Measuring aggregation in parasite populations.

Aggregation, a fundamental feature of parasite distributions, has been measured using a variety of indices. We use the definition that parasite-host system A is more aggregated than parasite-host system B if any given proportion of the parasite population is concentrated in a smaller proportion of the host population A than of host population B. This leads to indices based on the Lorenz curve such as the Gini index (Poulin's D), coefficient of variation and the Hoover index, all of which measure departure from a uniform distribution. The Hoover index is particularly useful because it can be interpreted directly in terms of parasites and hosts. An alternative view of aggregation is degree of departure from a Poisson (or random) distribution, as used in the index of dispersion and the negative binomial k. These and Lloyd's mean crowding index are reinterpreted and connected back to Lorenz curves. Aggregation has occasionally been defined as the slope from Taylor's law, although the slope appears unrelated to other indices. The Hoover index may be the method of choice when data points are available, and the coefficient of variation when only variance and mean are given.

Does moving up a food chain increase aggregation in parasites?

General laws in ecological parasitology are scarce. Here, we evaluate data on numbers of fish parasites published by over 200 authors to determine whether acquiring parasites via prey is associated with an increase in parasite aggregation. Parasite species were grouped taxonomically to produce 20 or more data points per group as far as possible. Most parasites that remained at one trophic level were less aggregated than those that had passed up a food chain. We use a stochastic model to show that high parasite aggregation in predators can be solely the result of the accumulation of parasites in their prey. The model is further developed to show that a change in the predators feeding behaviour with age may further increase parasite aggregation.

Parasites as valuable stock markers for fisheries in Australasia, East Asia and the Pacific Islands.

Over 30 studies in Australasia, East Asia and the Pacific Islands region have collected and analysed parasite data to determine the ranges of individual fish, many leading to conclusions about stock delineation. Parasites used as biological tags have included both those known to have long residence times in the fish and those thought to be relatively transient. In many cases the parasitological conclusions have been supported by other methods especially analysis of the chemical constituents of otoliths, and to a lesser extent, genetic data. In analysing parasite data, authors have applied multiple different statistical methodologies, including summary statistics, and univariate and multivariate approaches. Recently, a growing number of researchers have found non-parametric methods, such as analysis of similarities and cluster analysis, to be valuable. Future studies into the residence times, life cycles and geographical distributions of parasites together with more robust analytical methods will yield much important information to clarify stock structures in the area.

Parasites as indicators of movement and population connectivity of a non-diadromous, tropical estuarine teleost: king threadfin Polydactylus macrochir.

Temporal and spatial patterns in parasite assemblages were examined to evaluate the degree of movement and connectivity of post-recruitment life-history stages of a large, non-diadromous tropical estuarine teleost, king threadfin Polydactylus macrochir, collected from 18 locations across northern Australia. Ten parasites types (juvenile stages of two nematodes and seven cestodes, and adults of an acanthocephalan) were deemed to be suitable for use as biological tags, in that they were considered to have a long residence time in the fish, were relatively easy to find and were morphologically very different to each other which aided discrimination. Univariate and discriminant function analysis of these parasites revealed little difference in temporal replicates collected from five locations, suggesting that the parasite communities were stable over the timeframes explored. Univariate, discriminant function, and Bray-Curtis similarity analyses indicated significant spatial heterogeneity, with Bray-Curtis classification accuracies ranging from 55 to 100% for locations in north-western and northern Australia, 24 to 88% in the Gulf of Carpentaria, and 39 to 88% on the east coast of Queensland. Few differences were observed among locations separated by <200 km. The observed patterns of parasite infection are in agreement with concurrent studies of movement and connectivity of P. macrochir in that they indicate a complex population structure across northern Australia. These results should be considered when reviewing the management arrangements for this species.

Overdispersion in marine fish parasites.

A modification of Taylor's Power law was used to compare the degree of overdispersion in frequency distributions from 38 datasets of marine parasites, data that had originally been collected for fish stock discrimination. The results strongly indicate that the overriding factor contributing to overdispersion in these helminths and crustaceans is the number of hosts in the life cycle. This was particularly well shown by juveniles of Anisakis 1 from different fish species. Data on the cestode Otobothrium cysticum and the monogenean Pricea multae appear anomalous and lead to conclusions about their biology not at first evident from the literature.

Genetic population structure of grey mackerel Scomberomorus semifasciatus in northern Australia.

