1 64 A BIOLOGY OF CRUSTACEA 



Crustacea also play a part in the biology of water supply. 

 Reservoirs are often ideal places for the growth of small planktonic 

 algae, some of which can cause the water to become coloured, or 

 to taste unpleasant. Fortunately the algae are usually kept in check 

 by small planktonic Crustacea, particularly the Cladocera. These 

 in turn can cause a certain amount of trouble when they block the 

 filters at waterworks. There have been occasions on which a water- 

 works engineer has been heard to curse Daphnia longispina and 

 Bosmina while frantically searching for an alternate supply of 

 water to the suburban areas for which he is responsible. The 

 Cladocera had choked his filters and suburban man would be 

 watering his garden that evening, making heavy demands on the 

 supply. 



Onchocerciasis will serve as a final example of the indirect ways 

 in which Crustacea impinge on the affairs of man. The causal 

 organism of this disease is a nematode, Onchocerca, which lives 

 under the skin of man, and has the distressing tendency of migrat- 

 ing into the eyes, eventually causing blindness. Over a large part 

 of Africa this nematode is transmitted by the bite of a fly appropri- 

 ately called Simulium damnosum. The aim in controlling the 

 disease is to eliminate the fly by killing off its larvae. This can only 

 be done when the habits of the larvae are known; they live attached 

 to stones in swift flowing rivers, but in the hilly part of Kenya, near 

 Lake Victoria, the disease is transmitted by another species of 

 Simulium, S. neavi, and for many years the larva of this species 

 could not be found, although all the likely places were searched by 

 competent entomologists. Eventually, it was discovered that the 

 larvae habitually attach themselves to the backs of river crabs. 

 These crustaceans had innocently thwarted the efforts of medical 

 entomologists throughout the years of searching, and delayed the 

 beginning of control measures designed to save many people from 

 blindness. 



LITERATURE 



Anderson, A. M., Headly, D. & Tubb, J. A. (1953). Notes on the 

 fisheries industry of Brunei Bay and Labuan Island. Proc. Indo- 

 Pacific Fisheries Council. 4th Meeting, Sect. II, pp. 146-165. 



