All About Malawi Cichlids

Last year I had to do a mini-dissertation for evolution, I decided to do mine on "The Evolution and Speciation of Cichlids in Lake Malawi" I just realised that some TASA people might really enjoy some of the stuff so I'm going to post it here. Warning! Some of the stuff in this article is not intended for the average fishkeeper so you may not understand it Some of it is very basic though, so enjoy.

Index

Introduction......................................................... 3
Lake Malawi........................................................ 4
What is a Cichlid?................................................ 7
The Origin of Cichlids..........................................10
Feeding specializations.........................................11
Breeding................................................................12
Speciation..............................................................16
Origin of Speciation..............................................18
Is sexual selection driving cichlid speciation?......19
Malawian Cichlid Taxonomy................................22
Conclusions............................................................23
References..............................................................25












Introduction

“No lake in the world contains such a diversified and distinct community of cichlid fishes as Lake Malawi†(Ad Konings, 2007). Lake Malawi currently has about 1000 different species of cichlids (Turner 1994), boasting a fish species biodiversity diversity larger than any other lake in the world. There are over 500 endemic species already identified, this is more than all the freshwater fish in Europe and North America put together; and new species are still being discovered.
Lake Malawi is one of the three Great East African lakes that are found in the depression known as Africa’s Great Rift Valley. Each of the lakes (Lake Malawi, Lake Victoria, and Lake Tanganyika) has many fascinating aspects and unique characteristics that make them an interesting and exciting topic for all nature enthusiasts.
For many years Cichlids have been the most colourful freshwater fishes known (Ad Konings, 2007); the large diversity in colour and behaviour of cichlids added to their hardiness and willingness to breed in captivity, has made them extremely popular to aquarist around the world. This extreme interest in cichlids has funded many of the expeditions to discover more about these amazing fish, however there is still so much about cichlids that is unknown.
What really makes this topic interesting is the incredible extent and rate of evolution that these fish underwent described as “explosive evolution†(Liem, 1973). The popular example of evolution is the Galapagos finches often referred to as Darwin’s finches of which there are only 14 species on the islands (Lack, 1947). The finches would eventually provide evidence to support Darwin’s theory of evolution, cichlids exemplify his ideas. Although adaptive radiation has occurred in the non-cichlid fish species in the lake, it is clear that they have not undergone a fraction of the speciation of the family Cichlidae. The difference is so great, that many researchers have questioned whether the evolutionary processes were unusual and whether the cichlids have characteristics that are not shared by the other families with which they co-exist (Greenwood, 1981).
There is a common consensus that all endemic cichlid species and genera of Lake Malawi originated in the lakes proper (Mayr, 1984). The possibility of sexual selection has also aroused a large amount of interest.

 

Discussions:

