For many trematode species, individual reproductive parthenitae in first intermediate host colonies senesce, die, and are replaced by newly born parthenitae. The times involved in these processes are poorly understood. Here, we present an approach to estimate parthenita death rates and lifespans that uses readily obtainable data on senescent parthenita frequencies, brood sizes, and offspring (cercaria) release rates. The onset of parthenita senescence is often marked by the degeneration and disappearance of the germinal mass, its source of new offspring. Following germinal mass loss, the remaining viable offspring in a senescent parthenita finish development and are birthed before parthenita death. Therefore, a senescing parthenita’s remaining lifespan is the time it takes for all its viable offspring to mature and exit. We can estimate this time by measuring whole-colony (infected snail) cercaria shed rates, dissecting colonies to count reproductives, and then apply the per redia cercaria production rate to the observed brood sizes of senescent parthenitae. The per-capita parthenita death rate is then calculated as the proportion of parthenitae that are senescent divided by their average remaining lifespan. Reproductive parthenita lifespan is the inverse of this death rate. We demonstrate the approach using philophthalmid trematodes, first providing documentation of a free-floating germinal mass in 4 philophthalmids, and then, for 3 of those species, estimating parthenita senescence rates, death rates, and lifespans. This method should be broadly applicable among trematode species and help inform our understanding of trematode colony dynamics, social structure, and the evolution of parthenita senescence.

In a range of trematode species, specific members of the parthenitae colony infecting the molluscan first intermediate host appear specialized for defence against co-infecting species. The evolution of such division of labour requires that co-infection entails fitness costs. Yet, this premise has very rarely been tested in species showing division of labour. Using Himasthla elongata (Himasthlidae) and Renicola roscovita (Renicolidae) infecting periwinkles Littorina littorea as study system, we show that the size of emerged cercariae is markedly reduced in both parasite species when competing over host resources. Cercarial longevity, on the other hand, is negatively influenced by competition only in R. roscovita. Season, which may impact the nutritional state of the host, also affects cercarial size, but only in H. elongata. Hence, our study underlines that cercarial quality is, indeed, compromised by competition, not only in the inferior R. roscovita (no division of labour) but also in the competitively superior H. elongata (division of labour).