CALORIFIC VALUES IN THE PHYLUM PLATYHELMINTHES: THE RELATIONSHIP BETWEEN POTENTIAL ENERGY, MODE OF LIFE AND THE EVOLUTION OF ENTOPARASITISM

¹ Department of Zoology, University of Glasgow, Scotland, U. K
² Department of Pure and Applied Zoology, University of Leeds, England, U. K.

1. A comparison has been made of the calorific values (kcal/g ash-free dry weight) of 21 species of flatworms which exhibit, between them, the various life styles found in the phylum Platyhelminthes.

2. The grand mean calorific value for whole animals of all 21 species was 5.841 ± 0.230 kcal/g ash-free dry weight, which is close to the mean of 5.821 kcal/g ash-free dry weight reported for whole animals from a wide range of other phyla.

3. There is a direct relationship between mode of life and calorific value. Free-living flatworms have a mean value higher than the mean for whole animals in general, while entocommensal and entoparasitic species other than cestodes have lower values. The ectocommensal, ectoparasites and cestodes have values close to the mean.

4. In the cestode H. diminuta the values for immature and mature proglottids (regions comparable to whole flatworms from other classes) are less than the mean for whole animals from other phyla and approach those for the entocommensal and entoparasitic flatworms. Gravid proglottids have a higher mean.

5. The entosymbiotic flatworms, with high fecundity relative to free-living species, thus conform to the hypothesis that fecundity is linked with low potential energy per unit weight.

6. The dichotomy in food reserves in the Platyhelminthes, with storage of lipid in free-living species and of carbohydrate in the symbiotic forms is considered, therefore, to be basically an adaptation to different reproductive patterns necessitated by different life styles.

7. It is suggested that the emphasis on carbohydrate as an energy source, rather than lipid, pre-adapted entosymbiotes for partial or total anaerobic respiration. The relationship between high glycogen content, low environmental oxygen tension and modified respiratory physiology occurring in some entoparasites is thus proximate rather than ultimate in nature.

8. The evolution of entoparasitism is seen as a logical conclusion to the trend toward the formation of symbiotic relationships, which is a characteristic feature of the Platyhelminthes, in that it removes the need for long-term reserves and thereby allows total emphasis on progeny production. It follows that the parasitic habit, and especially entoparasitism, provides conditions necessary for high fecundity so that the latter can be regarded as a direct and automatic consequence of the parasitic mode of life, rather than as a specific adaptation to it.