48 THE BIOLOGY OF THE SEA-SHORE 



require periodic exposure) and every few inches below the 

 limit of low neap tide adds numbers of species to the list, 

 provided that physical conditions remain suitable (Farran, 

 1 9 14). As far as actual numbers are concerned the floating 

 population (Plankton) very easily take first place. Although 

 only temporary inhabitants of the shore they must play a 

 very important part as food for the sedentary and sessile 

 members of the shore communities during their periods of 

 activity, when covered by the tide. Herdman (19 19) states 

 that during March or April some millions of Diatoms may 

 usually be obtained by hauling a small silk tow net for about 

 five minutes through about half a mile of the surface water 

 of the Irish Sea ; similarly during July-August tens of 

 thousands (up to 214,000) Copepods may be thus obtained. 

 Estuarine faunas show a marked paucity of species, as com- 

 pared with rocky shores, although the actual number of 

 individuals present may be large. 



The number of individuals of certain species occupying 

 quite a small portion of the shore is frequently enormous. 

 Thus a square yard of a Sahellaria colony has been estimated 

 by Herdman (1920) to contain nearly 30,000 worms (see 

 Plate V). The same writer has shown that one square 

 foot of rock may carry 2940 barnacles {Balanus balanoides). 

 The wall of a cave at the south end of the Isle of Man was 

 covered with the small red Ascidian Styelopsis grossularia at 

 the rate of over 50,000 to the square yard. Counts of other 

 common shore animals were made by the same writer, 

 and gave the following results : Patelluy an average of 29 

 to the square foot ; Purpura (two counts), 72 and loi to the 

 square foot. The number of beadlet anemones {Actinia 

 equina) found in rock pools averages over 46 to the square 

 foot. Some idea of the numbers of lug-worms {Arenicola 

 marina) present on a sandy shore may be obtained from the 

 view shown in Plate VI. Ashworth (see Herdman, loc. 

 cit.) estimated the number of these worms present between 

 tide-marks in a portion of beach about a mile in length at 

 3 to 4 million. 



The Animal Associations of a Rocky Shore. — The animals 



