Studies on the ecology of Lymnaea tomentosa Pfeiffer 1855 and L. columella Say 1817 (Mollusca : Gastropoda), intermediate hosts of the common liver fluke Fasciola hepatica Linnaeus 1758 in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Massey University
Studies on the ecology of Lymnaea tomentosa and L. columella were carried out to provide a basis for further studies on the ecology of Fasciola hepatica infections in New Zealand. Intermediate hosts of F. hepatica in New Zealand are the native Australasian species L. tomentosa, an introduced American snail L. columella, and the European host L. truncatula. The literature on the systematics, geographical distribution, general biology and ecology of these snails, together with their relationships with F. hepatica, is reviewed. Field observations on snail habitats indicated that L. columella occupied ponds and spring fed marshes which remained wet even in dry seasons. L. tomentosa was found in similar marsh habitats. Stability appeared to be an important quality of the habitats of both species; factors unfavourable to the snails included rapidly flowing water, marked seasonal fluctuations in water level, and shade from tall vegetation. The calcium content of water in habitats ranged from 4 to 70 ppm. Snails were more often found on flocculent than on firm mud and this preference was more marked in the case of L. tomentosa. Snail population dynamics were examined on one pond habitat and three marsh habitats of L. columella and a marsh habitat of L. tomentosa. Age structure and density of populations fluctuated widely between and within each of the five years of the study. These changes were greater in populations of L. columella and not directly related to rainfall or temperature variations. Field and laboratory evidence showed that both species could breed throughout the year even when temperatures were as low as 5°C but populations were much larger in summer and early autumn. Studies of the fecundity of the two snail species and the relationship between temperature and rate of development of eggs showed that whilst L. columella has a higher adult mortality rate it also has a much greater reproductive potential than L. tomentosa. Eggs of L. columella develop over a wider range of temperatures, hatching up to 34.5°C whereas temperatures above 30°C are lethal to eggs of L. tomentosa. Eggs of both species underwent some development at temperatures between 5 and 10°C but the proportion that hatched at low temperatures was very small, particularly in the case of L. tomentosa. Both species showed similar responses to desiccation on filter paper at 16.5°C and 80 to 90% relative humidity; survival time was closely related to shell length, with large snails considerably more resistant than smaller specimens. When snails were subjected to desiccation on mud results were much less predictable and the mean shell length of survivors was often less than that of snails which died. A biometric analysis of shell shape showed distinct differences between large specimens of L. tomentosa and L. columella but shell dimension ratios were not diagnostic in specimens under 4 mm in length. Examination of a pond and a marsh population of L. columella showed no differences in shell shape attributable to habitats. Over a 5 year period there was no detectable relationship between the intensity of F. hepatica transmission from a L. columella habitat and snail population density. The only correlation between F. hepatica transmission and rainfall was a possible inverse relationship between uptake in tracer sheep and December/January (mid-summer) rainfall. There was no evidence of any overwintering infection of F. hepatica infection in L. columella. Uptake of flukes by tracer sheep was almost totally confined to the period between mid-summer and mid-winter. Both experimental and circumstantial evidence indicated that pugging by cattle rendered marsh habitats more suitable for snails, although exclusion of cattle from the experimental area coincided with an increase in the uptake of F. hepatica by tracer sheep.