Lake Malawi

Lake Malawi is the 3[SUP]rd[/SUP] largest lake in Africa and the 9[SUP]th [/SUP]largest lake in the world. It is often called Lake Nyassa by the natives and is of great importance to the surrounding human populations. Tens of thousands of tons of fish are taken from the lake each year; mainly Utaka (haplochromine cichlids), chambo (tilapiine cichlids), catfish, and usipa (lake sardines) (Ad Konings, 2007). The lake is approximately 600km long and at some areas 80km wide, with a maximum depth of 700 meters. It is surrounded by three countries: Malawi, Mozambique, and Tanzania. Its surface lies 472meters above sea level and covers an area of about 31,000km[SUP]2[/SUP]. Two important islands, Likoma and Chizumulu, lie in Mozambique waters but belong to Malawi.
The lake is in the tropics, which means the surface layers of water are much warmer than the deeper layers, preventing vertical circulation of water, although there is some. This means that only the first 200meters below the surface are sufficiently oxygenated to support organisms, other than anaerobic bacteria found below these depths. There is a south easterly wind that prevails throughout the dry season from June to August, which causes the colder waters to rise to the surface in the most southern parts of the lake, decreasing temperatures to around 20 °C. In the rainy season, temperatures may rise to as high as 30 °C; the average surface temperature is usually between 23°C and 28°C. The bottom profile of the lake is steep, descending rapidly to great depths. That coupled with the relative lack of wind, results in a nearly uniform water temperature to considerable depth (G Barlow, 2002).
The chemical composition throughout the lake is fairly uniform. Its pH ranges from 7.8 and 8.5. Visibility in the lake varies with season change, depending on temperature and rainfall. The visibility can be as bad as zero meters or as good as 20meters. Another important fact about the lake is its fluctuating water levels which will rise and fall by several meters from year to year, making habitats extremely variable near the shoreline. Research shows that 20,000 years ago the lakes levels were 400 meters below what it is now! These low levels are currently attributed to hot and dry climate at the time; this fluctuation of water level would have undoubtedly resulted in waves of speciation (and extinction) among the cichlids that inhabited the lake at that time.
The majority of cichlid species inhabit the shallower waters (around 600 known species) while the rest live offshore at varying depths. The lakes shoreline has three main categories: rocky shoreline (about one third); sandy beaches (the majority); and swampy areas with reeds (found around river estuaries). “Most cichlids occur in particular habitats and although none is totally restricted to its preferred environment, by far the majority are. The potential for geographical isolation due to alternation of gently sloping sandy or swampy shores with steep rocky coast has been, and still is, an important factor in the speciation of cichlids” (ad Konings, 2007). Added to this, heavy seasonal rains may cause fast flowing rivers may have an effect on the rock dwelling cichlids. These rivers often form a boundary between adjacent but morphologically different populations. Ad Konings opted to divide the lake into different habitats for convenience, but such a division is mostly artificial as many cichlids move between these habitats.
The habitats recognised are:
· The wave-washed upper rocky habitat. The upper 3-5meters of the rocky habitat consists of rocky outcrops, small islands and steep rocky coasts; usually characterised by clean turbulent water. This zone always has a substrate free of sediment, and the algal mat (aufwuchs) covering the hard substrate contains many firmly attached algal strands.
· The sediment-free rocky habitat. These areas are normally located on steep sloping-shores, and are normally populated by large numbers of cichlids. Rocks vary in size, but areas with smaller rock have larger populations of cichlids. Food is in abundance, so competition is normally for territory.
· The deep, sediment-rich rocky habitat. The cichlid population is much lower in these areas, probably because food levels are lower. This is due to algal growth being limited by low intensity light at that depth.
· The intermediate habitat (sand and rocks). This habitat represents the transition zone between rocky and sandy areas. It can occur at deep levels where entirely different cichlid communities can be found, with the riches species diversity but not necessarily density.
· The shallow intermediate habitat. In many areas the rocky coast is bordered by a shallow, gently sloping, shelf consisting of sand and rocks. A number of species are restricted to this habitat, but many others are found there as well.
· Sheltered bays with aquatic plants. This area is characterised by shallow water and muddy silt covering rocks. There are beds of plants which provide cover and food for many organisms including some which the cichlids eat.
· The sandy habitat. This covers well over half the shoreline, this biotope provides little to no cover for small cichlids and thus most are found in large schools. A rock or tree trunk will provide a reference point for several breeding colonies of cichlids. Empty snail shells also provide cover for smaller specimens.
· The Unknown depths. The deep, offshore areas have yet to be properly explored by divers, but hold a multitude of cichlids which have been caught by fisherman. It is uncertain what habitats and species will be found in this zone but it is expected that species will be limited due to cold and anaerobic conditions.

 

What is a Cichlid?

A cichlid is a perch-like fish that is a part of the very large family called Cichlidae; they are in the order Perciformes. The size of the cichlid family is still unknown as many are still being discovered and many have yet to be named, but what we do know is that they number in the thousands making them one of the largest vertebrate families in the world. The sheer size of the family makes it difficult to pinpoint an exact description of what a cichlid should look like; however there are some distinguishing features that they all exhibit:
· A single nostril on the forehead, instead of two.
· No bony shelf below the orbit of the eye.
· Division of the lateral line organ, into two sections.
· A distinctively shaped otolith.
· The small intestines left-side exit from the body, instead of the right.
· And the most important distinguishing feature of cichlids is the fused lower pharyngeal bones into a single tooth-bearing structure.


Illustration of a Cichlid skull showing the pharyngeal jaws which are so unique in this family of fishes. (http://palaeoblog.blogspot.com/2009/02/genetic-origin-of-1st-tooth.html)
Lake Malawi cichlids display an amazing diversity in trophic morphology, which is thought to be a result of the amazing ability of these fish to adapt to a large range of niches. This is reflected in the extensive alterations in the jaws and teeth, not to mentions body shape and finnage (Fryer, G. & Iles, T.D, 1972). As rich as they are in trophic diversity these fishes are even more striking in their array of colours and patterns (Fryer, G. &Iles, T.D, 1972).
Cichlids are found around the world and have evolved uniquely to suite their environments. Most people know some type of cichlid without ever knowing that it is a cichlid; for example, many species such as Angel fish and Oscars (common aquarium fish), Largemouth bass (a prized catch by many fishermen) are also cichlids.
Lake Malawi cichlids can be separated into two species flocks (group of closely related species inhibiting a geographically confined area, and endemic to this area); the first group are haplochromines, which is further divided into Mbuna and “haps” or Utaka. The second group is the tilapiine cichlids.
Mbuna is cichlids which are morphologically smaller than the haps, and live amongst the rocks. Most species are vegetarian, feeding off the “aufwuchs” that covers the rocky habitat. These colourful fish are usually found in very dense populations, as a result they are very aggressive and because food is plentiful, most aggression is caused by territory. This species flock has attracted the most attention. They are prominent residents of the reefs where they number 200 species or more. These are also the most popular species in aquariums because of their active behaviour combined with stunning colour.
Utaka or “haps” are larger more peaceful fish that usually live out in the open water of the Lake. They often form large shoals consisting or various species and thousands of individuals. Frequently, near underwater rock formations there is a slight current which carries planktonic crustaceans. This phenomenon causes many cichlids to sit in the water column facing the current waiting for their next meal. The fish drop their lower jaw, which pushes the upper jaw forward temporarily creating a short tube. As the jaw shoots forward the fish’s gills are closed and the water gushes into the fishes mouth, because of the slightly negative pressure created, and the zooplankton is carried with the water into the predator’s mouth. It has also been discovered that some species of Utaka have even adapted to breeding in open water. They do this by defending a mobile territory while trying to attract mates (Smith, 2000). They have many feeding specialisations which will be discussed in the following section.