This study used mtDNA sequence and microsatellite markers to elucidate the population structure of Scomberomorus semifasciatus collected from 12 widespread sampling locations in Australia. Samples (n = 544) were genotyped with nine microsatellite loci, and 353 were sequenced for the control (384 bp) and ATPase (800 bp) mtDNA gene regions. Combined interpretation of microsatellite and mtDNA data identified four genetic stocks of S. semifasciatus: Western Australia, north-west coast of the Northern Territory, Gulf of Carpentaria and the eastern coast of Queensland. Connectivity among stocks across northern Australia from the Northern Territory to the eastern coast of Queensland was high (mean F(ST) = 0·003 for the microsatellite data and Φ(ST) = 0·033 and 0·009 for control region and ATPase, respectively) leading to some uncertainty about stock boundaries. In contrast, there was a clear genetic break between the stock in Western Australia compared to the rest of northern Australia (mean F(ST) = 0·132 for the microsatellite data and Φ(ST) = 0·135 and 0·188 for control region and ATPase, respectively). This indicates a restriction to gene flow possibly associated with suboptimal habitat along the Kimberley coast (north Western Australia). The appropriate scale of management for this species corresponds to the jurisdictions of the three Australian states, except that authorities in Queensland and Northern Territory should co-ordinate the management of the Gulf of Carpentaria stock.

Stock structure of blue threadfin Eleutheronema tetradactylum across northern Australia, as indicated by parasites.

The parasite fauna of the blue threadfin Eleutheronema tetradactylum, collected from 14 sites across northern Australia, was examined to evaluate the degree of movement and subsequent stock structure of the fish. Univariate and multivariate analysis of nine 'permanent' parasite species [the nematodes Anisakis (type I) and Terranova (type II), the cestodes Otobothrium australe, Pterobothrium pearsoni, Pterobothrium sp. A, Callitetrarhynchus gracilis, Parotobothrium balli and Nybelinia sp., and the acanthocephalan Pomphorhynchus sp.] demonstrated little similarity between sites, indicating limited mixing and therefore long-term separation of post-juvenile fish. As such, the effects of fishing are likely to be localized within the current administrative boundaries, implying little need for interstate co-operative management. Within each jurisdiction, management of E. tetradactylum populations, including the establishment of harvest strategies and fishery regulations, should be conducted in a way that recognizes the resident nature of the fish.

Stock structure of blue threadfin Eleutheronema tetradactylum on the Queensland east coast, as determined by parasites and conventional tagging.

Blue threadfin Eleutheronema tetradactylum (Polynemidae) were examined from four areas (Princess Charlotte Bay, Trinity Inlet, Halifax Bay and Upstart Bay) in eastern Queensland covering a distance of c. 950 km of coastline. Parasites were used as biological markers to infer stock structure of E. tetradactylum. Parasites designated as 'temporary' biological markers were the copepod Thysanote eleutheronemi, the acanthocephalan Neoechinorhynchus topseyi, the nematode Philometra rajani and hemiurid trematodes. The larval nematodes Anisakis sp. Type 1 and Terranova sp. Type 2; and the larval cestodes Pterobothrium pearsoni and Callitetrarhynchus gracilis were considered 'permanent' biological markers. Both univariate and multivariate analyses demonstrated that there was little difference in temporary parasite abundance between the four areas. In contrast, the same analyses revealed that most areas had two or more significant differences in permanent parasite abundance, with the exception of Halifax Bay and Upstart Bay, which were significantly different only in the multivariate analysis. Biological markers predicted that Princess Charlotte Bay and Trinity Inlet consisted of distinct populations, whereas Halifax Bay and Upstart Bay were not clearly differentiated. Tag recapture data supported this hypothesis; the majority of recaptures were within 100 km of the initial tagging location. Geographical movement of E. tetradactylum may be limited due to their biology and ecology, as well as the distances and oceanographic boundaries that separate habitats. Contrary to current management definitions, the stock structure of E. tetradactylum on the east coast of Queensland appears to be geographically differentiated at a small spatial scale.

Detection of the initial infective stages of the protozoan parasite Marteilia sydneyi in Saccostrea glomerata and their development through to sporogenesis.

DNA probes were used in in situ hybridisation on histological sections of oysters exposed for defined intervals to Marteilia sydneyi infection to reveal the early development of the parasite in the oyster host, Saccostrea glomerata. The initial infective stages enter through the palps and gills whereupon extrasporogonic proliferation results in the liberation of cells into surrounding connective tissue and haemolymph spaces. Following systemic dissemination, the parasite infiltrates the digestive gland and becomes established as a nurse cell beneath the epithelial cells in a digestive tubule. Here, cell-within-cell proliferation results in the eventual liberation of daughter cells from the nurse cell into spaces between adjacent epithelial cells. None of these stages had previously been described. Proliferation is associated with host responses, including haemocytic infiltration of the connective tissue and diapedesis across tubule epithelia. The responses cease as sporogenesis begins.