NATURE ADRIFT 



Sheer numbers of indicator species, although labelHng the water mass the 

 more certainly, do not necessarily mean a proportionate increase in the vol- 

 ume of water being transported. They may do so, because the greater the vol- 

 ume of water the more slowly it will be affected by outside influences and the 

 favourable conditions will last all the longer. They may, however, be due 

 simply to an unusual abundance of the species at the source, or conditions 

 mav be favourable enough for the organism to reproduce whilst in transit. 

 It is sometimes possible to distinguish between the few older individuals 

 that have themselves been carried all the way, and the more numerous young 

 ones born en route. 



Some species are useful indicators in spite of a wide tolerance because 

 their origin is controlled by some other factor. A good example is the 

 medusa of Obelia (Plate XII). This is a very tolerant form, but is always 

 associated with a coastal origin because the hydroid stage in its life history 

 (p. 85) is coastal, whether an open Atlantic seaboard such as the west of 

 Ireland or the Hebrides, or perhaps in a very different type of water, such as 

 the coast of Kent. Similarly the larval stages of shore crabs and acorn bar- 

 nacles, and the empty cast skins of the barnacles, indicate coastal associations 

 of the water in which they have been. 



A small cladoceran (Fig. 19) Podoti polypJieiiioidcs can be used as an indica- 

 tor of Nile flood water as far as the coast of Israel. Turtles from Mexico, 

 Colopochelys keiiipi, were drifted to the west of Ireland in 1928 and 1934, and 

 it is probable that other turtles, Caretta caretta, found occasionally are also of 

 Mexican origin, as they are not known to breed in south European seas. 

 Leathery turtles, Dcniiochelys coriacea, reached the northern North Sea and 

 close to western Norway in 1956. 



Deep-living species can be used to indicate places of upwelling. Hosts of 

 examples from aU over the world could be given. 



There are five main water masses in the neighbourhood of the British 

 Isles and each can be distinguished by its plankton content. So as not to make 

 this section too cumbersome only a few indicators are mentioned here as 

 examples, some of them are depicted in the figures associated with Chapters 

 4 and 5. 



I. The local water of the North Sea, Irish Sea and similar local areas, 

 (Plate XXXVII) with: 



Sai^ittii sctosa arrow-worm 



Lahidoccrn wollastoiii 1 , 



^copcpods 

 hi as ciaviceps j 



Tinia hairdii 

 Etitoiiia ill diem IS 



zA 



medusae 



142 



