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Dicrossus filamentosus: There's Something Fishy Going On
|By Ted Judy, 1997.|
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(This article was originally published in "The Southern Colorado Aquarium Society Journal" 11/97. It is here reproduced with the permission of author Ted Judy).
Tragedy strikes. Four hours after releasing five Dicrossus filamentosus into their new home, the male decides that the fishroom floor looks nicer than the painstakingly planted tank. I am beside myself. Checkerboard cichlids are not all that easy to find. I had to wait several months to get this group. I think that this situation qualifies for one of Charlie Grimes "Murphys Fish Laws". All the fish in the group are young adults, with the male being full tailed and very colorful. The females are plump and very red in the pectoral fins. The water is soft and acid. The worm and daphnia cultures are going strong. This group is supposed to be the group that finally produce baby checkerboard cichlids in my fishroom. I guess that a few more months are not too long to wait for a new male.
Fast forward two weeks. I do not give the checkerboard females a lot of attention since they are awaiting a new male. I am feeding the little darlings and counting heads when I notice that three of the four look just as before, but one of the females looks a bit different. She is not as black as the other three. Her fins are no longer red, and her tail and dorsal fins are getting a lot of iridescent spangles. She is very aggressive towards the other females. A few days later she looks even less like a female and is starting to look a lot like a male!! She has a tail that is starting to fork. The black squares on her flanks are gone. There is a lot of blue iridescence and she is vigorously defending a territory with the help of the next largest female. The definite female is beautiful! Her checkerboard has turned to solid black stripe and her pectoral fins are nearly blood red. The two are stubbornly defending a piece of bog wood from the other two females.
Two days go by before eggs are laid. Now the once female is obviously a male. The eggs turn out to be viable, and the new male continues to change until he is as pretty a male checkerboard cichlid as I have ever seen. What has happened? Was the new male a cross-dresser from the very beginning? Time to do a little research.
Many fish species, the vast majority of which are marine reef fishes, are hermaphroditic. A hermaphrodite is an organism that has all the parts that they need to perform the reproductive role of either the male or the female. Many invertebrate organisms are hermaphroditic. Snails and earth worms, for example, can copulate with any other individual that they meet. This is a real advantage. Imagine how fast a population could grow if every individual in the group can produce eggs and any other individual can fertilize them. When two earth worms copulate they both go away with fertilized eggs! Fish are not quite that advanced as hermaphrodites.
There are two types of fish hermaphrodites that have been identified: protogynous and proteandrous. Protogynous hermaphrodites have populations in which every individual starts out life as a female. When the fish reach sexual maturity the strongest and most aggressive individual becomes the functional male. This phenomenon is well documented in marine fish such as wrasses, groupers, anthias and angelfish. Protogyny is also the type of hermaphroditism that my Dicrossus filamentosus seem to exhibit.
Protogyny is very common in marine reef fishes. The classic example involves the Caribbean bluehead wrasse, Thallosoma bifasciata. In this species the fish live in colonies of one male with a large group of females. The largest and strongest of the group is always the male, often called the super male. His job, reward for being the only breeding male, is to defend his harem from predation. He must meet all challenges. When the challenge is too great he is the one predated upon. When this happens the next largest fish in the colony, the largest and most aggressive female, will become the next male. The male breeds with all of the females. This is beneficial to the population because it guarantees that the strongest fish is a part of every breeding event.
The first freshwater fish to be identified as a protogynous hermaphrodite is Crenicara punctulata, a close relative of the checkerboard cichlid. The species of the genus Dicrossus were, until very recently, in the genus Crenicara. A German zoologist named Dietrich Ohm worked out the mode of hermaphroditic reproduction in C. punctulata. Professor Ohm kept groups of 4 to 10 young fish together in small tanks. Males in the groups became dimorphic from the rest of the group as the groups matured. Professor Ohm conducted an experiment by removing the males as they appeared, and noting how long it took for another male to mature. He found that when he left the initial male in the group, no other males appeared. When he removed a male, no matter how long he left the male in the tank after it had matured, the most aggressive female would become the next male. No tanks had two males develop at the same time.
