Marine Biology: Cultured Prawns

Japanese Ichthyologist Motosaku Fujinaga was still a senior in Tokyo University when he decided on his life’s work: a study of the life and loves of the 6-in., shrimplike creature known as the kuruma prawn. Dr. Fujinaga’s selection was more than an exercise in esoteric biology. Kuruma prawns are Japanese delicacies and are usually kept alive until the very moment when they are either deep fried as tempura or skinned alive and eaten raw as sushi.

Trouble is, demand has drastically diminished the kuruma supply. Japanese fishermen working home waters last year netted only 3,000 tons; another 4,000 tons were imported. But the imports were far from fresh by the time they arrived. The price of local prawns soared to $5 per Ib. Then, last week. Dr. Fujinaga announced that he was about to ease the culinary crisis. After 30 years of study, he has finally learned how to raise captive kuruma prawns in commercial quantities.

Brutal Female. Before his experiments began to pay off. Dr. Fujinaga had to go back to the beginning—he had to pry into the prawns’ most intimate secrets. For reasons known only to themselves, the little creatures mate only between midnight and 3 a.m. on perfect summer nights in calm, untroubled water. Night after night Dr. Fujinaga waded hip-deep in his experimental saltwater pond, wielding only a flashlight. Not until 1940 did he see the first prawn mating ever witnessed by man. “The ritual is truly bewitching,” he reported. “The male prawn first chases the female; then she molts, or undresses for him. The male next embraces the naked female, and she. in somewhat brutal fashion, absorbs his sex organ entirely, breaking it off. He is incapacitated until he grows a new one.”

His scientific voyeurism taught him little of practical value, and Dr. Fujinaga continued to spy on his prawns. After testing countless kinds of marine microorganisms, he found that during the first four days after hatching, larval kuruma prawns eat only microscopic Skeletonema costatum, a kind of diatom. When he learned how to grow his own Skeletonema in glass-covered tanks, his prawns survived their infancy. But Dr. Fujinaga could not manage to keep them alive longer than that.

In 1955, as head of Japan’s Fisheries Agency Research Department, he went to a fisheries conference in Washington. There he heard about brine-shrimp eggs, on which American fanciers feed finicky tropical fish. When he fed the eggs to infant prawns back in Japan, he brought them safely through infancy into reasonably hardy youth.

Plankton & Clam Larvae. In 1959, modestly financed by fisheries companies, Dr. Fujinaga set up a pilot prawn ranch in abandoned salt-evaporation ponds at Iku-shima on Shikoku Island. He now has 30 employees, and the place is jumping with prawns. The tiny just-hatched kurumas are coddled in indoor tanks and eat yellowish-brown Skeletonema plankton that have been grown in filtered sea water doped with chemicals. Other kinds of plankton, also specially cultured, carry them through the next stage. When they are one-quarter-inch long, they graduate to outdoor tanks and are fed clam eggs and larvae or brine-shrimp eggs. Then they move to the salt ponds, where they grow to delicious maturity on chopped trash fish and are fit for conspicuous consumption at elaborate geisha parties.