 

Origin of the Cichlids

The original colonisers of the newly formed lakes were, almost certainly, riverine species. Those which have adapted most successfully to the new conditions in the developing lakes are the cichlids, particularly the haplochromines, which constitute the vast majority of contemporary species.
Although all haplochromine species are believed to have had a single common ancestor (Greenwood, 1979) it seems that different, but closely related, ancestral forms colonised each lake. Regan (1921), Trewavas (1935) and Fryer & Iles (1972) consider the endemic haplochromines of Lake Malawi to have had a common ancestor with many features in common. The cichlid fishes of Lake Malawi are composed of two main lineages: the tilapiines, comprising of 6 Oreochromis spp. and 2 Tilapia spp., and the haplochromines comprising numerous genera and 400-500 species.
The phylogenies of the haplochromines are largely unknown, but several contemporary groups may be recognized : (a) the 10 Mbuna genera ; (b) the genus Cyrtocara which comprises a number of distinct groups which will probably be recognised as separate genera when a revision of the genus is completed; (c) the others which comprise the Lethrinops spp., Aulonocara spp. and Trematocranus spp. which are closely related to some Crytocara spp., as well as the more distantly related Rhamphochromis spp., Astatotilapia calliptera (Haplochromis callipterus), a contemporary widespread species which lives in a variety of water bodies, including rivers and streams, as well as in Lake Malawi. However, Greenwood (1979) believes that the Malawi species are likely to have had a polyphyletic origin and that lineages related to Thoracochromis (Greenwood, 1979), Serranochromis and Chetia (Greenwood, 1979) may have contributed to the flock in addition to Astatotilapia calliptera. However no evidence is given to support this suggestion.
The haplochromines of Lake Victoria appear to have had a monophyletic ancestry (Astatoreochromis alluaudi exception) originating from a Haplochromis bloyeti-like ancestor (Green-wood, 1974).
Lake Tanganyika poses a more difficult problem. Its cichlid species are older and more markedly differentiated than those of the other two lakes and it is not now possible to define the ancestral forms. Indeed, there is at present considerable doubt as to whether certain species should be assigned to the haplochromines or tilapiines (Wickler 1963; Fryer & Iles, 1972; Greenwood 1978) and some species may not belong to either (Greenwood, 1978). A great deal of study is required to accurately define the origins, and trace the phylogenies of these fishes.

 

Feeding specializations

Cichlids are among the best fish to inhabit a lake because of their ability to adapt to different conditions with ease. A large lake like Lake Malawi has an enormous range of niches that form across its expanse; cichlids are capable of occupying these niches. “Their feeding equipment- maxillary and pharyngeal jaws and dentition- is able to evolve rapidly to cater for a particular new food source, and this gives them an advantage over other types of fishes that lack dietary flexibility (Ad Konings, 2007). “Explosive radiation” was the way in which Geoffrey Fryer described the amazing radiation into different species within the East African Lakes;(Fryer & Iles, 1972) it also refers to the incredible array of feeding specializations that occur, especially in the “Hap” species flock. These cichlids feed on a wide range of food items and include algae-feeders, macrophyte-feeders, piscivores, planktivores, snail-crushers, scale-eaters, and sand-sifters, as well as many other trophic groups. What is also amazing is that many of these specialized feeders will readily eat other food that is available; showing the true diversity of these fish. These cichlids are extremely “plastic” when it comes to eating.
One possible explanation as to why these cichlids are such specialized feeders, yet appear to have random feeding habits at times, is that during time of high environmental pressure, certain food groups were limited and thus threatened the survival of a species, forcing them to evolve more specialized feeding techniques in order to survive (Pyke, 1977).
When it comes to cichlids, the process of survival is a little more complicated than “changing their diet to survive”. The cichlids must be able to harvest food efficiently without losing is territory or life. It is thus the nature (the type and degree of competition) of the local community that influences the survival rate of a particular species at a particular site (Ad Konings, 2007).
Fish in the wild are constantly under threat from environmental pressures, predators, and competitors, all of which limit their growth and morphological features (e.g. fin length). Fish in captivity have access to large amounts of food with little stresses, allowing them to evolve into bigger, brighter and more distinguished species (assuming they do not cross breed).

