SELECTION OF THE POND-BRED MOLLUSCIVOROUS CICHLID TREMATOCRANUS PLACODON (REGAN, 1922) FOR FOUR GASTROPOD SPECIES (THREE BULINUS SPECIES AND MELANOIDES TUBERCULATA) AND THEIR SIZES

Abstract

Pond-bred Trematocranus placodon (Regan, 1922), a molluscivorous cichlid in Lake Malaŵi, was studied under laboratory conditions to investigate its selection for different gastropod species of various sizes relative to their own sizes. This was done to evaluate its potential as an agent for the biological control of intermediate host snails. Bulinus globosus (Morelet, 1866) and Bulinus nyassanus (Smith, 1877), both intermediate hosts of Schistosoma haematobium in Lake Malaŵi, Melanoides tuberculata (Müller, 1774), the most abundant snail species in Lake Malaŵi and Bulinus tropicus (Krauss, 1848), a common species found outside the lake were used as prey. In addition to species preference, the relationship between its size and pharyngeal bone development in comparison with field collected T. placodon, and snail size preyed were investigated. Three different size classes based on total length (TL) of pond-bred fish were used. These were 70 – 80 mm TL, 100 – 110 mm TL and 120 – 130 mm TL. Bulinus globosus and B. tropicus were separated into three sizes, < 7 mm, 7 – 10 mm and > 10 mm shell height while < 7 mm, 7 – 9 mm and > 9 mm were the classes for B. nyassanus. Melanoides tuberculata, however, was tested using 5 – 10 mm, > 10 – 15 mm and 15 – 20 mm shell height classes. In both field and pond-bred T. placodon, there was a significant positive relationship between the standard length (SL) of the fish and the pharyngeal bone length (PBL), pharyngeal bone width (PBW), pharyngeal bone height (PBH), pharyngeal bone keel (PBK) and number of molariform teeth. Positive relationships between pharyngeal bone length, pharyngeal bone height, pharyngeal bone keel and pharyngeal bone width, vii and the number of molariform teeth were found. Given single snail species, pond-bred T. placodon preferred smaller snails (< 7 mm) to other snail sizes. A significant statistical interaction was found between fish size and snail size indicating that large fish consumed more large snails than small fish. Fishes of all sizes preyed on B. tropicus primarily and to some extent on B. globosus. Choice experiments in which fishes were provided with the four snail species of the smallest size group simultaneously showed that fish in the smallest size category preferred B. tropicus, while medium and largest fishes chose both B. tropicus and B. globosus. The study revealed that pond-bred T. placodon is able to eat snails probably due the development of pharyngeal jaws hence has a potential in schistosomiasis control in both natural and artificial water bodies.