BIOLOGY OF THE SEA-HOW THEY LIVE 29 



Youth, Maturity, Old Age, and Death 



"The interaction out of which the organism develops ... is between the 

 organism and environment! And the organism is different at each different 

 stage of its development." (Lehrman, 1953.) 



It is rather obvious to us the way environments change through the years 

 or with the seasons. It is also obvious that animals grow, but it is not very 

 widely appreciated what a drastic effect on the life of an animal growth has. 

 Animals start out life as a helpless egg, which soon hatches or develops into only 

 a slightly less helpless hatchling or newborn. This tiny creature goes through 

 a growth period in which increase in size is rather rapid through juvenile 

 and subadult stages. At the onset of sexual maturity, the growth rate levels off, 

 that is, becomes slower and, though growth continues past the onset of maturity, 

 it tends to be slower and slower as time goes on or even to stop. As old age is 

 reached, vigor is decreased, breeding stops, and some animals (man, for instance) 

 even shrink a little in size. Rarely, in nature, do animals die by "natural causes." 

 Predators, parasites, or disease usually spell the end. 



There are several fascinating questions that may be raised concerning this 

 cycle of life which every living thing goes through. Some forms of life such 

 as fishes seem never to stop growing. In most animals, aging does not begin 

 until growth ceases and no one seems to know how long fishes could live if 

 it were not for predators, parasites, and disease. Some fishes live one hundred 

 vears or more, but it is not known when they begin to grow old. But these 

 questions are not the main interest here. What is of the greatest significance 

 is that this cycle means that as an animal grows and matures, it must pass 

 through a series of stages, each one of which is different from the others. In 

 general, animals are most vulnerable in the beginning of the cycle when they 

 are small, and therefore the process of elimination of the individuals with the 

 greatest negative selective value is strongest in the young before maturity is 

 reached, a fact of evolutionary significance. Those individuals that do survive 

 do so only because they have been able to find food and elude enemies through- 

 out all stages of their life, and in this rather simple statement, we believe, lies 

 the greatest proof of the great delicacy and complexity of adaptation. For instance, 

 the jewfish is a huge, powerful, carnivorous animal when it is mature. But each 

 jewfish starts life as a tiny egg which hatches into a small hatchling. When the 

 voung fish starts to feed, it is a fraction of an inch long and must have all 

 the adaptive behavior necessary to allow it to live the life of a tiny, secretive, 

 invertebrate-eating carnivore. Soon it grows to a deep-bodied, little reef fish with 

 many of the habits of other small, spiny-rayed fishes that live alert, watchful 

 lives in rocks and reefs. As it grows, its prey and enemies and ways of life 

 continually change, and, at maturity, breeding behavior adds another facet to 

 its life. 



Therefore, every animal may be thought of as passing through a series of very 

 different stages each one of which must be finely adapted to the environment. 

 The greater the animal's difference in size between youth and maturity, the 

 greater and more significant will be its adaptive changes through life. This is 

 the reason why so many fishes have young that are different in so many respects 

 from their parents and are sometimes mistakenly listed as separate species. 



