OF SOME COMMON MARINE ANIMALS. 257 



and weighings at Port Erin, but the results vary greatly with the amount 

 o£ moisture retained by the Algre. The general conclusion, however, is that 

 a very large amount of organic food must be present in the region of the 

 coarse brown sea-weeds, and especially in the Laminarian zone, and therefore 

 it is not surprising that shoals of young fishes are found feeding there. 



ZOSTERA. 



The '" Grass-wrack " (Zostera marina) is a flowering plant allied to the 

 sedges, which grows in muddy sand at and below low-water mark. It is 

 enormously abundant in certain places, and in the South of Australia I have 

 seen banks of it several feet high extending for miles along the sandj- 

 beach. Petersen says that about 2000 square miles are covered by Zostera 

 in Danish seas, that in the Kattegat alone it amounts to 24 million tons, 

 and that it produces annually four times as much dry vegetable matter as 

 all the hay on the land of Denmark. It is evidently .the fundamental plant- 

 food for fishes and other animals in the Danish seas, and no doubt ife serves 

 the same important purpose on a smaller scale in our seas. 



There is a small Zostera bed in Port Erin Bay, and three patches at the 

 mouth of Port St. Mary Harbour which are just accessible by wading at the 

 lowest spring-tides, and there are much larger beds elsewhere further north 

 on the shores of the Isle of Man. 



A Zostera bed always supports a large fauna or epifauna associated with 

 its roots, rhizomes, and leaves. Characteristic animals are the anemone 

 Anthea cereus (■= Anemonia sulcata), various species of Trochus, Littorina 

 and Lacuna, some nudibranchs, many worms, zoophytes, polyzoa, and 

 compound ascidians, especially the transparent Diplosoma. In addition, 

 there are delicate filamentous Algse (Confervaj) and enormous masses of 

 Diatoms attached to the older withered or decaying leaves. Even after the 

 Zostera leaves have decayed and gone to pieces, they contribute to the 

 important organic detritus on the bottom, upon which many creeping 

 animals on the surface of the sand are nourished. So that both directly and 

 indirectly, through the organisms it attracts and supports, the Zostera bed is 

 an important source of food to fishes and to many invertebrates. 



The long blades or leaves, which may extend from 3 to 5 or G feet above 

 the sand, arise in shoots from a creeping rhizome, and in my counts I find 

 from 4 to 6 leaves in a shoot and about 20 shoots in an area of 3 inches by 

 1 inch — that is, about 1000 shoots in the square foot. The waving forest so 

 produced, clothed in turn with many other organisms large and small, is one 

 of the densest masses of living food-matter that I know of in the sea. The 

 value of the Zostera bed in nature, both directly from the food that it 

 furnishes to the animals living on it, and also indirectly from the enormous 

 quantities of Diatoms which cover its decaying leaves, is a part of the 



