Small worms, big appetites: Ratios of different functional morphs in relation to interspecific competition in trematode parasites

Animals living in colonies or collectives composed of highly-related individuals often produce morphs that are physically and behaviourally specialised to perform specific tasks. Because such morphs are often sterile, their production represents a fitness cost for the colony and there should be an optimal ratio of the numbers of sterile specialists and reproductive members that may be adjustable to environmental conditions. Trematode parasites undergo asexual multiplication within their snail intermediate host, resulting in large numbers of clonal stages known as rediae or sporocysts, depending on the trematode species. In areas with high prevalences of infection, the host can be infected by multiple species, which can lead to intense competition for limited resources. Here, we describe the existence of specialised 'mini-rediae' in the trematode 'Philophthalmus' sp. that are morphologically and functionally specialised for interspecific competition. Mini-rediae were observed feeding on the sporocysts of a co-occurring trematode species - 'Maritrema novaezealandensis'. In addition, in larger snails - which are less likely to have 'M. novaezealandensis' infections - 'Philophthalmus' sp. produces relatively fewer mini-rediae than expected. Our findings support results from a prior study which demonstrated the existence of morphs that perform specialised functions in antagonistic interspecific interactions in trematodes, and additionally shows that the number of these morphs in each host is associated with the likelihood of encountering other species within the same host. Trematodes may thus provide interesting models for studying morphological specialisation in colonial organisms.