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Aim: Interspecific variation among metazoans often follows a latitudinal pattern, with species at higher latitudes being larger bodied than related species from lower latitudes (Bergmann's rule). For parasitic species, body sizes within any higher taxon are often correlated with the body sizes of their hosts (Harrison's rule). Whether temperaturedriven latitudinal effects or host-driven resource constraints act independently or additively to shape interspecific variation in parasite body sizes remains unknown. We take a comparative approach to test the effects of latitude and host body size on parasite body sizes in two taxa of parasitic worms showing convergent life cycles. Location: Global.Time period: Contemporary.Major taxa studied: Hairworms (Nematomorpha) and mermithids (Nematoda) parasitic in arthropods. Methods:With 223 records for mermithids and 258 for nematomorphs world-wide, we used linear mixed effects models to test the effects of latitude and host body size on parasite length, intraspecific length variation, parasite egg diameter and variation in egg diameter. Furthermore, we modelled parasite length with local mean annual temperature as the predictor instead of latitude, as a direct test of underlying mechanisms. All models took into account host and parasite taxonomic structure within the datasets.Results: For both taxa, host body size was clearly the main determinant of parasite body length, with neither latitude nor local temperature (annual mean or range) having an effect. No predictor affected intraspecific length variation, whereas egg diameter was positively associated with parasite length, and variation in egg diameter was negatively associated with latitude.Main conclusions: Our results support a strong role for host traits in shaping the evolution of parasite body sizes (Harrison's rule), but no role for latitude (Bergmann's rule), although these parasites infect ectothermic hosts. At a mechanistic level, the evolutionary driving force of external temperature on parasite physiology seems to be eclipsed by the availability of resources from the host.

Parasites can play a critical role in mediating inter-species interactions. Potential effects induced by parasites can range from species-wide traits to functional alterations in host community structure. One of the most intriguing host–parasite interactions pertains to adaptative host manipulation, an evolutionary occurrence where parasites alter the phenotype of their host to increase their own fitness. This study aims to address this phenomenon in a marine setting by investigating the effect of a philophthalmid trematode, Parorchis sp., on the vertical upward movement and phototactic behaviour of their intermediate hosts whilst simultaneously addressing the host specificity of these effects. These behaviours could impact the odds of trematodes successfully transmitting from their intermediate snail host to their definitive shorebird host, a crucial step in the life cycle of these parasites. Most trematode species exhibit strong specificity for snail hosts, typically infecting only a single snail species. In this rare system, however, the trematode infects a pair of sympatric and congeneric littorinid sea snails found in the intertidal zone of New Zealand’s rocky shores: Austrolittorina cincta and A. antipodum. Precisely, experiments were conducted in a controlled, laboratory-based setting, extending over a period of six weeks, during which vertical displacement and response to light were measured. Our results demonstrate that vertical upward movement amongst infected snails increased for A. cincta, but not for A. antipodum. No difference in response to light between infected and uninfected groups was evident across either species. Our findings highlight the complex nature of parasitic infections, where trait-specific behavioural effects are dissimilar across even congeneric host taxa.

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