USING Trematocranus placodon AS A BIOLOGICAL CONTROL AGENT OF SCHISTOSOMIASIS SNAIL HOSTS

A breeding program for Trematocranus placodon, a Lake Malawi molluscivorous cichlid that can be used as a biological control agent of snails in schostosomiasis control programs, was initiated at Lilongwe University of Agriculture and Natural Resources in 2003. Initial experiments using hatchery-bred fish in snail control revealed that snails were quick to sense the presence of the predator or predation on their conspecifics in laboratory settings, which improved their protective ability. An experiment was conducted to quantify the behavioural responses of snails in the presence of cues from the predator (T. placodon) or killed conspecifics.

Sixty Bulinus tropicus snails were put in each of the nine 30-litre glass aquaria. Bulinus tropicus was used because it is not a host of Schistosoma haematobium, a parasite that causes Schistosomiasis. There were three treatments: no chemical cues were applied (treatment 1), cues from killed conspecifics  (treatment 2) and cues from the predators (treatment 3). Each treatment was replicated thrice in a completely randomized design. Chemical cues from killed conspecifics were prepared by killing five snails after administering an anaesthetic and leaving them in a 1 litre beaker of water for an hour. The water was then added to Treatment 2. Chemical cues from the predator were prepared by putting five 40.32 ± 0.27 grams fish in 30 litre tanks for 24 hours and removing them just before the snails were put in. The aquaria had half of the bottom covered with stones. Twenty snails were then put in each of the nine aquaria. After one hour, the number of snails that were on the open bottom, hiding under stones and outside the water was recorded.

The result showed a significant (p-value = 0.0001 ) association between application of cue and behavioural response of Bulinus tropicus. Cue application resulted in snails moving away from the open water. Hiding behaviour was higher when the snails sensed the cues from killed conspecifics than from the predators. We suggest that ues from conspecifics may have alarm signals to alter snail behavioural response.