WHEN a school of reef fish loses its single male, the largest female begins acting like a male within a few hours and will produce sperm within 10 days. Some other species repeatedly switch back and forth between the production of eggs and of sperm during a single mating. Among deep sea fish that only rarely encounter potential mates, reproduction is often possible only if one changes sex.
Such opportunistic sex changes in fish, once thought to be a rare oddity, are proving far more common than supposed. Conversions from female to male are now known to occur in species belonging to at least 14 families, while conversions from male to female are known in eight families.
Nevertheless, the process by which fish change sex remains a mystery to scientists. ‘‘No one has ever come up with an answer’’ as to how they do it, Dr. Klaus D. Kallman of the New York Aquarium, a fish geneticist, said recently.
Nevertheless, according to Dr. N. Robin Liley of the University of British Columbia, the sex of a fish or of a reptile is known to be more susceptible to change than that of a mammal or bird. As examples, the sex of a maturing reptile may be determined by environmental temperature, while a school of trout can be made entirely male simply by adding a certain hormone to the water.
Dr. Kallman said a fish’s original sex seems to be genetically determined by inherited chromosomes. All of the species that change their sex, he said, have evolved from those whose sex was fixed for life. It is therefore a capability that evolved, presumably because it made for greater reproductive success.
These new discoveries regarding sex changes in fish have been summarized by Dr. Robert R. Warner of the University of California at Santa Barbara in recent issues of American Scientist and the journal Evolution. Dr. Warner and Dr. Eric A. Fischer of the University of Washington, assisted by local Kuna Indians, have been studying reef fish mating behavior at the San Blas Field Station of the Smithsonian Tropical Research Institute in Panama.
The process whereby both fish repeatedly switch back and forth between male and female behavior is known as simultaneous hermaphroditism. Dr. Warner sees it as a device to prevent ‘‘cheating’’ by either partner.
It is as though one fish says ‘‘I will give you four eggs to fertilize if you, then, will give me four eggs,’’ Dr. Warner said in a recent telephone interview. The fish whose turn it is to release sperm places itself above the other, its body cupped to catch the buoyant eggs. The partner, having only partially discharged its eggs or sperm, is not tempted to wander off in search of another mate. This switching also helps to equalize the energy demand, Dr. Warner said, because egg production is more energy-consuming.
In his studies of the bluehead wrasse, a common reef fish, Dr. Warner said he has found that mating behavior depends on the size of a reef and the density of its population. On small reefs with less than 200 wrasses, large males stake out their own spawning area, taking advantage of their size to defend it. In one day as many as 150 females may visit such a nest to deposit their eggs and have them fertilized.
Apart from these dominant males, most of the fish on so small a reef are, or become, females because that is their best chance of reproducing. Small males account for less than 20 percent of the population. They grow twice as fast as males on a large reef, however, diverting energy from reproduction efforts in order to quickly become large males.
On a large reef with more than 400 fish, large males find it difficult to prevent ‘‘sneaking’’ by small males who dash in and deposit sperm in the midst of the dominant male’s courtship. Being small is therefore less of a disadvantage.
Furthermore, in parts of the reef swarming with wrasses of both sexes, the smaller males collectively and indiscriminately deposit their sperm within the milling crowd of females. As many as 100 males may participate.
On so large a reef these smaller males account for 25 percent to 50 percent of the population. Their gonads become large enough to represent 20 percent of their body weight. Such outsized reproductive organs probably evolved, Dr. Warner said, because males releasing the most sperm into the melee are the most likely to reproduce.
A very different situation may exist close by, where small parrotfishes live in beds of sea grass. Short-range visibility prevents large males from dominating, so small males can reproduce effectively and in some species no sex changes appear to occur.
Harems Convert to Male
Dr. Warner’s colleague, Dr. D. R. Robertson, has studied a species of wrasse that sets up ‘‘cleaning stations’’ on the reef to which other fish come to have the wrasses clean their gills, mouth and skin. Each station contains a male wrasse that fights off intruders and five or six females with whom the male mates daily.
There are virtually no small males in these populations. When the male dies, one of the harem converts to masculinity. Harems are common among wrasses, angelfishes, parrotfishes, damselfishes, basses and gobies.
The initial male-female ratios in populations of such fishes are unknown, according to Dr. Warner. No sex determinations have been made of the tiny drifting larvae from which the fish develop. Those fish that become dominant males may either mature from juvenile males or convert from females.
Males Become Females
Conversions from male to female, known as protandry, are less common than the reverse switch, or protogyny. But they occur, for example, in the small damselfish that live under the protection of a stinging anemone. Such colonies have been studied in the Red Sea and Indian Ocean by Hans and Simone Fricke of the Max Planck Institute in Seewiesen, West Germany.
The fish, who seem immune to the anemone poison, typically form a group consisting of a large adult female, a smaller adult male and several juveniles. If the female is removed the male becomes female and one of the juveniles, apparently relieved of some form of growth repressor, matures into a functional male.
Dr. Douglas Y. Shapiro of the University of Puerto Rico, studying one species of bass (Anthias squamipinnis) in the Philippines, has found that their schools seem programmed to maintain a specific number of males. Forty-eight social groups were studied, the smallest consisting of two males and 13 females and the largest of 50 males and 294 females, all adults.
The two sexes are easily distinguished by color, behavior and reproductive organs, all of which change when they alter their sex. As many as nine males were removed from each group and an equal number of females then became male. They did not, however, begin doing so simultaneously but at intervals of about two days. All told, after 58 males were removed, 57 females changed sex.
The changes are presumably under hormonal control. Dr. Liley is trying to trace the process by recording the levels of blood-borne hormones in fish at various stages of a sex alteration.
Fish carry many of the sex hormones found in mammals, although their roles are not identical. They have been converted into males by injecting a variant of the human male hormone.
Dr. Liley has collected and frozen blood samples from reef fish off Belize in Central America and brought them back to Vancouver. His analysis has just begun and no results are yet available, he said.
