Fish Tales
In the human mode of reproduction we come into this world either male or female and, short of some drastic surgical intervention, are destined to remain so throughout our lives. But the situation is much different in other animals. In some species, individuals start off life as males and later change to females, or the other way around. In others they are true hermaphrodites, playing the male and females roles simultaneously. In a few species there are, alas, no males, the females producing eggs that develop into new individuals without the benefit of fertilization by sperm. Ethologists have shown that each of these patterns of reproduction is advantageous in some way to the animals that have them. I would like to describe just three piscine examples here that I hope will be of interest to you.
The first example occurs in the blue-headed wrasse (Thalassoma bifasciatum) that lives on coral reefs of the western Atlantic. The blue-headed males are brightly-colored with blue heads, as the name suggests, separated from their greenish bodies by two vertical black bands. They defend breeding territories on the reef. Females, which are yellowish in color, choose the largest and most brightly colored males to breed with so the largest males may spawn more than 40 times a day during the breeding season while small males have little, if any, breeding success. Under these conditions it would pay an individual to start off life as a female, when it can breed, and then change into a male later on when it becomes larger. This, indeed is just what they do.
It has been shown that this sex change is socially controlled. Thus, if the large, territorial males are removed from a reef the largest females change into males to take their places. The sequence of events is somewhat different on large and small reefs.
On large reefs there are many females present so that each male can attract a large number to his territory. This makes it possible for small males to occasionally sneak in and spawn with a female before the territorial male can intervene. Also, small males sometimes swim around in gangs, harassing a female until she spawns with them. Under these circumstances some individuals enter the population as small males (called primary males) and remain so all their lives. The large males, who started life as females are called secondary males.
On small reefs, there are only a few wrasses and so there is little chance for a small male to successfully sneak into a territory to mate with a female. Here all individuals start off life as small females and all the males are secondary males having changed sexes when the previous dominant male was removed. This is understandable since under these conditions of intense competition only the largest males are successful and starting off life as a female can enhance an individual's reproductive success. This situation where individuals start life as females and later change to males is called protogynous hermaphroditism.
The second example involves clown fish, or anemone fish (Amphiprion akallpisos), that live in close symbiosis with sea anemones on coral reefs in the Indian Ocean. The fish live in monogamous pairs since there is usually only enough room for two fish on each anemone. In this case the reproductive potential of both fish of a pair is limited by a female's ability to produce eggs since even a small male can produce more than enough sperm to fertilize all her eggs. Under these circumstances it is most advantageous for the female to be as large as possible since larger females lay more eggs than small ones. As one might suspect clown fish start off life as males and only become females later in life when they are larger. Here again, sex change is socially controlled. A male changes into a female only when his female partner is removed and he is joined by a smaller fish. This is called protandrous hermaphroditism.
The third example, which involves the apex of sexual flexibility, occurs in some groupers, wrasses, and sea basses. When two of these fish meet during the breeding season one individual, usually the larger, takes on the female role while the smaller plays the part of a male and actively courts her. After they have shed their eggs and sperm into the water the two fish almost immediately switch roles, and after an appropriate courtship, spawn again. The advantages of this system seem obvious especially where the individuals are solitary, distribution is patchy, or populations are spreading into new areas.
All this raises an interesting question: Why doesn't hermaphroditism occur in other vertebrates particularly where size confers a significant advantage on one sex? In sea elephants, for example only the largest males are able to breed while the younger, smaller ones are completely without success and don't even try. Why don't they start out life as smaller females who can breed and then change into males when they get large enough to compete successfully? The answer probably lies in the observation that all the species of fish that are known to change sex have simple reproductive systems and external fertilization. Mammals such as sea elephants, on the other hand, have the more complex reproductive systems necessary for internal fertilization, gestation, and lactation. The investment of time and energy needed to change sexes is probably too great under these circumstances and can be better spent in growing and learning the techniques needed to successfully defend a harem.
Sexual reproduction is a fascinating study for ethologists and the students of evolution. Nobody knows how it arose in the first place, although there are many theories, and there is much discussion as to what its advantages might be.
References:
Photo: Cliff, Kashmere H.S.
J.R. Krebs and N.B. Davies: An Introduction to Behavioral Ecology (3rd ed). Blackwell Science, Ltd, 1993.
E.O. Wilson: Sociobiology. The Belknap Press of Harvard University, 1975.