Cichlidae is among the most speciose fish families in the world. Their diversity is especially wide in Africa. Zoologists are therefore understandably curious to know why this is so, and why the speciation happened so fast, since some of the great African lakes are less than one hundred thousand years old.
Among the obvious suspects is geographical discontinuity. Populations are more likely to differentiate into new species when they are confined to a small area and prevented from interbreeding with other populations. The barriers do not have to be absolute, as species will still develop even if there is a small amount of genetic interchange. This also applies to species developing as a result of specialising in different food types, where there can still be a bit of overlap in diet during speciation . A third method of speciation, demonstrated in Lake Victoria cichlids , occurs when gender preferences for colour displays drifts apart. One group of males tends to prefer a certain pattern displayed on another group of females, another group a different pattern, and so forth. The incipient species may still interbreed, but as time goes by they can drift apart genetically into distinct species, purely on the basis of mating habits, not food niche or isolation. This is not a case in which a new species suddenly occupies one certain area, or eats only a certain food that no other cichlid does.
One stumbling block faced by zoologists is that first the researcher must determine what species are in the study area. It is seldom the case that there are clear and distinct fish. More commonly a continuum of populations exists where the fish at either end are obviously different but there is a gradual transition in between. Aquarists will receive the extremes of wild fish from collectors and think of them as distinct, yet someone who sees them in habitat will not be so confident.
Labeotropheus fuelleborni is a Lake Malawi cichlid kept by many aquarists. A study published in 1999 showed that this cichlid migrates readily enough along rocky shorelines, but more than two km of sandy beach or deep water will intimidate it and act as a barrier to gene flow. The study was carried out along the Nankumba Peninsula shoreline at the south end of Lake Malawi . Males at the northwest end of the peninsula were darker blue with darker grey bars, while males at the south-east end were lighter in colour with stronger red-orange fin pigmentation. It can be seen that the fish are two extremes of a single species, but if isolated by geographical change long enough, they would eventually develop into distinct species.
In the aquarium, such extremes are often considered as two species. Against this, however, “interbreeding” in the tank has been common, especially where the females are drab and easily confused. A lost cause of the American Cichlid Association is its valiant fight to eliminate cichlid hybrids from the hobby. The danger of hybrids is that by muddying the gene pool they make it impossible to reconstruct the original species by line breeding. This never used to matter because aquarists could start over again with freshly imported specimens from habitat. Now in some cases those specimens are gone, because the wild species has been wiped out. The most prominent examples of this are the Lake Victoria cichlids, exterminated by the introduction of Nile perch into the lake.
Cichlids have a general reputation for being aggressive in the home tank, but I think this unfair. In habitat, aggression seldom leads to fatal consequences because the loser can flee the area. This is not possible in the aquarium. The two most common reasons for this are that the tank is too small and that the aquascaping allows one or a few fish to dominate the entire tank.
Just how much room is needed can be estimated from studies such as one carried out on Lamprologus ocellatus, a snail-dwelling cichlid. The male keeps a harem of females. He prepares empty snail shells, keeping one or two open and burying the others nearby for future use. The largest female will dominate the harem and will be quite intolerant of other females. In habitat, the females of a harem will establish themselves about 90 cm apart . One can see that this would not be possible in the average home aquarium, but aquascaping to block females’ views of each other could allow a small harem. Otherwise the tank may only support a single pair of fish, rather than a harem. Whatever the case, it is easy to see that in habitat the snail cichlids have room to spread out, but in a small tank they end up killing each other because the losers can’t get away. From this is then made the claim that “cichlids are aggressive”. Confine six aquarists in a small utility room and see if they don’t become aggressive as well!
In the converse, however, it will not pay a cichlid to have too much territory, more than it can reasonably defend. A study on convict cichlids (Cichlasoma nigrofasciatum) showed that large cichlids defend food patches against smaller convicts, and experience a growth surge because of the better food supply. The food patch can be so large that the owner spends more time defending it against intruders than enjoying the benefits. As a result, the growth rate of the convict declines because it is using up too much energy . In habitat, cichlids must obtain an optimal sized territory. Too small, and it won’t provide a living. Too big, and it will be indefensible. The cichlid must strike a balance between food and energy expended in defence. By extrapolation, in the aquarium, fish which spend most of their time defending territories are burning up energy which could be used for breeding. Something to think about when stocking a tank or wondering if it should be culled.
