The results of this study emphasise the importance of castrating parasites as potential agents of population regulation in host species with limited dispersal ability.

The impact of parasitism on population dynamics and community structure of marine animals is an area of growing interest in marine ecology. The effect of a microphallid trematode, Maritrema novaezelandensis on the survival of its amphipod host, Paracalliope novizealandiae, was investigated by a laboratory study combined with data from field collections. In the laboratory, the effect of infection level on host mortality was investigated. Four groups of individuals were exposed to 0 (control), 5 (low), 25 (moderate) and 125 (high) cercariae, respectively, and their survival was monitored during a 10-day period. The distribution and migration of unencysted cercariae within the host were examined during dissections 6 and 48 h post infection. Parasiteinduced mortality under field conditions was investigated by quantifying the relationship between parasite load and host body size. In the laboratory experiment, a highly significant decrease in host survival was observed for amphipods in the moderate and high infection groups relative to that of control amphipods. Parasite-induced mortality was most pronounced in the first two days post infection suggesting that the increased mortality is due to penetration of host cuticula and migration of cercariae within the host. Field data showed a monotonic increase in the mean parasite load with the body size of the amphipods, indicating that M. novaezelandensis does not severely affect P. novizealandiae-populations under normal field conditions. However, a decrease in the variance-tomean ratio for the largest size-classes indicates that heavily infected individuals are removed from the population as predicted by the experimental infections. The results from the laboratory study in conjunction with our knowledge of the transmission strategy of the parasite emphasize the potential

Maritrema novaezealandensis n. sp. is described from Otago Harbor, South Island, New Zealand, on the basis of adult specimens collected from the Red-billed gull, Larus novaehollandiae scopulinus, and excysted metacercariae obtained from crabs. It belongs to the "eroliae group" and differs from other related species mainly in the shape, size, and patterns of distributions of the spines on the cirrus, the shape of the metraterm, the presence of an unlobed ovary, and the complete ring of the vitelline follicles. Based on morphometric features of metacercariae and adult specimens, the trophic relationships among invertebrate and vertebrate hosts, experimental infections, and previous reports of species of Maritrema with similar transmission patterns, the life cycle of M. novaezealandensis n. sp. is described. A 3-host life cycle is proposed for this parasite. The first intermediate host is the mud snail, Zeacumantus subcarinatus, in which the cercarial stage is produced in sporocysts located within the gonad of the snail. At least 3 crab species (Hemigrapsus crenulatus, Macrophtalmus hirtipes, and Halicarcinus whitei) and several species of amphipods act as second intermediate hosts, with metacercariae encysted in the body cavity of the crustacean host. Finally, the definitive host, the gull, L. n. scopulinus, harbors the adult worms in its intestine.

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 eggproduction 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. q

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This data set presents food webs for three North American Pacific coast estuaries and a ''Metaweb'' composed of the species/stages compiled from all three estuaries. The webs have four noteworthy attributes: (1) parasites (infectious agents), (2) body-size information, (3) biomass information, and (4) ontogenetic stages of many animals with complex life cycles. The estuaries are Carpinteria Salt Marsh, California (CSM); Estero de Punta