 

Breeding

All the species of cichlids in Lake Malawi, except non-endemic species, are maternal mouthbrooders. This means the male is able to fertilise the eggs of several females and play no role in the birth or growth of the fry. Thus the male’s main concern is attracting the females by having the best morphological features or territories. Most ‘haps’ are sexually dimorphic; the females are dull in colour and pattern while the males are brightly coloured with interesting morphological features. In this case it is easy to tell the sexes apart; the problem comes in with the Mbuna species flock (and some ‘haps’), where the males and females look very similar. In the wild, males are very rarely seen displaying to females of different species and it seems that males and females are picky about which mate they choose (McKaye, 1990).
In some species (i.e. Cyrtocara moori) which are non-territorial and the males and females look similar, courting occurs upon encounter and males and females can tell each other apart by shape, size, and behaviour. When a male thinks he has found a suitable female, he displays while simultaneously releasing a fluid that carries a species-specific scent (Plenderleith, 2005). These males can display their breeding colours at any time because encounters with females can occur randomly.
In territorial cichlids, the male exhibits breeding colours to show he has secured a prospective spawning area; in some species this display is reinforced by the construction of a bower (Ad Konings, 2007). The males of these species normally begin displaying together; this is because as one male becomes ready to mate he becomes more colourful, this triggers the neighbouring males to also display breeding colours. This mutual stimulation results in groups of healthy males competing over territory and effectively ensuring that only the strongest males are able to reproduce. This group of males has another advantage because they are more likely to attract females than a single male. This type of communal display, which is called arena-breeding or lek-breeding, is favourable for the survival of a strong and healthy population but imposes heavy demands on the males (Ad Konings, 2007).
The cichlids of Lake Malawi exhibit two types of mouthbrooding: less advanced species fertilize the eggs outside the female’s mouth; the more advanced species fertilize the eggs inside the female’s mouth, this is the majority. The Male secures a spawning site and begins courting a female. The female decides if she wants to breed with that particular male, if she does, then she will remain in his territory. Even while the male chases intruders away, she remains there patiently waiting. The male takes the lead and tries to entice the female to lay the eggs, there are often a few “dry runs” before she lays her eggs, but once she does the male fertilises them and the female scoops them up. They repeat this process until the female has laid all her eggs or until they are disturbed.
In species where the eggs are fertilized inside the female’s mouth, the process is different. The male leads the female to the spawning site and begins quivering its fins and body, particularly the anal fin. The male begins releasing sperm and the female mouth his anal fin, picking up sperm before laying a single egg. Next, the male begins circling the female vibrating his anal fin which is dragged over the spawning site. After a couple rounds the female slows down and deposits a couple eggs and without hesitation picks them up. The male stays alongside her, keeping the eggs between the male and female. After the female has collected her eggs the male begins quivering again and the females ingests his sperm again. The eggs of most fish are a nutritional meal to any other species and are therefore very vulnerable; this type of internal fertilization limits exposed to the environment and thus increases the chances of a successful breed. Malawi cichlids are not monogamous so the female will actually have her eggs fertilised by many males, increasing the gene pool. Once she has fertilised the eggs with a few males she retreats to a quite area or joins a school of mouthbrooding females.
During incubation the female will not eat for the first eight to ten days (Ad Konings, 2007). After that she may eat, but only a little, and very cautiously. Some larger species of cichlids do not eat at all over the incubation period which is an average of 24days, depending of species and temperature.
Once the offspring have been born, most species abandon them to their own devices. These newly borne fry seek refuge in shallower waters or in the nests of catfish. In most species that do not practise lek-breeding females may protect their fry for up to 6 weeks before abandoning them. Such parental devotion is often abused by juveniles of other species, which join the guarded broods (Ribbink, 1980). Some Mbuna females have been observed releasing their fry into several different crevices in the aquarium. Sometimes females are seen defending a small site and exhibiting male breeding colours, this suggests that the male breeding colours may in fact be aggression colours.

This picture shows a male Cyrtocara moorii with the fibrous hump on its head which becomes larger with age. (http://diszhal.info/english/cichlids/en_Cyrtocara_moori.php)

 

