The influence of population density on patterns and physiology of growth in Hymenolepis diminuta (Cestoda:Cyclophyllidea) in the definitive host

Abstract

The normal growth of Hymenolepis diminuta in its definitive host has been described by means of gross measurements of length and weight, by correlation of morphological development with growth, and by description of the changes taking place in the major chemical compounds through the prepatent period. The simultaneous study of the growth of worms from populations varying in intensity has shown the effects of crowding on the different growth phases, maturation, and chemical composition. The initial length of H. diminuta was 0.175 mm, and little growth was measured up to 24 hours postinfection, but the worms grew rapidly thereafter, increasing in length about 2.5 times during the first 48 hours. The period of most rapid specific growth in length was 1 to 7 days postinfection, after which occurred a phase of retardation until patency at 16 to 17 days. The effects of population density became apparent during this latter phase where a higher population density caused a lower growth rate. Characteristics of growth in fresh and dry weight were similar to those found for length, with similar effects of population density. A time scale of maturation was presented, and this was little affected by population size. The number of immature proglottids was unaffected by population density, the mature proglottids were reduced in number in worms from large populations, and the gravid proglottids were most severely affected. The effects on increase per worm of carbohydrate, protein, lipid, and phospholipid by population density were similar to those noted for length and weight. The effect of population size was greatest on the amount of carbohydrate per worm, and it was least on lipid. The carbohydrate concentration increased from 20 to 40% in worms from small populations, but little or no increase was noted in larger populations. The smaller percentage of carbohydrate in worms from crowded infections was partially made up by an increased percentage of lipids. “Protein” fell from 60–70% to about 30%, and the phospholipid fell from 10 to 3% in all populations. Calculation of the heterauxetic rate constants with relation to increase in dry weight showed that protein and phospholipid were bradyauxetic, carbohydrate was tachyauxetic and had a tendency toward isoauxesis with increasing population size, and lipids were bradyauxetic or isoauxetic with a tendency toward tachyauxesis with increased population. Among the most important conclusions emerging from this study are the following: (1) the earliest growth is the most rapid, (2) population effects appear in the retardation phase of growth, (3) the rate of maturation is essentially independent of the rates of growth in length and weight and the rate of proglottisation, (4) population density apparently has a direct influence on the germinative region, (5) crowding affects especially the carbohydrate and lipid concentrations in the worms, and (6) there may be some fundamental difference in the metabolism of worms from different population densities.