Morphological plasticity among polystomatid flatworms (Monogenea: Polystomatidae)

Abstract

Polystomes (Monogenea: Polystomatidae) are parasitic platyhelminths infecting a wide variety of hosts, including anurans, freshwater chelonians, caecilians, the Australian lungfish, salamanders, and the hippopotamus. Although polystome genera are collectively distributed across all zoological realms, most individual genera are restricted to one realm or even island. Although molecular tools are now commonly used for the systematics and classification of polystomes, traditional morphological studies and morphometrics are still important. Yet there has been surprisingly little study of the morphological plasticity they display. Since it has been suggested that a high degree of morphological plasticity may have profound effects of the current classification of polystomes, this study aimed to evaluate the degree of plasticity displayed by some polystomatid genera occurring in both amphibians and chelonians. Several morphological features of polystomes were assessed in this study, with the primary focus being on the sclerotized hooks, since classification of these soft-bodied parasites rests mainly thereupon. The effect of different chemical fixatives on the marginal hooklets was evaluated and found to be minimal. However, it seemed as though the age and life-stage of the parasite might have some influence on the sizes of these hooks in some species. The validity of the genus Metapolystoma was evaluated based on morphology and molecular tools. The molecular analyses yielded similar results to those of previous studies suggesting that the genus is a junior synonym of Polystoma. The definite morphological differences between the two genera may be attributed to a high degree of plasticity very dependent on the ecology of the host. Finally, the morphology of chelonian polystomes was also studied in an attempt to partially resolve the generic paraphyly displayed by previous molecular studies. Several morphological features have proven valuable for separation of species occurring in one of the three microhabitats inhabited by these polystomes. The most important features included the respective shapes of the eggs and testis, and the number and sizes of the genital spines, hamuli, and marginal hooklets respectively. This study conclusively suggests that polystomes display a higher degree of morphological plasticity than previously suspected. However, the full extent still needs to be discovered.