CRUSTACEA AND MAN 1 6 1 



ment of such a stock, taking care to leave a few nauplii to reach 

 maturity and keep up the breeding strength. 



Some Crustacea are the direct cause of economic loss to man. The 

 fouling of ships by barnacles might at first sight seem a trivial 

 matter, but a heavy layer of fouling can increase, by about fifty 

 per cent, the amount of fuel needed to maintain a given speed. Such 

 increased costs of transport will be reflected in the increased price 

 of goods when they eventually appear for sale in shops. 



The important species causing fouling are Balanus crenatus in 

 temperate water, B. amphitrite and B. tintinnabulum in warmer 

 waters. As a general rule the tropics are the worst areas for the 

 fouling of ships; the polar regions are comparatively clean in this 

 respect. Settlement only occurs when ships are in port; when a ship 

 is moving the cypris larvae cannot establish themselves and the 

 hull remains clean. Natural control of fouling can be achieved by 

 avoiding long stays in tropical ports and by arranging that the 

 ship visits fresh-water ports, where the barnacles are killed. The 

 death of the barnacles does not get rid of their tough outer shells; 

 this can be done by visiting a scouring port. In such ports, for 

 instance Calcutta, the water carries a lot of sand which effectively 

 scours the side of the ship and wears down the barnacles. This 

 method has to be used with caution, obviously the scouring action 

 will not restrict itself to the barnacles, and too long a stay in a 

 scouring port may necessitate repainting the hull to resist corrosion. 



The painting of ships' hulls is done primarily to prevent corrosion 

 of the metal by sea water, and secondarily to reduce fouling by 

 poisoning the organisms concerned. Anti-fouling paints are de- 

 signed to liberate poisons in sufficient amounts to kill any cyprids 

 settling on the surface. Such paints cannot of course last for ever; 

 the best ones are those which liberate the poisons at a rate which 

 is only just sufficient for their purpose and so make for a longer 

 interval between repaintings. 



The poisons used in anti-fouling paints are often compounds of 

 copper or mercury. Mixtures of compounds of these two heavy 

 metals have been found to give better results than compounds of 

 each metal used separately. Organic compounds of mercury are 

 often much more toxic than its inorganic compounds; n-amyl 

 mercuric chloride is twenty times as toxic as mercuric chloride to 

 the nauplii of Elminius modestus. To some extent the toxicity of 

 organomercuric compounds is related to their solubility in fats: this 

 may be important, because the poisons appear to act by penetrating 



M 



