Parasites as Biological Tags of Fish Populations: Advantages and Limitations

A century of parasitology in fisheries and aquaculture

Fish parasitological research associated with fisheries and aquaculture has expanded remarkably over the past century. The application of parasites as biological tags has been one of the fields in which fish parasitology has generated new insight into fish migration and stock assessments worldwide. It is a well-established discipline whose methodological issues are regularly reviewed and updated. Therefore, no concepts or case-studies will be repeated here; instead, we summarize some of the main recent findings and achievements of this methodology. These include the extension of its use in hosts other than bony fishes; the improvements in the selection of parasite tags; the recognition of the host traits affecting the use of parasite tags; and the increasingly recognized need for integrative, multidisciplinary studies combining parasites with classical methods and modern techniques, such as otolith microchemistry and genetics. Archaeological evidence points to the existence of parasitic problems associated with aquaculture activities more than a thousand years ago. However, the main surge of research within aquaculture parasitology occurred with the impressive development of aquaculture over the past century. Protozoan and metazoan parasites, causing disease in domesticated fish in confined environments, have attracted the interest of parasitologists and, due to their economic importance, funding was made available for basic and applied research. This has resulted in a profusion of basic knowledge about parasite biology, physiology, parasite-host interactions, life cycles and biochemistry. Due to the need for effective control methods, various solutions targeting host-parasite interactions (immune responses and host finding), genetics and pharmacological aspects have been in focus.

Mosaic or melting pot: The use of monogeneans as a biological tag and magnifying glass to discriminate introduced populations of Nile tilapia in sub-Saharan Africa

The origin of introduced Nile tilapia stocks in sub-Saharan Africa is largely unknown. In this study, the potential of monogeneans as a biological tag and magnifying glass is tested to reveal their hosts' stocking history. The monogenean gill community of different Nile tilapia populations in sub-Saharan Africa was explored, and a phylogeographic analysis was performed based on the mitogenomes of four dactylogyrid species (Cichlidogyrus halli, C. sclerosus, C. thurstonae, and Scutogyrus longicornis). Our results encourage the use of dactylogyrids as biological tags. The magnifying glass hypothesis is only confirmed for C. thurstonae, highlighting the importance of the absence of other potential hosts as prerequisites for a parasite to act as a magnifying glass. With the data generated here, we are the first to extract mitogenomes from individual monogeneans and to perform an upscaled survey of the comparative phylogeography of several monogenean species with unprecedented diagnostic resolution.

Parasites as conservation tools

Parasite success typically depends on a close relationship with one or more hosts; therefore, attributes of parasitic infection have the potential to provide indirect details of host natural history and are biologically relevant to animal conservation. Characterization of parasite infections has been useful in delineating host populations and has served as a proxy for assessment of environmental quality. In other cases, the utility of parasites is just being explored, for example, as indicators of host connectivity. Innovative studies of parasite biology can provide information to manage major conservation threats by using parasite assemblage, prevalence, or genetic data to provide insights into the host. Overexploitation, habitat loss and fragmentation, invasive species, and climate change are major threats to animal conservation, and all of these can be informed by parasites.

Ecological Analysis of the Helminth Community of the Wood Mouse, Apodemus sylvaticus, along an 18-Year Post-Fire Regeneration Period in a Mediterranean Ecosystem

Simple Summary

After a wildfire, especially in Mediterranean ecosystems, the vegetation starts to recover gradually but appears regenerated after a decade. However, the study of helminth parasites of the wood mouse, the most important post-fire recolonizer mammal in the western Mediterranean forests, allowed us to elucidate the reality of this process more accurately, mainly due to the, sometimes complex, life cycles of the helminths using other hosts (vertebrates and invertebrates) to complete them. Thus, the comparative analysis of various aspects (biodiversity and kinds of life cycles) of the helminth community of the wood mouse, as well as the influence of some external factors (related to the environment and its periodical post-fire regeneration) and internal factors (related to the population of the wood mouse), between the burned area and a non-burned area used as control, reveals the biological indicators (indicators of the situation/evolution of a process) of the post-fire regeneration process and its true state. The current study confirms the important role of the helminths of the wood mouse as biological indicators and the influence that climate variables exercise on the quality and evolution of the post-fire regeneration process in Mediterranean ecosystems.

Abstract

The role of helminths of the wood mouse, Apodemus sylvaticus, as biological indicators of the post-fire regeneration process in Serra Calderona Natural Park, a Mediterranean forest ecosystem located between the provinces of València and Castelló (Valencian Country, Spain), has been analysed for almost twenty years. The helminth ecological analysis of 917 A. sylvaticus (675 originating from the burned area and 242 originating from the control area) has been carried out between the 2nd and 18th post-fire years. The influence of intrinsic (host population density, sex and age) and extrinsic (site, period and year of capture, climate variables) factors on the post-fire evolution of the helminth community of the wood mouse, and the biodiversity, species richness and life cycle of the helminth species was studied. Taking into account the most important results obtained, various aspects of the helminth community dynamics of the wood mouse are confirmed as biological indicators of the post-fire regeneration process in Mediterranean ecosystems. The still existing differences between the two areas are mainly related to the influence of climate variables on the post-fire regeneration process. Moreover, the important role that helminth parasites of the wood mouse play as biological indicators of this process in Mediterranean ecosystems is demonstrated.