Speciation

Speciation is an evolutionary process whereby new species arise. It is important to realise that evolution can occur without speciation, but speciation cannot occur without evolution.
With the recent discovery that Lake Malawi underwent periodic water fluctuation, the allopatric speciation theory has gained considerably credibility with regard to Malawi cichlids. According to this theory speciation takes place after a small number of individuals are separated from the ‘mother’ population and founded a new one. Little or no speciation will occur in the new or ‘daughter’ population if there are sufficient individuals present because there is enough selection to ensure a stable population. Another theory suggests that many rock-dwelling species are in fact isolated from one another and therefore thousands of genetically different populations can be distinguished (Van Oppen, 1997). This however does not mean that these all different species, because a degree of genetic variation can occur without speciation taking place. It does however lend strength to the idea that populations separated by an inhospitable barrier of sometimes less than 500meters have the potential to evolve into different species.
Most cichlids spend the majority of their time near the bottom and cannot easily cross deep open water to settle in another area. It is therefore not hard to imagine that a long stretch of open water will effectively act as a barrier between populations. If this were not the case, rock-dwelling species would be found at each suitable area of rocky coast in the lake. Large parts of the shoreline are separated from the opposite shoreline by deep water, which cannot be crossed by cichlids. If the cichlids wish to migrate to the opposite side they must move along the entire circumference of the lake to get to that spot; hence the opposite side of the lake is often the most distant point in terms of cichlid dispersal (Ad Konings, 2007).
Despite this fact, many cichlid species are found on both sides of the lake, without their range being continuous. This can be explained by the fluctuation of water levels that were experienced in the past. If the lake was small enough then the cichlids would be able to access all the habitats of the lake. This can be seen in the smaller Lake Kivu where the cichlid species are present across the entire shoreline (Snoeks, 1994). When the water levels in Lake Malawi dropped, species were able to move up and outward from their locations, remaining at their preferred depth. Even though cichlids on the eastern and western side of the lake are separated by the deep water, they remain the same species, at least there are no apparent differences to be found between them (genetically they may be very different). This point is most convincingly illustrated by the discontinuous distribution of those species which are found on either side of the lake but not in suitable habitats North and South of their range. These include Aulonocara stuartgranti, Copadichromis sp. ‘kawanga’, Cynotilapia sp. ‘Lion’, Labeotropheus trewavasae, Labidochromis caruleus (white form), Tropheops sp. ‘red fin’. These species were probably present in the most recent such paleo-lake rather than having evolved after the lake had risen to its current level. One problem with this idea is that most people believe speciation take longer than the 25,000-50,000 years it took for the level of the lake to rise. However there is evidence to support this theory with the existence of 100 new species found around the islands of the lake, which could not have been there around the time of low water level because that entire area was the dry land (Owen, 1990). These species must either be the sole remnants of the paleo-lake species that have survived only in these new habitats, or have developed from species that dispersed from the paleo-lake populations into this area-which is, the more likely explanation (Ad Konings, 2007).

 

Origin of Speciation

As the fossil record of the fishes of the Great Lakes is virtually non-existent, extrapolations from the contemporary status are necessary to form hypotheses regarding the origin of diversification. The zoogeography and ecology of the majority of endemic species is largely unknown. Nevertheless the general picture which has been emerging over the past thirty years is that the extant species are tropically specialized, most are geographically restricted e.g. for some species of Mbuna the entire area occupied by its members is no more than a few thousand square meters (Ribbink, 1983), and most species have clearly defined microhabitat preferences, particularly during periods of breeding (Lowe, 1952).
The observations that cichlids are sedentary and habitat specific makes it easy to envisage how intralacustrine speciation could occur if a contemporary species were divided into two or more sub-populations by geographic barriers. Gene flow between such populations would cease and divergence would follow as each population adapted to its new environment. The major conceptual problem of cichlid speciation in the Rift Valley Lakes, however, is to appreciate how the specializations and philopatric tendencies (i.e. tendencies to stay in one area, usually the birth place) arose originally. If for example, a generalized ancestor colonised the paleo-lakes, then by virtue of its generalized attributes, it might be expected to occupy all habitats with equal distribution right around the lake. Gene flow would be uninterrupted, but perhaps retarded by distance as the lakes enlarged.
Fryer (1959, 1977) suggested that the original colonisers of Lake Malawi had undergone a degree of specialization when they first entered the lake so that those preferring sandy regions would not be in competition with those preferring other habitats such as rocky substrata. With time the specializations for the respective habitats of the groups increased so that habitat discontinuities came to constitute formidable barriers to dispersion. Eventually the fauna was split into innumerable isolated populations and the stage was set for an accelerating allopatric speciation. We shall never know whether these speculations are correct, but it is true that extant riverine species are adapted to different habits (e.g. piscivory, herbivory) and do occupy species characteristic habitats.
It is possible, therefore, that different lines colonised the lake as postulated by Fryer (1959, 1977). However, it is not necessary to postulate such an oligophyletic ancestry if one considers the sedentary nature of cichlids. Foyer & Iles (1972) postulate that a single Haplochromis-like ancestor colonised Lake Malawi which is not consistent with the hypothesis that several ancestors invaded the lake. Although fishes would appear to be highly mobile it is apparent from the zoogeographical data emanating from each of the Great Lakes that the out breeding one might expect from high mobility is not a feature of lacustrine cichlids.
On the contrary, the majority of cichlids are sedentary. One of the most important reasons for their sedentary nature is the fact that cichlids practice parental tare (Poll, 1956), which tends to increase philopatry and reduce dispersal sharply (Mayr, 1963). Furthermore, Dobzhansky (195 1) believed that evolution and speciation occur most rapidly in those species in which parental tare is developed. Since parental tare is well developed in extant riverine species, including those which are believed to resemble the ancestral forms, it is almost certain that it was a feature of those cichlids which colonised the lakes originally.
Consequently there is justification for assuming that the colonisers were philopatric. It follows that the increasing size of the growing lakes would have isolated populations as a result of the greater distances it placed between them. If, as Fryer (1959, 1977) suggests, the colonisers also exhibited habitat preferences, then fragmentation of populations would have been greater and was probably accompanied by an increase in the rate and extent of speciation. Parental tare, particularly the aeration of eggs and larvae, in cichlids makes it possible for many species to live, breed and rear their offspring while remaining within a particular habitat (or micro-habitat). Thus, without the need to return to rivers for breeding, as do anadromous fishes (Lowe-McConnell, 1969, 1975), a cichlid population may adapt to the conditions of a single habitat only, which is presumably conducive to ecological specialisation and to speciation of allopatric populations. There are, of course, exceptions such as species with lake-wide distribution and those which undergo extensive migrations (Lowe-McConnell, 1969, 1975; Fryer & Iles, 1972), but these species are relatively few in number.

