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Introduction
Latin America has experienced an economic growth rate between 2.5 and 5% in the past 10 years (http://data.worldbank.org/indicator/NY.GDP.MKTP.KD.ZG/countries). However, this growth has occurred without proper sustainable development for both urban and rural environments, resulting in land degradation, deforestation, overfishing, extinction of plant and animal species, and carbon emissions leading to climate change (Castro-Diaz & Kuna, 2007). Furthermore, these environments are constantly receiving pesticides, heavy metals and persistent organic pollutants (POPs), such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and polybrominated diphenyl ethers (PBDEs), as a consequence of human activities (e.g. Vidal-Martínez et al., 2003, 2006). As there is an obvious need for effective control of these environmental issues and proper disposal of pollutants and contaminants (for FAO definitions, see Sciortino & Ravikumar, 1999), we must understand and prevent the outcomes of harmful environmental degradation processes and toxic exposures. One strategy to obtain this information is the use of bioindicators. These are species that reflect the environmental impact because they respond to habitat alterations with changes in their numbers, physiology or chemical composition. Bioindicators can be indicators of effect or accumulation. Effect bioindicators can range from functional molecular or physiological changes to changes in population size; accumulation bioindicators must accumulate substances from the environment efficiently without showing adverse effects (Sures, 2003). These bioindicators measure exposure to pollutants at different levels of organization, from the subcellular to the ecosystem level, and are useful indicators of different effects (Amiard-Triquet et al., 2013).
Traditionally, ecologists have used free-living organisms as bioindicators to assess the environmental quality of both terrestrial and aquatic environments (e.g. Ortega-Ãlvarez & MacGregor-Fors, 2011; Tweedley et al., 2014). However, there are limitations to the use of free-living organisms as bioindicators, such as: (1) the large number of samples (and funding) required for quantitative sampling; (2) the seasonal variation affecting the distribution and abundance of the organisms; (3) the large number of methods (and indices) available for the analyses, suggesting a large heterogeneity in the interpretation of the results (Warwick, 1993; Wright, 2010).
The helminth parasites of both terrestrial and aquatic organisms are also useful bioindicators because they are affected by anthropogenic and natural environmental factors (Vidal-Martínez, 2007; Sures, 2008; Vidal-Martínez et...