The growth of knowledge and research practices in any discipline is characterised by a trade-off between depth and breadth: we can either invest efforts to learn a little about many things, or learn a lot about few things. In parasitology, breadth of knowledge corresponds to research on biodiversity and taxonomy: the discovery and description of an increasing number of new species. In contrast, depth of knowledge comes from focused research on a few model species, about which we accumulate much detailed information. Breadth and depth of knowledge are equally important for progress in parasitology. In this essay, focusing on trematodes, I demonstrate that current research is rapidly broadening our knowledge (high rate of new trematode species being discovered) but not deepening that knowledge at a comparable rate. The use of model species, with caveats, appears to offer a promising avenue for deeper knowledge. I present a case study illustrating how it is possible to develop new model trematode species at low cost to increase the depth of our understanding in areas including host-parasite ecological dynamics, co-evolution, and responses to environmental and climatic changes. The take-home message serves as a call to action to parasitologists, emphasising the need to focus as much effort on depth of knowledge as we currently invest in breadth of knowledge.

New, well-known and predicted life cycles for trematodes of the Haploporoidea (Haploporidae and Emprostiotrematidae) and three families of the Lepocreadioidea (Enenteridae, Gorgocephalidae, Gyliauchenidae) involve encystment of the metacercaria in the open (usually on vegetation) followed by ingestion by a range of herbivorous or detritivorous fishes. These life cycles appear among relatively highly derived plagiorchiidan trematodes in which three-host life cycles incorporating an animal second intermediate host are dominant. We hypothesise that the two-host life cycles in the Haploporoidea and Lepocreadioidea arose by secondary truncation of a three-host cycle; the second intermediate host was lost in favour of encystment in the open. Modification of a three-host life cycle effective for the infection of carnivores is consistent with the understanding that fishes arose as carnivores and that multiple lineages have secondarily become detritivores and herbivores. Four of the five trematode families involved infect fishes relating to multiple orders, suggesting a complex history of host-switching. In contrast, the Gorgocephalidae, the smallest of the families, has been found only in a single family, Kyphosidae. The timing of the evolutionary events leading to this putative life cycle truncation is yet to be deduced, but the rich developing understanding of the history of the fishes creates a strong template for future analysis.