 

Is sexual selection driving cichlid speciation?

That sexual selection operates in cichlid fishes was suggested very early in the study of the group (Kosswig, 1947). Although critical evidence to support this idea is not abundant, many investigators have assumed in recent reviews that female mate choice on male coloration is a common phenomenon in cichlids (McKaye 1991, Meyer 1993, Ribbink 1994, Sturmbauer 1998, Turner 1994). While interspecific discrimination has been repeatedly demonstrated (Knight and Turner 1999, Knight et al. 1998, Seehausen and VanAlphen 1998, Seehausen et al. 1997, Seehausen et al. 1998), strong evidence for intraspecific mate choice is limited.
Models of cichlid speciation by sexual selection fall into two major categories: rapid divergence of sexual characters in allopatry (Dominy 1984, McKaye 1991), and disruptive or divergent sexual selection in sympatry (cichlid refs: Seehausen and VanAlphen 1999, 1998, Turner and Burrows 1995, VanDorn et al. 1998). Both scenarios employ the process of “Fisherian runaway” sexual selection. The distinction of the Fisherian process from other mate choice scenarios, such as 'good genes' or 'handicap' processes, is that the female preference for a sexually selected trait increases in frequency as a result of co-inheritance of trait and preference, rather than via direct selection owing to increased fitness of females who choose quality males (Andersson 1994). Assumptions of the runaway process include (1) heritable genetic variation in male trait(s), (2) heritable genetic variation in female preference, (3) increased mating success of males with exaggerated trait, and (4) genetic covariance of trait and preference (Andersson 1994).
Allopatric models show that geographical variation in sexual characteristics (i.e. coloration and bower form) is generated by the original cichlid effects and maintained by limited dispersal. “Sympatric models have had to overcome the theoretical restriction imposed by the effects of recombination on reproductive traits and preferences and the elimination of intermediate genotypes” (Johnson and Gulberg 1998). This has been done by assuming genetic dominance in female preference traits (Turner and Burrows 1995), variation in habitat choice (Van Dorn et al. 1998), and extreme mating preferences (Higashi et al. 1999).
The association of species diversity with exaggerated sexual characteristics is appealing to many researchers. A recent body of literature has addressed this phenomenon among bird species by identifying sexual dimorphism or ornamentation as a direct influence on species richness (Barraclough et al. 1995). However, this result begs the question of the relationship between these two factors and the necessary elements to determine reason for this species richness. Unfortunately, despite the need for rigorous hypothesis testing and the data that are required to support sexual selection as a cause for speciation, the ability to recreate mechanisms causing species diversity are limited by the time scales in which divergence events occur. Barraclough and his co-workers (1998) suggest several methods for recreating past divergence events using species level phylogenies. Thus, the absolute need for such reconstructions among closely related African cichlid species is clear.
Female cichlids are able to discriminate males of their own species and closely related species by body colour (Holzberg 1978, Knight and Turner 1999). This ability is important in the maintenance of diversity, especially where closely related species are sympatric, and not ecologically differentiated (Seehausen et al. 1997). Documentation of interspecific mate choice, though, does not help assessment of competing hypotheses of divergence by sexual selection. Current data does not allow rejection of a completely allopatric divergence scenario in which sexual selection plays no role.
Intraspecific mate choice has been recorded in field observations (Karino 1997, McKaye 1991) and field experimentation (Hert 1991). Though all of these studies suffer from small sample size and limited replication, they present compelling evidence for the existence of mate choice within species. The male traits under selection in these studies were bower size (McKaye 1991), pelvic fin length and symmetry (Karino 1997)' and number of egg spots on the anal fin (Hert 1991). In general, studies have not demonstrated heritability of male traits, female preferences, or the co variation of those traits through breeding experiments. An alternative hypothesis of the evolution of the extent of sexual dimorphism deserves some attention. For instance, intrasexual selection via male contest competition is very likely to operate in cichlids and remains largely unstudied (Karino 1996). Divergence of male colouration in allopatry may be as likely under intrasexual as it is under intersexual selection, and intrasexual selection does not require a change in female preference traits. For example, observations previously considered supportive of female choice (McKaye et al. 1993) could be explained by the effects of male competition.
McKaye’s documentation of female mate choice on bower size and position in Lake Malawi cichlids depends on the use of bower morphology as an 'extended phenotype' of males. Two later studies of mate choice in Lake Tanganyika (Karino 1996) and Lake Malawi (Kornfield et al. 1991) bower building cichlids failed to find evidence for choice in relation to bower morphology, and further found that males are often chased from their bowers by males who defend territories in the absence of a bower (Karino 1996). These additional data suggest that male competition is important in the acquirement and defense of a breeding territory. Size has been observed to effect competition between males (Barlow, 2002) but the use of conspicuous male coloration as a signal between males is also an alternative use.

