Physiological and biochemical alterations in Littorina saxatilis infected with larval trematodes were investigated and compared with the metabolism of non-parasitized snails. Oxygen consumption rates of infected snails differed from those of non-infected controls in medium sized individuals (30 to 130 mg) but not in very large infected individuals (> 200 mg). Small snails (0.5 to 8.5 mg) were seldom infected by parasites, and this size-class consisted only of non-infected specimens. The specific oxygen consumption rate of infected snails was not dependent on their mass and remained constant over the size ranges investigated. Alterations in the snail metabolism appeared to be connected to injuries to digestive gland tissues caused by the parasites. The glycogen concentration and fatty acids of neutral lipids and phospholipids in the digestive gland were determined. Infected snails differed from uninfected snails in the complete absence of glycogen in digestive gland and had proportionally higher quantities of eicosenoic (20:1) acid in the total phospholipids. It remains unclear whether infection by trematodes activates enzymes in the snail's digestive gland to synthesize eicosenoic (20:1) acid, or whether the sporocysts themselves possess these enzymes. The role of phospholipid fatty acids in the regulation and maintenance of the parasite's metabolism is briefly considered. Biochemical alterations observed in the fatty acid composition may have an adaptive significance, by helping to stabilize the host-parasite system. KEY WORDS: Littorina saxatilis · Metabolism · Trematodes · Digestive gland · Phospholipids · Fatty acids Resale or republication not permitted without written consent of the publisherDis Aquat Org 60: [223][224][225][226][227][228][229][230][231] 2004 tems of Lymnaea stagnalis and cause castration and gigantism in this species (De Jong-Brink 1995). Trematode parasitism does not have a significant effect on the growth rate of Littorina saxatilis but stunts growth in Littorina obtusata, and enhances growth rate and size at maturity in Onoba aculeus (Gorbushin & Levakin 1999). Infected Littorina littorea either show reduced growth, or the parasites do not effect growth rates (Mouritsen et al. 1999). Co-adaptivity within the host-parasite system has been suggested as an explanation for gigantism in parasitized individuals of longlived snail species (Minchella 1985). According to Curtis et al. (2000), however, the oversized snails in long-lived Ilyanassa obsoleta were the result of greater age rather than of faster growth induced by parasites. Thus it is probable that the co-adaptivity theory is not always valid. Most likely, parasites have different effects on growth rate in different species.With regards to energy metabolism, several studies have shown that alterations in oxygen consumption rates in poikilothermic animals induced by parasitic invasion are inconsistent. For example,the presence of larval trematodes in Physa occidentalis induced higher requirements for oxygen in the host (H...

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Quantitative measures of feeding, respiration, growth and reproduction were brought together to achieve an overview of differences in physiology between 2 sympatric species of Anisus (Gastropoda, Planorbidae) in subarctic Lake Krivoye, northwestern Russia. This is a resource-limited lake with a short summer growing season where one might expect intense competition for resources among the numerous snails. Because coexistence is a multidimensional process, we have aimed to quantify bioenergetic variables related to several different major physiological functions. Using experiments that manipulated detritus and periphyton, we showed that both species consume detritus in combination with algae, but used different feeding strategies. Anisus contortus has a steeper growth allometry, greater specific growth rate and faster reproductive cycle. Anisus acronicus, however, has a faster absolute growth rate and longer life cycle, which results in its dominance in biomass and numbers. Although bioenergetics in general cannot be used to test directly for competition between closely related species, we have shown alternative growth and energy-metabolism strategies used by different species in a common habitat. Indicators of niche partitioning, maintained by balance calculations, included differences in preferences, quality and consumption rate of consumed food. We expect that these differences are not only important in proximal coexistence, but may also present alternatives for surviving long-term changes in the lake's productivity and habitat types.