Marine biologists have been dumbfounded by the diversity of methods used by sharks to bring forth their young. Even closely related species depend on very different techniques. Some sharks nourish their young internally through a primitive form of placenta. Others produce single eggs of huge dimensions.

Mr. Springer is currently studying a shark of the genus Centrophorus that produces a single egg the size of a softball. Its nutrients are sufficient to enable the embryo within that egg to grow to one-third the length of its five-foot mother before being born.

Other species produce small eggs but enough of them to provide food for the more successful embryos. One species of tiger shark may give birth to 80 pups at a time. Some sharks bear egg cases that drift in the sea until the babies hatch, like the ''mermaids' purses'' (skate egg cases) sometimes found on beaches. Horn shark egg cases occur in a variety of exotic spirals, some with twisted appendages. The whale shark egg case is larger than a basketball.

In a review article on shark reproduction in Oceanus, journal of the Woods Hole Oceanographic Institution, Dr. Gilbert has told how two families - the requiem sharks and hammerheads - use a placenta, in the form of a yolk sac, to nourish their young after the pups have exhausted the food supply in their eggs. As in placental mammals, such yolk sacs transfer wastes from the offspring's bloodstream to that of the mother and carry nourishment in the opposite direction.

Dr. Gruber, in addition to his research on threshers, has been keeping track of free-swimming lemon sharks as part of his behavioral research. Some 1,500 of them have been tagged in Florida waters and 90 of these have already been recaptured.

Dr. Gruber is also working to develop a powerful new shark repellent and ways to test its effectiveness. The Shark Chaser developed under Navy auspices during World War II proved of little value, but in the 1970's it was found that a Red Sea fish, the Moses sole, exudes a substance that repels at least some shark species.

Sharks have been seen to charge a Moses sole with open jaws, only to stop within inches of the fish. A strong dose of the active substance, called pardoxin, may cause the shark to behave erratically or curl on the bottom of a test tank, belly up. The substance, a chain of 162 amino acids, is difficult and costly to synthesize. It quickly deteriorates and is not as powerful as might be desired.