 

Malawian Cichlid Taxonomy

The cichlid species flock in Lake Malawi is one of the most complex assemblages of species in existence, and, since the majority of the species are still undescribed, it is often difficult to determine their phylogenetic relationships. Over the years several taxonomists and biologists have attempted to determine the phylogenetic relationships of these fish, many of their finding have been changed or reworked, only to be changed and reworked again at a later stage. At this point in time the phylogenetic relationships are still not known, and a lot of work will be required to create a system to show all the relationships and how their ancestors.

The picture above shows a small group of cichlids from Lake Malawi. I put it here only to show the reader the complexity of the cichlid radiation in this lake.

 

Conclusions
This dissertation has considered cichlids as a whole and more specifically cichlids in Lake Malawi. The aim of this project was to investigate the wide array of species in Lake Malawi and try to determine a possible reason for such unbelievable radiation within a family.
The study has shown that various factors play a role in the evolutionary history of these fish. These findings show in general that, fluctuations in the level of the lake over the past 20-50 thousand years was probably a major cause in the speciation of these fish.
The most obvious finding to emerge from this study is that this family of fishes are capable of incredible adaption to environmental changes. Many species have been subjected to environmental pressures that have causes their extinction, but these fish have prevailed the conditions in an extraordinary manner. Conditions in a specific area of the Lake can be very different from conditions in another area only a few hundred meters away. This difference has caused a number of niches to be formed and the populations of cichlids in their own area have adapted to fill the niche. As a result each area contains populations that are unique to neighbouring populations.
The ability of these fish to adapt to a new niche is also largely attributed to their variance in trophic specialisations. This study revealed that these fish to not conform to one type of food despite having very specific adaptions to suite a specific food. This is very likely another reason they have been successful as a species, when conditions were unfavourable, species were forced to develop specific eating habits to survive. When these conditions changed again the species did not die, but instead adapted to the new conditions. This is evidence that these fish are some of the best survivors on Earth.
Another aspect of cichlids which seems to be a major factor in the success of this species is there highly advanced breeding techniques. This study showed that all species of cichlids within Lake Malawi are Mouthbrooders. The commitment to protecting the eggs, by the female, ensures that the probability of the juvenile’s survival is increased. If the eggs were left out in the open, it is almost certain that a predator would not hesitate to eat them all because of their nutritional value. Therefore it seems that a large portion of the fish’s success in Lake Malawi can be attributed to the fierce paternal instincts of the females.
The research aimed at possible origins of speciation had some profound implications. It is theorised that all the species in Lake Malawi arose from a single ancestor species. It is thought that this ancestor species was lacustrine (from the river) originally. The issue with this idea is that, if one ancestor species flowed from the rivers into the lake and proceeded to undergo speciation, by the very nature of these fish, the ancestor species should have occupied all the habitats in the lake and genetic variation would be limited. However this is not the case. The finding suggested that a possible reason for this is that, interruptions in the flow of the river would have carried lacustrine species into the lake at different times over thousands of years. This would create geographical barriers and therefore favour allopatric speciation.
It was also found that the nature of the cichlids could have been a driving force in their speciation. This is due to the individual fish’s preference as to what kind of habitat and habits it chooses. This idea can be seen in lacustrine species which have taken to eating different foods and living in different areas of the river for no apparent reason, other than that’s what they chose. Whether this theory is true or not has yet to be proven, but many researchers readily accept it as a likely factor of cichlid speciation.
The last section in this study which was looked at was sexual selection as a possible driving force in speciation. The theory is that speciation was increased because of the preferences of potential mates to specific characteristics. This type of sympatric divergence has been witnessed in other species such as birds and insects, so it is possible that it may occur in fish. Many researchers have attempted to prove this theory but it is no easy task. This is because this type of divergence takes place over a long period of time and recreation of these events is inconsistent. The other reason this type of divergence is uncertain is because the current theory of allopatric speciation as the only reason for divergence cannot be rejected. Observations in the field have provided compelling evidence to the existence specific mate selection. The high levels of sexual dimorphism found within the cichlids of Lake Malawi also suggest that sexual selection may play a role in speciation. Until further evidence has been provided sexual selection as a driving force for speciation in cichlids will remain a likely, but uncertain factor.
The last section which was considered in this study was the taxonomy of the fish in the lake. The findings were inconclusive, and due to the complexity of the phylogenetic relationships within the lake a lot more research is required to make any useful connections between species.
The evolution of cichlids in Lake Malawi is an extraordinary topic which is the best example of adaptive radiation witnessed on Earth. The reasons for such incredible speciation within a relatively small area may never be certain. What is certain is that such a unique place will remain as an amazing example of evolution.

