16 UNDERWATER GUIDE TO MARINE LIFE 



environment for life found on earth so it is not surprising that the greatest 

 diversity of hfe lives there. The tropics are characterized largely by endemism, 

 that is, a great many groups live there and nowhere else. All phyla of the animal 

 kingdom are represented. High temperature is the basic reason for this, but other 

 factors enter in. For instance, there is an abundant supply of food in the tropics 

 so nourishment is not usually a major problem. 



Not only are the tropics characterized by endemism, but animal groups seem 

 to be more diverse there than anywhere else. The most bizarre shapes and habits 

 and the greatest variation in behavior within any group are usually present in 

 tropical members of that group. A review of the fishes adequately reveals this fact. 



The tropics are further distinguished by the presence of the lowest tides 

 and the clearest water of all seas. They are the home of the coral reefs and 

 dazzling white coral sands. 



Tropical seas are divided into these three regions: the Indo-Pacific, the 

 Adantic, and the East Pacific, in order of their richness of species and size. 

 Many animals and plants such as the hammerhead shark, all the sea turtles, 

 and many bony fishes are found in all three, that is, they are circumtropical. 



Atlantic Tropics. These waters extend north to southern Florida and thence 

 to Bermuda. The whole of the Gulf of Mexico is included. The warm Caribbean 

 and Gulf currents characterize this region. There is good coral growth here, 

 probably richest in the Bahamas, but these are neither generally as lush nor as 

 extensive as those of the Indo-Pacific. The sub-tropics of the Atlantic extend 

 up the eastern coast of Florida to Cape Hatteras. There is very little coral 

 growth here because of low temperatures and the lack of the hard bottom to 

 which coral must anchor (Ekman, 1953). 



East Pacific Tropics. Tropical waters reach north to the lower tip of Baja 

 California on the west side of that peninsula, and the whole of the Gulf of Cali- 

 fornia on the east is tropical. The subtropics reach north to San Diego. Upwelling 

 of deep bottom water cools almost the whole of this zone rather drastically at 

 times, and the warm waters are rather shallow in extent. Even in the tropics, 

 water of 54° Fahrenheit lies only 600 feet down. The lower limit of the tropical 

 water is a shallow 300 feet. 



This is a distinct zone which bears a few resemblances to the Atlantic tropics, 

 but which, because of the isolation forced on it by the barrier of the Isthmus of 

 Panama and the depauperate central Pacific, has many endemic groups and 

 lacks several tropical groups completely. Ekman (1953) lists the kelp basses, 

 Paralahrax, as endemic and says that porgies have only one species, butterfly 

 fishes two species, while parrot fishes, common eels, and manatees are completely 

 lacking. 



TEMPERATE SEAS 



These are the waters of the greatest temperature variation. Temperatures do 

 not generally rise above 70° Fahrenheit, but they do sometimes reach as 

 much as 80°. However, these zones, being subject to wide seasonal variation, 

 may have temperatures only a little above freezing to the north in winter. 



The temperate zones show a wide variety of bottom types from rocky to 

 sandy or muddy. The shores are often covered with dense beds of brown algae, 

 which are scarce in the tropics. 



