P ALIGN=”LEFT”>NOW YOU KNOW WHY YOU CAN’T GET A BREEDING PAIR
One of the basic stumbling blocks facing aquarists trying to spawn their cichlids is the difficulty in getting a breeding pair. Trying to identify which is the male and which is the female is not always easy. Some species are dimorphic, with obvious males and females, usually due to coloration or shape. Others lead aquarists into frustration trying to determine if they have a pair or if they are just wasting fish food on two of the same gender.
Recent evidence is suggesting that it is not always the aquarist’s fault in not being able to identify gender in cichlids. Some cichlid species are known to be able to change gender as a result of environmental conditions around them. The Midas cichlid Cichlasoma citrinellum, for example, develops as a female by default but changes into a male during juvenile development based on its relative size to other juveniles in the brood [1]. Males are larger in this species.
This justifies the traditional advice that if you can’t buy a known breeding pair of cichlids, then get a half dozen young and let them sort themselves out as they grow up. This advice was actually based on probability odds, as out of six fish there is a very strong chance that at least one pair exists. Now one can see the other reason, for if gender can be altered during maturation, then at least one cichlid might switch to a male if all of them are default females to begin with.
Another study using Crenicara punctulata verified this, where it was confirmed that the dominant female in a batch of juveniles will become a male [2]. A female in isolation will turn into a male.
The tilapia Oreochromis mossambicus has its gender determined during its first ten days of life [7]. If during that time, the larvae are in cool water (20o to 24oC), most of them will develop as females. In warmer water (28C to 32C) they will be mostly males.
As anyone knows who reads the aquarium literature, there is all manner of conflicting advice and non-reproducible experiments in the hobby. It can be seen that part of the confusion on how to select and maintain breeding pairs comes about because the aquarist writing the article may not be aware that one or more of the fish has changed gender. It’s enough to make one take up stamp collecting instead.
GOOD BREEDING MATTERS
As stocks of wild-caught cichlids dwindle from habitat extinction, it becomes more important to maintain breeding stocks in captive populations. One problem is the loss of genetic diversity in captive stocks, and the accumulation of defective or sickly individuals. This is often blamed unfairly on inbreeding, but aquarists should realize that inbreeding is neutral. The reason that it gets blamed is because those doing the inbreeding fail to cull out the inadequate fry. Lethal or sub- lethal traits then accumulate, and the problem is then unjustifiably said to be ‘inbred fish’. All breeds of domesticated animals, whether they be Charolais cattle, German shepherds, or roller canaries, were initially established by inbreeding for desired traits. Unfortunately, too many fish breeders can sell whatever they produce no matter what garbage the fish are, which is why you see parrot cichlids and crick-back goldfish in pet stores. But it is not the fault of inbreeding per se, just the failure to select and cull.
Decline in genetic diversity is a more serious problem. It should be understood that low genetic diversity does not mean subnormal quality. Animals in a low-diversity population can be quite healthy and vigorous. However, when the environment changes, or a new disease sweeps through, high genetic diversity allows the species to survive no matter how great the mortality rate. Random variability of the genes ensures that at least a few of the individuals happened to have resistance.
In the aquarium hobby, it is therefore important to maintain high genetic diversity within a species population. The average aquarist cannot maintain large stocks of a species, though. This eliminates the idea of keeping genetic diversity high by the brute-force method of having thousand of cross-breeding individuals. Zoos get around this problem by regularly exchanging individual animals between themselves to keep the genes flowing between sub-populations.
A recent study on haplochromines out of Lake Victoria, carried out at the Ohio State University, showed that the best procedure to keep a cichlid population diverse is to periodically remove dominant males and to maintain more sub-populations [6]. Removing a dominant male after he has bred a few times will allow another male to get a chance to spread his genes about. Sub-populations are subject to random mutation of genes, which then have a better chance of establishing instead of being swamped in one large population.
Genetic diversity doesn’t just mean color patterns or shape. It also means resistance to disease or environmental shock. Behaviour is known to be an inheritable trait. There are many other traits invisible to the aquarist but which have a very real impact on the population.
AGGRESSIVE BEHAVIOUR
The reputation of cichlids for nasty behavior is more the aquarist’s fault than the fish. In the wild, the loser of a fight flees the scene and there are seldom fights to the death. In the aquarium, the loser can’t get away, which frustrates the winner and escalates what would have been a brief skirmish into deadly combat.
How do winners know they have won? Obviously, the loser backs off, but another reason is color change. Losers often change color and pattern to submissive or at least aggression- inhibiting colors. An example recently studied is the oscar Astronotus ocellatus, where the loser goes to near black body color with irregular white bars [3]. This sends a signal to the victor, and cools him down.
Victors of a fight have no reason to continue it out of spite, for injuries and wasted metabolism of food energy can be costly to them, albeit not as much as to the loser [4]. The winner of a fight must expend energy that could be used for other things such as spawning or building up food reserves. If a winner continues to beat up a fish in a tank, it is because it is not perceiving that it has actually won the contest. Depending on the species, it may expect submissive coloring from the loser or for the loser to leave the scene. Aquarists who have trouble with murder and mayhem in the tank should bear this in mind, and look to remedies such as netting out the loser. Don’t make life hell for both of them by leaving them jammed in together in a glass box that is obviously too small for them.
COLOR
Cichlids are the most speciose group of fish, particularly in eastern Africa. There has long been controversy as to why this would be so. The countless color patterns of Rift cichlids have been the obvious point of departure for enquiry, but experimental results have been confusing and contradictory. Wallace Dominey published an hypothesis in 1984 that speciation was driven by sexual selection, which is to say that females prefer bright colorful males of a certain pattern.
While this is believed to be true in general, there have been exceptions. A new study by a group of Dutch ichthyologists at the University of Leiden [5] suggests that the problem in understanding cichlid colors is that each species has two sets of colors and patterns evolving under different pressures. Nuptial or breeding colors are based on sexual selection, where the prettiest male gets the females. Vertical bars, however, are camouflage for structurally complex habitats such as rock piles. Horizontal stripes are associated with piscivorous feeding and/or shoaling behavior. Sexual selection has no effect on bars or stripes. This would therefore explain why previous studies have been at odds with each other, because they thought all three sets of patterns and colors were one.
REFERENCES
1] Francis, R.C., and G.W. Barlow (1993) Social control of primary sex differentiation in the Midas cichlid. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES USA 90:10673-10675
2] Carruth, L.L. (2000) Freshwater cichlid Crenicara punctulata is a protogynous sequential hermaphrodite. COPEIA 100:71-82
3] Beeching, S.C. (1995) Colour pattern and inhibition of aggression in the cichlid fish Astronotus ocellatus. JOURNAL OF FISH BIOLOGY 47:50-58
4] Neat, F.C., A.C. Taylor, and F.C. Huntingford (1998) Proximate costs of fighting in male cichlid fish: the role of injuries and energy metabolism. ANIMAL BEHAVIOUR 55:875-882
5] Seehausen, O., P.J. Mayhew, and J.J.M. Van Alphen (1999) Evolution of color patterns in East African cichlid fish. JOURNAL OF EVOLUTIONARY BIOLOGY 12:514- 534
6] Fiumera, A.C., P.G. Parker, and P.A. Fuerst (2000) Effective population size and maintenance of genetic diversity in captive-bred populations of a Lake Victoria cichlid. CONSERVATION BIOLOGY 14:886-892
7] Wang, L.H., and C.L. Tsai (2000) Effects of temperature on the deformity and sex differentiation of tilapia, Oreochromis mossambicus. JOURNAL OF EXPERIMENTAL ZOOLOGY 286:534-537 ?