Acanthobothrium quadripartitum, a tapeworm parasitic in the intestine of Raja naevus, is not found in the closely related R. radiata. The tapeworm survives in vitro for over 24 h in fresh serum from the natural host, but fresh serum from R. radiata is extremely toxic; about 80% are killed within 2 h with the formation of a characteristic external precipitate. The toxicity of fresh R. radiata serum was abolished by procedures known to inactivate complement and restored by the addition of fresh serum from the natural host R. naevus. This observation suggests that both species of ray possess a similar complement system in fresh serum but only R. radiata contains a natural serum antibody toxic to A. quadripartitum. No toxicity could be demonstrated in R. radiata mucous in vitro, probably due to the serum factors being present in low concentrations.
The results are discussed in relation to the observed inability of A. quadripartitum to survive in the intestine of R. radiata.