This is the first systematic review of helminth parasitofauna in Spanish anurans over 120 years. Although the number of papers generated during this period was low compared with that in other disciplines of biology, the number of authors involved in these papers was unevenly distributed, with the greatest number of authors being from Javier Lluch (n = 26, 42.62%), Pilar Navarro (n = 23, 37.70%), and Vicente Roca (n = 11, 18.03%) from the Universitat de València. However, it should be noted that the pioneering publication was presented by Carlos Rodríguez López-Neyra de Gorgot in 1916 with the identification and description of the trematode Haematoloechus variegatus in R. iberica. Publications were available mostly in either Spanish or French in contrast to the rest of Europe, where English is the dominant language. More recent publications are in indexed journals, which has improved their availability in widely used indexed academic search engines.
One difficulty that could be identified in this review was the changes in the names of the helminth species in the period studied. Changes in nomenclature may result in the incorrect assignment of a parasite species to one or several hosts. In this study, thirteen synonyms and redescriptions were identified. Trematodes presented the greatest number of changes.
This study included information on the parasitic helminth fauna of 12 species of anurans (Table 1). The parasite-specific richness was greater in Ranidae and lower in Pelobatidae. This may be due to the greater number of studies focusing on frog species across much of the range of the different species. R. perezi had the highest parasite-specific richness.
The most common helminths in this study were trematodes, as they were found in all species. A. muletensis, E. calamita, H. merididionalis, and P. cultripes did not harbor these parasites. However, the most cited helminth species are two nematodes, C. ornata and O. filiformis. This can be partially explained by their complex life cycles. Oswaldocruzia is known to be acquired passively (by ingestion of infective larvae), while Cosmocerca larvae enter through the skin (Anderson, 2000). Trematodes are the most diverse group of parasites, but are more closely linked to aquatic clades, such as Pelophylax and Rana. This finding fits perfectly with the pattern for aquatic species described by Campiao et al. (2015) and Cañizales (2020). In the immature stage of anurans (tadpoles) with exclusively aquatic habits, they acquire some trematode species through the skin, and during development they acquire other species passively by ingesting intermediate invertebrate hosts (Anderson, 2000). Acanthocephalans and cestodes need invertebrates (crustaceans, acari, or insects) as intermediate hosts, which are consumed by anurans as part of their natural diet. Monogeneans have a direct life cycle in which the immature infective (oncomiracidium) forms attach to tadpole gills and remain on them, reaching the adult stage in the adult frog (Mehlhorn, 2016).
Differences related to the geographical distribution of parasites are directly related to the number of anuran species included in each study. The high number of parasites reported in Castilla and Leon is because each study involved the largest possible number of anuran species simultaneously. Cantabria, the Canary Islands, and the Balearic Islands recorded the lowest number of parasite species, as only a single anuran species was considered in all cases.
Variations in the degree of similarity in shared helminthofauna among anuran families (Sørensen's similarity index) show that Bufonidae and Ranidae together account for more than 80% of the anuran species studied, being the only host families harboring all major helminth groups (Fig. 3).
The conformation of heterogeneous groups (Morisita's similarity index) indicates that these species share few or no parasite species (e.g., P. cultripes and P. saharicus). Although cluster analysis shows low similarity, it is still a valuable tool in the visual representation of the structure of the data. Despite the results obtained, caution must be exercised in their interpretation, as these relationships are determined by the available information. This finding reinforces the need for a detailed assessment of parasite diversity in all anuran species according to locality and range.
As mentioned by Poulin & Morand (2000, 2004), the habitat/activity interaction relationship appears to be a determinant of parasite richness. In this study, despite the amount and bias of available data, the influence of the habitat/activity pattern of anuran hosts on parasite richness was statistically significant. Statistical models in parasitology generally quantify the effect of different biological factors on a parasitic infection. Intuitively, the easiest way to increase the statistical power of a test is to increase the sample size, but unfortunately, there is no general rule that can be applied to all studies or all species. In conclusion, there was a reasonable difference between the groups.
The overall network topology comprises nodes with random connections, with each node having approximately the same number of connections as the others. Variation in the degree of specificity among parasites with different life strategies is an important determinant of network structure. These results can be attributed to the effects on their local abundance and diversity, as well as on their habitat type and pattern of activity. Terrestrial anurans are susceptible to acquiring parasites whose infective stages are in soil (direct life-cycle nematodes). Aquatic anurans acquire infective parasites in water (such as trematodes), and arboreal anurans are infected mostly by parasites transmitted through ingestion. The richness of helminths was more similar among sympatric hosts. This suggests that host biology, including evolutionary history, is the main factor determining the colonization success of the parasite species.
In the European context, the general situation is as follows: Bailinger & Chanseau (1954) reported for the Bordeaux region, France, a total of 17 helminth species (Monogeneans: 1, Nematodes: 6 and Trematodes: 10) in Rana esculenta, R. agilis, R temporaria, and H. meridionalis. Comas et al. (2014), in the region of Calabria, Italy, recorded seven helminth species (Acanthocephalans: 1, Nematodes: 3, and Trematodes: 3) in Pelophylax kl. hispanicus. Cedhagen (1988) reported 12 helminth species in Rana arvalis and R. temporaria in Sweden. Okulewicz et al. (2014) recorded 17 helminth species (Acanthocephalans: 2, Cestodes: 1, Nematodes: 7 and Trematodes: 7) in Pelophylax esculentus, P. lessonae, R. arvalis, and R. temporaria in Poland. The highest species richness of helminths (13 species) was observed in R. arvalis, whereas the lowest (5 species) richness was observed in P. lessonae. Herczeg et al. (2016) reported a total of eight helminth species (Acanthocephalans: 1, Nematodes: 2, and Trematodes: 5) in Pelophylax esculentus in Hungary. In Turkey, Yildirimhan et al. (2006) recorded ten helminth species (Acanthocephalans: 1, Monogeneans: 1, Nematodes: 3, and Trematodes: 5) in Rana holtzi and R. macrocnemis. The highest species richness of helminths (10 species) was observed in R. macrocnemis, whereas the lowest (4 species) was observed in R. holtzi. Saglam & Arikan (2006) identified a total of nine helminth species (Acanthocephalans: 1, Nematodes: 5, and Trematodes: 3) in Rana ridibunda. Düsen et al. (2009) reported nine species (Acanthocephalans: 1, Monogeneans: 1, Nematodes: 4, and Trematodes: 3) in R. dalmatina. According to the above results, Spain ranks first in terms of parasite diversity in anurans with a species richness of 78 taxa reported and obtained in this study.
There is no doubt that parasitic organisms are an important part of the biodiversity inventory. Documenting the diversity of parasites in each host (vertebrate or invertebrate) faces several challenges, all related to obtaining biological samples from the host species. There are many species of parasites that have yet to be found and described; unfortunately, only studies carried out on captured or collected animals can provide a true approximation of the parasitofauna of a species or locality to be studied. In this sense, animal slaughter should be subject to the legal and sanitary regulations in force for these cases. Here is the importance of biological collections.