The life history and development of Amplicaecum robertsi, an ascaridoid nematode of the carpet python (Morelia spilotes variegatus). II. Growth and host specificity of larval stages in relation to the food chain

Abstract

Observations are reported on the life history of Amplicaecum robertsi, in relation to the mode of infection as it occurs under natural conditions. Some of the natural intermediate hosts are listed, as compiled from observations on natural infection with third-stage larvae in Queensland mammals. Various natural intermediate hosts were experimentally infected by means of eggs, and observations on the growth and development of the larvae are described.It was found that the second-stage larvae have a wide range of hosts, including earthworms, snails, fish, tadpoles, reptiles, birds, and mammals. Development to the third-stage was only observed to occur in birds and mammals. Third-stage larvae exhibited a wide variation in the degree of growth attained in different animals. In birds, growth was slight, not extending beyond 3–4 mm.; in mammals, growth varied considerably in different species; it was greater in the indigenous species, reaching 70–80 mm. No development beyond the third-stage occurred, except in reptiles.Second-stage larvae in the tissues of invertebrates and lower vertebrates could be transferred by feeding to reptiles, birds and mammals. Development proceeded in these hosts to the same extent as occurred following egg infection.Third-stage larvae could be transferred to certain snakes and lizards by feeding liver of infected rodents, but third-stage larvae over 3 mm. could not be transferred to mammals. Third-stage larvae were found to be uninfective at all stages of growth for birds.The third moult occurred in the carpet snake when fed with third-stage larvae in the liver of Trichosurus caninus, Rattus assimilis and Melomys cervinipes infected 1 month previously. This was in contrast to previous findings with laboratory rodents, in which maturation of the third-stage larva took considerably longer.Besides the carpet snake, the third and fourth moult was observed to occur in the goanna (Varanus spp.), the blue-tongued lizard (Tiliqua scincoides) and the bearded dragon (Amphibolurus barbatus). No growth in length was observed and no eggs were evident. As far as is known, none of these lizards is a definitive host of A. robertsi.The results are discussed in relation to the food chain involved in the natural habitat of the carpet snake. The snake is depicted as the apex of a food pyramid, whose base comprises a variety of animals ranging from earthworms to herbivorous mammals. It is concluded that A. robertsi has adapted its life history in such a way as to enable the parasite to ascend the pyramid by transference through a series of predatory episodes, which culminate in the infection of the natural prey of the snake.The life history is thus regarded, not as a life cycle, but as a life pyramid; development proceeds according to a pattern of diminishing host-specificity. Host-specificity is wide at the base of the pyramid, so that second-stage larvae occur in a wide variety of paratenic hosts. Host specificity narrows at the second moult which may occur in birds and mammals. It narrows still further in the third stage, because this larva, though it will survive in reptiles, birds and mammals, will not grow to a length at which it is capable of further development in the snake, except in certain mammals. At the third moult, host specificity shifts to certain reptiles, but becomes eventually restrictive to the carpet snake, because this host alone appears to provide a suitable environment for maturation of the eggs.This work was financed by a research grant from the University of Queensland. The writer's sincere thanks are due to Miss Ann Pritchard and Mr J. Ballantyne for their valuable assistance.