Proteandrous hermaphrodites are the opposite of protogynous hermaphrodites. In proteandry all of the fish are born male and later turn into females. The most noted example of this group are the marine anemone fishes. This sex change was discovered by professor Hans W. Fricke while studying the anemone fish Amphiprion bicinctus in the Red Sea. Professor Fricke was studying the complex social organization of anemone fish. This species lives in large groups in a symbiotic relationship with a sea anemone. The entire population is broken down into territories associated with individual anemones (which exist in large colonies). Each territory is lorded over by a breeding pair of anemone fish and a few non-breeding sub-adults. Anemone fish do not have any external sexual differences. This is a case of the female is the one that lays the eggs.
Professor Fricke began to experiment with the social behavior of the anemone fish by removing specific individuals from a territory, and then observing what happens. When he removed the breeding female, the breeding male would take over the female role and be able to produce viable eggs within 24 days. When he removed the breeding male, the next most dominant non-breeding fish in the group courted the female and became the breeding male. When he removed one of the non-breeding fish there was no affect on the breeding pairs behavior. Fricke suggests that the reason that this method of mate selection evolved is that it is a response to the limited number of anemones in the colony. If all anemone fish try to pair and breed during breeding seasons, some of them will be forced off of their protective anemones. The anemone fish population benefits if every fish is not able to breed every season, since evicting fish from the territories will result in predation of the sub-dominant fish.
Freshwater proteandry is very rare. The fish that can accomplish the feat do so only under very specific circumstances. Hans Peters, another German scientist, discovered that many mbuna from Lake Malawi have the ability to switch from male to female. Microscopic examination of the gonads of male mbuna show that there are primitive eggs lying in stasis within the sperm producing testes. Damage to the testes from trauma or disease might wipe out the fishs ability to be fertile as a male. The eggs, in some cases, can then take over, produce hormones that will affect the way the fish appears, and cause the male to become a female. These new females are probably not as reproductively successful as a normal female, but a sex change has taken place.
There is one other type of hermaphroditism that is expressed in at least one fish species. This is parthenogenesis: an individual, always classified as a female, that can produce viable eggs without the need for fertilization by another individual. A brackish water killifish from the sub-tropical and tropical Atlantic coastlines of North, Central and South America has this distinction: Rivulus marmoratus. This diminutive killifish is not much to look at, but its method of reproduction is amazing to see. A single fish, that can be kept in very modest accommodations (such as a jar), will parent a large number or viable offspring, each genetically identical to its mother.
Sexual determination in non-hermaphroditic fish is not as cut and dry as it is with humans. We have a set of chromosomes designated the sex chromosomes. This pair of chromosomes either have two of the same chromosomes (female), or one each of two different types (male). Fish often have sex genes that exist on chromosomes other than the sex chromosomes. Sexual determination is based upon the balance between the number of female sex determinants and male sex determinants in the fish's entire genome. If there are more male genes that female genes then the fish will be male, and vice versa. Environmental conditions have been shown to play a role in sex determination, or in the appearance of secondary sexual characteristics. Pelvicachromis pulcher is well known as a species of West African dwarf cichlid that can have the sex ratio of an entire spawn skewed by pH. Behavioral response to aggression in cichlids, especially dwarf cichlids of the genus Apistogramma, can suppress the expression of secondary sexual characteristics in sub-dominant males. Apistogramma species that get sneaker males, fish that look like females but breed like males, in groups that have a dominant male in them are an example of this phenomenon. Not everything is as cut and dry as fancy guppies.
I have always wanted to work with Dicrossus filamentosus in order to have the experience of working with a species that is known as a troublesome breeder with very specific requirements. My attention is focused on more than just the water chemistry and live food selection now. Discovering that they are hermaphroditic only enhances my enjoyment of the species. I hope that I will get opportunity to work with some of the other hermaphroditic species in the future. Next time a male of some poorly understood species takes a dive out of a group of females in your fishroom, watch carefully. Maybe you will not have to wait several months for a new male.
|Dicrossus filamentosus female in the aquarium. Photo by Ad Konings.|
© Copyright 1997 Ted Judy, all rights reserved
Judy, Ted. (April 29, 1998). "Dicrossus filamentosus: There's Something Fishy Going On". The Cichlid Room Companion. Retrieved on May 22, 2013, from: http://www.cichlidae.com/article.php?id=91.