Trematode infections in frogs: Do all cercarial morphotypes infect and induce effects on the common hourglass tree frog, <i>Polypedates</i> <i>cruciger</i>?

Abstract

Trematode infections are known to induce malformations and increase mortality in amphibians. However, not all the larval trematodes induce effects on the host. Nine types of cercariae (A-I) belonging to five morphotypes (oculopleurolophocercous, distome, gymnocephalous, xiphidiocercariae and pleurolophocercous) shed by the freshwater snail, Melanoides sp. were tested. Tadpoles of the common hourglass tree frog Polypedates cruciger at 10 days post-hatch (Gosner stage 27/28) were exposed to the cercariae in a mesocosm experiment. First exposure was carried out to identify which morphotypes induce effects on the host and later the exposures were repeated using selected cercariae, who induced effects on the host. Survival, growth and development of malformations were recorded in 15 and 30 days post-hatch tadpoles and metamorphs. Of the nine cercariae types, only three types., gymnocephalous (Type C) and two pleurolophocercous types (H and I), induced effects on the host with a single exposure. The repeated exposures showed that the cercariae type H significantly reduced the survival of tadpoles (4 replicates; Chi square test, X2 = 15.172, p<0.001) but not the cercariae type I (7 replicates; X2 =2.105, p= 0.147). Both cercariae types induced axial malformations such as kyphosis and scoliosis. The exposures incurred no changes to the size of tadpoles, but types H and I prolonged their growth period. Cercariae type C caused open wounds in the belly area of the host. This type of malformations has not been recorded in frogs before. However, cercariae type C was not shed by any of the snails collected later and therefore replicates were not carried out to confirm its effect. The results show that only some cercariae types induce effects on the frog host, possibly the cercariae types that use the tadpoles as the intermediate host to enter the definitive host and to complete its life cycle. Previous studies have reported two cercariae types induce frog malformations, hence collectively four cercariae types have demonstrated trematode induced effects on P. cruciger.

Keywords

• trematode infections
• amphibians
• malformations
• <i>polypedates</i> <i>cruciger</i>