Summary Despite the strong impact of ultraviolet (UV) radiation on invertebrates, it is unknown whether it affects immune function across metamorphosis. More generally, the mechanisms on how larval stressors bridge metamorphosis and shape adult fitness in animals with a complex life cycle remain poorly understood. We studied whether cuticular melanin content is upregulated under UV exposure in the larval stage of the damselfly Coenagrion puella and whether this is traded off across metamorphosis against a key component of the invertebrate immune response, the melanotic encapsulation response, in the adult stage. Larvae exposed to UV increased the melanin content in their exoskeleton and metamorphosed later and at a smaller mass than animals reared without UV. Across metamorphosis, this was associated with a reduced melanotic encapsulation response, thereby constituting the first proof for a UV‐driven impaired immune response in an invertebrate. The demonstrated costs of UV exposure in terms of age and mass at metamorphosis and reduced adult immune response likely translate into reduced adult fitness. Path analysis indicated that the immunosuppressive property of larval UV exposure was not mediated by age and mass at metamorphosis, but instead that the adult immune response was traded off against larval cuticular melanin investment. Melanin‐based trade‐off across metamorphosis provides a new pathway by which effects of larval stressors are carried over to the adult stage and thereby advances our understanding of the still largely enigmatic mechanisms of carryover effects of larval stressors across metamorphosis. Given the mechanistic basis, this carryover effect of larval UV exposure on adult immune function is expected to be general and may constitute a widespread and important cost of UV exposure in invertebrates.

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Freshwater snails act as intermediate hosts (IH) for schistosomiasis, a tropical disease affecting over 200 million people worldwide. Despite their medical importance, an extensive understanding of IH snail ecology remains absent. Especially data on the tolerance limits to different abiotic factors is fragmented and incomplete. Consequently, the construction of accurate species distribution models to identify snail habitats and guide targeted snail control efforts remains difficult. Here, we compiled a summary on the tolerance limits to abiotic factors of African IH snails of human schistosomiasis. A systematic search on Web of Science, Pubmed, and Embase identified 45 relevant studies. Synthesis of these studies indicates that research efforts differ greatly between IH snail species, life stages and abiotic factors. The importance of each abiotic factor in determining snail presence and abundance is discussed. Furthermore, attention was drawn to knowledge gaps and the lack of standardised experimental designs, which impedes comparisons between studies. This in turn prevents us from making firm conclusions and calls for best practices adopted by all malacologists. In doing so, IH snail ecological data could serve as a basis to assess schistosomiasis risk and guide snail control efforts in order to support schistosomiasis control.