26 Routes from the Sea 



Southgate, & Bassindale, 1932). But Fraser (1932) at the mouth 

 of the Mersey River found the clam, Mya arenaria, only where sand 

 and gravel were mixed with mud. "Mud of a very liquid nature 

 apparently contains no fauna." 



Suspended materials are constant factors which influence the lives 

 of animals at the mouths of rivers. Estuarine animals often show 

 special adaptations which enable them to live in water which is 

 heavily charged with silt. Sessile animals must grow in length to 

 rise above accumulating sedimentary deposits. Robson (1925) states 

 that the frontal width of an estuarine crab (Carcinas) decreases as 

 water becomes more silty. Some estuarine species lack eyes; certain 

 crustaceans are colored like their relatives in the deep sea, which 

 also live on soft, muddy bottoms. River waters generally contain 

 more organic matter per unit of volume than the neighboring ocean 

 (Johnstone, 1908) . Some estuarine animals, such as oysters, depend 

 for a considerable portion of their food on small organisms carried 

 in flowing water. 



About the mouths of many streams in the tropics there are exten- 

 sive growths of mangroves. Such areas are particularly favorable 

 for various animals, which find food, shelter, and suitable conditions 

 for reproduction among the aerial roots and branches of these 

 peculiar plants. Though the mud in mangrove marshes is usually 

 soft and without oxygen, the plants furnish strata which are oc- 

 cupied by a variety of animals. Mangroves are not xerophytes, as 

 are many plants which are found in salt or brackish water. They 

 have a thicker epidermis in salt water; some species excrete salt 

 through the leaves and are thus able to absorb salty water without 

 raising the concentration in their tissues. In many ways mangroves 

 are adapted for life on the soft muddy bottoms of estuaries. 



Altogether, an estuary is usually a region where, though there 

 may be wide variations in environmental factors, there are more or 

 less extensive areas where marine, fresh-water, or brackish-water 



