Advances and Trends in the Molecular Systematics of Anisakid Nematodes, with Implications for their Evolutionary Ecology and Host–Parasite Co-evolutionary Processes

Simonetta Mattiucci*; Giuseppe Nascetti†    * Department of Public Health Sciences, Section of Parasitology, “Sapienza”—University of Rome, P.le Aldo Moro, 5, 00185 Rome, Italy
† Department of Ecology and Sustainable Economic Development—Tuscia University—Via S. Giovanni Decollato, 1, 01100 Viterbo, Italy

1 Introduction

Adult nematodes of the genera of most species of the anisakid nematodes Anisakis, Dujardin, 1845, Pseudoterranova Krabbe, 1878 and Contracaecum Railliet and Henry, 1913 are parasites of the alimentary tract of aquatic vertebrates. They display indirect life cycles in aquatic ecosystems and involve various hosts at different levels in food webs. Marine mammals (cetaceans and pinnipeds) and fish-eating birds serve as definitive hosts; fish, squids and other invertebrates serve as intermediate or paratenic hosts; and crustaceans serve as first intermediate hosts. In humans, several larval anisakid nematodes cause the zoonotic disease, presently known as ‘anisakidosis’ or ‘anisakiosis’, when consumed in raw or undercooked fish. Anisakis is considered as the most important anisakid genus with regard to human infection (Audicana et al., 2002, and references therein), but some species of Pseudoterranova also have been implicated in human infections (Adams et al., 1997; Oshima, 1987), and a few species of Contracaecum are potentially infective (Vidal-Martinez et al., 1994). This zoonosis has a history starting from the first report (Van Thiel, 1960) of a larval nematode from herring in the gastro-intestinal tract of humans in the Netherlands. Some aspects of human anisakidosis have recently been reviewed by Audicana et al. (2002). Because of the existence of several excellent reviews by others authors on the history and clinical aspect of the anisakidosis (Audicana et al., 2002; Chai et al., 2005; Umehara et al., 2007), this introduction will not summarize the major events that mark the history of anisakid nematodes as causative agents of anisakidosis, but instead will highlight major landmarks involved in our understanding of their taxonomic status and ecology based on genetic markers. This field has had a major impact during the last 20 years on our knowledge of these parasites, including their host-specificity, geographical range and the possible identification of human cases of anisakidosis.

Starting from the knowledge summarized in the exhaustive revision by Smith and Wootten (1978), this review treats the explosion in the literature that has accompanied research activities on these nematodes and resulted in a series of discoveries over the last 25 years on the systematics and ecology of anisakid nematodes using molecular genetic tools applied to their taxonomy. Indeed, based on the advances in our understanding of the basic biology of Anisakis, reviewed by Smith and Wootten (1978), this introduction starts from their concluding remarks highlighting knowledge gaps and perspectives for investigations over the next 30 years. One major gap concerned the identification of species. Indeed, in the case of the species belonging to the genus Anisakis, they wrote: ‘…despite Davey's revision (1971), there are still taxonomic problems, including the distinction between A. simplex and A. typica … …The generic diagnosis of Anisakis should be re-examined …. Anisakis larva (I) from North Atlantic waters has been cultured in vitro and shown to develop into A. simplex: There is need to culture the other “larval types” in order to confirm that they do, in fact, represent Anisakis and, if so, to determine which species they represent ….’. In 1978, Smith and Wootten underlined the major gap in our knowledge of nematode biology at that date, as represented by the genus Anisakis but also relevant to other anisakid species, that is, the identification of biological species. Indeed, species identification, based on morphological characters only, is difficult for adults, but even more difficult for larval stages.

The need for species identification was especially important for larval stages of Anisakis because they have been implicated as causative agents of human anisakidosis. Indeed, Smith and Wootten (1978) concluded their review as follows: ‘…in these circumstances there is need for reliable methods of differential diagnosis. There is much scope for future investigation’. Thus, this ‘future investigation’ started with the application of molecular genetic methodologies in an attempt to address the systematics of these anisakids. The prospect of assessing anisakid nematode biodiversity based on molecular genetic markers as the preferred diagnostic tools seemed promising because the unambiguous identification of specimens causing human disease (anisakidosis) is essential for a proper epidemiological survey.

The power of resolution of molecular genetic methodologies in detecting anisakid species revolutionized the taxonomy of these species during the following 25 years. Most descriptions of parasite species conformed with what can be regarded as the ‘morphological or typological, species concept’. Because genetic speciation is not always accompanied by corresponding morphological change, the actual number of biological species is likely to be greater than the current tally of nominal species, most of which are delineated based on morphological grounds.

Detecting biological species of anisakid nematodes challenged parasitologists via an expected genetic variation and heterogeneity within the nominal species and has led to the definition of anisakid species according to the Mayr's (Mayr, 1963) ‘biological species concept’ (BSC).

The aim of this review is to gather together the many and varied aspects concerning the biology and evolutionary ecology of those anisakid species detected genetically and recognized as members of Anisakis, Pseudoterranova and Contracaecum, including (a) species presently accepted as senior synonyms based on the application of different molecular genetic markers; (b) current molecular genetic approaches to identify anisakid species; (c) ecological evidence relating to the geographical distribution of detected species, their host preferences and life cycles; (d) the use of anisakid nematodes as biological tags for fish stock identification in a multidisciplinary approach; (e) correlation between the values of genetic variability and the levels of parasitic infection in definitive and intermediate/paratenic hosts as indicators of the integrity of marine food webs; (f) estimates of genetic divergence at intraspecific and interspecific levels; and (g) estimates of their genetic relationships based on different clustering approaches inferred from different molecular genetic data sets and their use to infer phylogenetic hypothesis...