Does the Infestation by Trematode Parasites Influence Trade-Offs Between Somatic Condition and Male Reproductive Traits in a Viviparous Fish?

Life history theory predicts that a trade-off may occur when an increased investment in one fitness component causes a reduced investment in another. Parasites generate changes in the optimal life history traits of organisms, causing compromises for their hosts. The objective of this research was to determine whether trematodes Clinostomum sp. and Uvulifer sp. inflict measurable damage on the males of the viviparous fish Poeciliopsis infans, generating trade-offs between somatic condition and reproductive traits (gonad mass and gonopodium length). We found (i) a negative relationship of large encysted metacercariae and non-encysted metacercariae on somatic condition. Interestingly, individuals with a higher parasite load had a worse body condition, but bigger gonads and longer gonopodium; and, (ii) a bond between small encysted metacercariae and non-encysted metacercariae with smaller gonopodia, only in fish with poorer somatic condition. The strongest correlation was given by the non-encysted metacercariae, probably due to mechanical damage during migration through the body cavity. We did not find any trade-off effects of Uvulifer sp. We consider that the statistical effects found on somatic condition can be attributed to a greater reproductive investment, generating energetic costs that compromise their defences against infection, allowing a greater parasite load.