32 UNDERWATER GUIDE TO MARINE LIFE 



The limits of distribution are complicated greatly by the factors of vagility, 

 or the relative power of dispersal of an animal. Sea turdes swim actively and 

 so are found in all tropical seas— they have high vagility. However, many of the 

 groups common to the coral reef communities of the Indo-Pacific are rare or 

 lacking in the east Pacific because their powers of vagility are not great enough 

 to allow them to cross the huge and rather barren central Pacific. Many sessile 

 animals, such as coral and sponges, have pelagic larvae which increase the 

 vagility of the species. Mackerels migrate and have high vagility. Many sedentary 

 fishes or fishes of coral reefs have pelagic eggs or young which serve to spread 

 them to new habitats. In fact, it is relatively common to find young reef fishes 

 far from their parental homes, but rare to find adults of these species far from 

 the tropics. Many Caribbean reef fishes have their distributions listed "north 

 to Cape Cod." This northern extention of range is due mainly to wandering 

 young in the Gulf Stream. Of course, pelagic eggs and wandering young 

 represent wasteful methods of dispersal because many never find suitable habitat 

 and die. For instance, many Caribbean fishes and corals are not found in 

 Bermuda because the wandering young do not live long enough to cross the 

 wide expanse of open sea between these two places. Other methods of dispersal 

 are adventitious (incidentally acquired), for example, sessile animals on ships 

 and driftwood or whale barnacles attached to whales. In summary, the greater 

 the vagility of a species, especially when vagilitv is bv swimming free of the 

 influence of currents (nektonic animals) the more widespread a species is liable 

 to be. 



An important factor influencing distribution and dispersal concerns barriers. 

 Barriers are like fences which keep animals from moving out of an area. Bar- 

 riers are rather obvious on land and consist of rivers, mountains, plains, or 

 deserts that a species can not cross. But barriers in the sea are harder to see 

 and much more variable. Barriers in the sea are also usually of a less severe 

 nature than on land so that most marine animals have wider distributions than 

 are common for terrestrial animals. The most obvious barrier is a land mass. 

 For instance, the isthmus of Panama prevents most Caribbean species from 

 reaching the east Pacific. Land masses separate the major divisions of the sca- 

 the oceans. Other barriers are salinity, temperature, and oceanic currents. How 

 severe a barrier is depends a great deal on the vagility and tolerance of the 

 animal. For instance, powerful swimmers like tuna can skirt around such bar- 

 riers as land masses, a feat a blenny could not accomplish. Therefore, tunas 

 have wide ranges and blennies small ones. 



Barriers seem to the human eye to be of a rather permanent nature, but this 

 is not true. Barriers vary widely in geologic time. The earth's temperature is 

 an example. Not too many million years ago Panama lay beneath the ocean's 

 surface. Because of this impermanence of barriers, distribution is influenced by 

 a third factor besides valence and vagility. This is the geologic age of a group. 

 A family of fishes that has existed for 75 million years has obviouslv had more 

 time to spread itself throughout the seas than a family that has existed for 10 

 million years. This is why the squirrel fishes, derived from relativelv ancient 

 stock, are found in all tropical seas, whereas the grunts of the genus Haemnlon, 

 relative Johnnies-come-latclv, arc found onlv in the New World. 



Two more factors concerning distribution must be understood. First, when 

 the range for the green turtle is given as "cosmopolitan in tropical seas," this 