RAISING THE FRY
It is not unusual for parental cichlids to be guarding fry of other parents, or even other species. Some cichlids, such as the Tanganyikan Microdontochromis, actually release their fry into other broods not even the same species . Lamprologus ocellatus females are known to accept other fry if they are not too dissimilar and are smaller than hers . Smaller, because large fry will cannibalise small fry if the size difference is too great. For those aquarists who want to maximise their output of spawns, this suggests that fry should regularly be culled and sorted on the basis of size, to keep each group at about the same average size and thereby avoid cannibalism.
One might expect parents to resent such unwanted foster care, but, without being able to read the thoughts of a cichlid, there seem to be some reasons why this is tolerated. To pick out individual fry in a swarm is a difficult energy-consuming task, one which would also attract the attention of predators who would swim over to see what all that thrashing is about. (Predators are often attracted to thrashing because that may indicate a prey in trouble and thus easier to take.) The dilution effect reduces the chances of a parent’s fry being taken by a predator. If, for example, one-third of the cloud of fry are not the parent’s fry, then the odds are one-third that a predator will snatch someone else’s kid.
Mouth brooding cichlids normally don’t eat while carrying fry. This is not an absolute rule, as Microdontochromis is known to be able to feed and brood simultaneously . The parent feeds on zooplankton floating through the water, and so do the brooded fry, thus enabling them to grow bigger and faster.
Aquarists dealing with a newly-obtained cichlid for which they are unable to find information can often determine what type of food it eats and how it eats by looking at the mouth structure. Aquarists often forget that it is important to know not only what type of food the fish eats, but how it eats. A mid-water feeder will ignore the algae growing on a rock, while the algae scraper depends on that food source. Algae biters have stronger jaws, such as the Lake Victorian haplochromine Neochromis .
Quality of the food makes a difference not only to the general health of the fish but to its success in territorial defence and breeding ability. It is common in many species for the female to chose a more brightly coloured male, all else being equal, because the pigmentation of fish depend on their diet. The better the diet, the brighter the colours. The brighter the colours, the better the male must be as a father. It isn’t just humans who judge by image!
This was demonstrated by a study on male fire-mouth cichlids (Cichlasoma meeki) where half the test fish were given a high-carotenoid diet and the other half a low-carotenoid diet . The more carotenoid, the brighter the red pigment of the fish. The brighter the fish, the more likely it was to win aggressive interactions with other males.
1] Arnegard, M.E. et al (1999) Population structure and colour variation of the cichlid fish Labeotropheus fuelleborni Ahl along a recently formed archipelago of rocky habitat patches in southern Lake Malawi. PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON 266B:119-130
2] Yanagisawa, Y., H. Ochi, and A. Rossiter (1996) Intra-buccal feeding of young in an undescribed Tanganyikan cichlid Microdontochromis sp. ENVIRONMENTAL BIOLOGY OF FISHES 47:191-201
3] Bouton, N., F. Witte, J.J.M. van Alphen, A. Schenk, and O. Seehausen (1999) Local adaptations in populations of rock-dwelling haplochromines (Pisces: Cichlidae) from southern Lake Victoria. PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON 266B:355-360
4] Brandtmann, G., M. Scandura, and F.Ttrillmich (1999) Female-female conflict in the harem of a snail cichlid (Lamprologus occellatus): Behavioural interactions and fitness consequences. BEHAVIOUR 136:1123-1144
5] Praw, J.C., and J.W.A. Grant (1999) Optimal territory size in the convict cichlid. BEHAVIOUR 136:1347-1363
6] Genner, M.J., G.F. Turner, S. Barker, and S.J. Hawkins (1999) Niche segregation among Lake Malawi cichlid fishes? Evidence from stable isotope signatures. ECOLOGY LETTERS 2:185-190
7] Seehausen, O., J.J.M. van Alphen, and R. Lande (1999) Color polymorphism and sex ratio distortion in a cichlid fish as an incipient stage in sympatric speciation by sexual selection. ECOLOGY LETTERS 2:367-378
8] Evans, M., and K. Norris (1996) The importance of carotenoids in signaling during aggressive interactions between male firemouth cichlids (Cichlasoma meeki). BEHAVIORAL ECOLOGY 7:1-6 ?