NATURE ADRIFT 



rich micro-fauna associated with the dock water, which is essentially the 

 same as that outside. In the Mersey estuary, however, the mud is constantly 

 being churned up by strong tidal currents and chokes the filtering mechanism 

 of any squirts that try to grow on the tug. 



Fouling not only occurs on ships' bottoms, piers and the like, but inside 

 pipes. Here, indeed, some of the worst fouling occurs as conditions are 

 usually ideal. The pipes, particularly the stationary pipes to shore installations, 

 are usually handling the same kind of water, and the water in which the larvae 

 lived is also right for the adults. The constant stream of water into the pipe 

 brings a continued supply of oxygen and food. Mussels and other fouling 

 organisms grow bigger and so reduce the effective diameter of the pipe until 

 it is eventually blocked (Plate XXXIII). At times a mass of mussels may break 

 away and be carried along the pipe to jam valves or choke the smaller 

 subsidiary pipes. This is especially liable if the current is suddenly increased, 

 e.g. during a fire, and the blockage then occurs when the water is most 

 wanted. If the mussels are killed by poisoning, the dead shells remain and 

 the trouble is not relieved, and the only solution is the expensive dismantling 

 of the pipes. The settlement cannot be prevented by filtering off the planktonic 

 larvae as these are too small to be retained by any filter that will allow a reason- 

 able water-flow to pass. The only preventative is the constant addition of 

 a poison such as chlorine to the pipe for the whole period of the 

 planktonic stage, and as different species breed at different seasons this is, in 

 effect, the whole year. 



To return to the main subject of this chapter — we find that temperature 

 is not only an important factor in seasonal distribution but it is also responsible 

 for determining the limits of the geographical distribution of many species. 

 Just as on the land there are some cold-living species, but many more that 

 need warmth, there are planktonic species confined to tropical and sub- 

 tropical seas. They include many exotic species that are a real joy to the natur- 

 alist, but to the fish or other animals looking for food on the whole less of a 

 joy than are the less bizarre species of the temperate and colder waters. If the 

 cold-water lovers can withstand life in the cold, dark waters of the deep oceans 

 they are found there also, and are then usually found in the upper layers of 

 water in both the Arctic and the Antarctic. If they cannot, then they are 

 usually confined to cither one pole or the other, though no doubt they could 

 live at either given the chance to get there in sufficient numbers to start a 

 successful reproduction. 



Then there arc other species whose habitat is confined to the depths, and 

 here it is not a temperature-controlled factor as these species arc not found in 

 the surface polar regions. They form an important part of the world's total 

 plankton as 84 per cent of the sea area is deeper than 2,000 metres — indeed 



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