Marine parasites as biological tags in South American Atlantic waters, current status and perspectives

Many marine fisheries in South American Atlantic coasts (SAAC) are threatened by overfishing and under serious risk of collapsing. The SAAC comprises a diversity of environments, possesses a complex oceanography and harbours a vast biodiversity that provide an enormous potential for using parasites as biological tags for fish stock delineation, a prerequisite for the implementation of control and management plans. Here, their use in the SAAC is reviewed. Main evidence is derived from northern Argentine waters, where fish parasite assemblages are dominated by larval helminth species that share a low specificity, long persistence and trophic transmission, parasitizing almost indiscriminately all available fish species. The advantages and constraints of such a combination of characteristics are analysed and recommendations are given for future research. Shifting the focus from fish/parasite populations to communities allows expanding the concept of biological tags from local to regional scales, providing essential information to delineate ecosystem boundaries for host communities. This new concept arose as a powerful tool to help the implementation of ecosystem-based approaches to fisheries management, the new paradigm for fisheries science. Holistic approaches, including parasites as biological tags for stock delineation will render valuable information to help insure fisheries and marine ecosystems against further depletion and collapse.

Parasites as biological tags to assess host population structure: Guidelines, recent genetic advances and comments on a holistic approach

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Parasites as biological tags to assess host population structure.

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Recent molecular advances support incorporation of parasite genetic data.

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Guidelines for selection of a parasite species as a tag candidate updated.

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Holistic approach allows for robustness and support of observed result.

We review the use of parasites as biological tags of marine fishes and cephalopods in host population structure studies. The majority of the work published has focused on marine fish and either single parasite species or more recently, whole parasite assemblages, as biological tags. There is representation of host organisms and parasites from a diverse range of taxonomic groups, although focus has primarily been on host species of commercial importance. In contrast, few studies have used parasites as tags to assess cephalopod population structure, even though records of parasites infecting cephalopods are well-documented. Squid species are the only cephalopod hosts for which parasites as biological tags have been applied, with anisakid nematode larvae and metacestodes being the parasite taxa most frequently used. Following a brief insight into the importance of accurate parasite identification, the population studies that have used parasites as biological tags for marine fishes and cephalopods are reviewed, including comments on the dicyemid mesozoans. The advancement of molecular genetic techniques is discussed in regards to the new ways parasite genetic data can be incorporated into population structure studies, alongside host population genetic analyses, followed by an update on the guidelines for selecting a parasite species as a reliable tag candidate. As multiple techniques and methods can be used to assess the population structure of marine organisms (e.g. artificial tags, phenotypic characters, biometrics, life history, genetics, otolith microchemistry and parasitological data), we conclude by commenting on a holistic approach to allow for a deeper insight into population structuring.

Parasites as biological tags of fish stocks: a meta-analysis of their discriminatory power

The use of parasites as biological tags to discriminate among marine fish stocks has become a widely accepted method in fisheries management. Here, we first link this approach to its unstated ecological foundation, the decay in the similarity of the species composition of assemblages as a function of increasing distance between them, a phenomenon almost universal in nature. We explain how distance decay of similarity can influence the use of parasites as biological tags. Then, we perform a meta-analysis of 61 uses of parasites as tags of marine fish populations in multivariate discriminant analyses, obtained from 29 articles. Our main finding is that across all studies, the observed overall probability of correct classification of fish based on parasite data was about 71%. This corresponds to a two-fold improvement over the rate of correct classification expected by chance alone, and the average effect size (Zr = 0·463) computed from the original values was also indicative of a medium-to-large effect. However, none of the moderator variables included in the meta-analysis had a significant effect on the proportion of correct classification; these moderators included the total number of fish sampled, the number of parasite species used in the discriminant analysis, the number of localities from which fish were sampled, the minimum and maximum distance between any pair of sampling localities, etc. Therefore, there are no clear-cut situations in which the use of parasites as tags is more useful than others. Finally, we provide recommendations for the future usage of parasites as tags for stock discrimination, to ensure that future applications of the method achieve statistical rigour and a high discriminatory power.

Where are the parasites in food webs?

This review explores some of the reasons why food webs seem to contain relatively few parasite species when compared to the full diversity of free living species in the system. At present, there are few coherent food web theories to guide scientific studies on parasites, and this review posits that the methods, directions and questions in the field of food web ecology are not always congruent with parasitological inquiry. For example, topological analysis (the primary tool in food web studies) focuses on only one of six important steps in trematode life cycles, each of which requires a stable community dynamic to evolve. In addition, these transmission strategies may also utilize pathways within the food web that are not considered in traditional food web investigations. It is asserted that more effort must be focused on parasite-centric models, and a central theme is that many different approaches will be required. One promising approach is the old energetic perspective, which considers energy as the critical resource for all organisms, and the currency of all food web interactions. From the parasitological point of view, energy can be used to characterize the roles of parasites at all levels in the food web, from individuals to populations to community. The literature on parasite energetics in food webs is very sparse, but the evidence suggests that parasite species richness is low in food webs because parasites are limited by the quantity of energy available to their unique lifestyles.