 

References

Andersson M. 1994. Sexual Selection. Princeton: Princeton University Press. 599
Barraclough T, Harvey P, Nee S. 1995. Sexual selection and taxonomic diversity of passerine birds. Proc. R. Soc. Lond. B 259: 211-2 15
Barraclough T, Vogler AP, Harvey PH. 1998. Revealing the factors that promote speciation. Phil. Trans. R. Soc. Lond. B 353: 241-249
Bowers N, Stauffer JR, Kocher TD. 1994. Intra- and interspecific mitochondria1 DNA sequence variation within two species of rock-dwelling cichlids. Mol. Phyl. Evol. 3: 75-82
Dominey W. 1984. Effects of sexual selection and life history on speciation: species flocks in African cichlids and Hawaiian Drosophila. In: Echelle AA, Kornfield I, eds. Evolution of Fish Species Flocks. Orono, Maine: University of Maine Press. 23 1-249.
Eccles DE, Trewavas E. 1989. Malawian Cichlid Fishes: the Classification of Some Haplochromine Genera. Herten: Lake Fish Movies. 334
Fryer G. 1959. The trophic interrelationships and ecology of some littoral communities of Lake Nyasa with special reference to the fishes, and a discussion of the evolution of a group of rock-frequenting Cichlidae. Proc. 2001. Soc. Lond. 132: 153-281
Fryer, G. 2001 on the age and origin of the species flock of Haplochromine cichlid fishes of Lake Victoria. Proc. R. Soc. Lond. B 268: 1147- 1152
Fryer G, Iles TD. 1972. The Cichlid Fishes of the Great Lakes of Africa. London: Oliver and Boyd. 641
Greenwood PH. 1974. The cichlid fishes of Lake Victoria, East Africa: the biology and Evolution of a species flock. Bull. Br. Mus. Nut. Hist. Zoo1 :1-134
Greenwood PH. 1984. African cichlids and evolutionary theories. In: Echelle AA, Kornfield I, eds. Evolution of Fish Species Flocks. Orono, Maine: University of Maine Press. p. 141-154
Greenwood PH. 199 1. Speciation. In: Keenlyside M. ed. Cichlid Fishes: Behaviour, Ecology, and Evolution. London: Chapman and Hall. p. 84-102
Higashi M, Takimoto G, Yamamura N. 1999. Sympatric speciation by sexual selection. Nature 402523-526
Karino K. 1996. Tactic for bower acquisition by male cichlids, Cyathopharynx furcifer, in Lake Tanganyika Ichthyol. Res. 43: 125- 132
Karino K. 1997. Female mate preference for males having long and symmetric fins in the bower-holding cichlid Cyathopharynx furcifer. EthoIoay 103:883-892
Knight ME, Turner GF. 1999. Reproductive isolation among closely related Lake Malawi cichlids: can males recognize conspecific females by visual cues? Anim. Behav. 58: 761-768
Kocher TD, Conroy JA, McKaye KR, Stauffer JR. 1993. Similar morphologies of cichlid fish in Lakes Tanganyika and Malawi are due to convergence. Mol. Phylo. Evol. 2: 158-165
Konings, A. 2001 Malawi Cichlids in Their Natural Habitat. El Paso, Texas: Cichlid Press. 351
Konings, A. 2007 Malawi Cichlids in Their Natural Habitat. E4 El Paso, Texas: Cichlid Press. 4: 8-25
Kornfield I. 1978. Evidence for rapid speciation in African cichlid fishes. Experientia 34: 335-336
Kornfield I. 1991. Genetics. In: Keenlyside M, ed. Cichlid Fishes: Behaviour, Ecology, and Evolution. London: Chapman and Hall. p.103-1 28
Markert JA, Arnegard ME, Danley PD, Kocher TD. 1999. Biogeography and population genetics of the Lake Malawi cichlid Melanochromis auratus: Habitat transience, philopatry and speciation Mol. Ecol. 8: 1013-1026
Mayr E. 1984. Evolution of fish species flocks: a commentary. In: Echelle AA, Kornfield I, eds. Evolution of Fish Species Flocks. Orono, Maine: University of Maine Press. p. 3-12
McElroy DM, Kornfield IL, Everett J. 1991. Coloration in African cichlids: diversity and constraints in Lake Malawi endemics. Nether. J .Zool. 41: 250-268
McKaye KR. 1991. Sexual selection and the evolution of the cichlid fishes of Lake Malawi, Africa. In: Keenlyside M, ed. Cichlid Fishes: Behaviour, Ecology, and Evolution. London: Chapman and Hall. p. 241 -257
Meyer A. 1993. Phylogenetic relationships and evolutionary processes in East African cichlid fishes. Trends Ecol. Evol. 8: 279-285
Owen RB, Crossley R, Johnson TC, Tweddle D, Komfield I, et al. 1990. Major Low levels of Lake Malawi and their implications for speciation rates in cichlid fishes. Proc. R. Soc. Lond. B 24O: 519-533
Ribbink AJ. 1986. The species concept, sibling species and speciation. Ann. Mus. Roy. Afr. Centr. 251: 109- 116
Ribbink AJ. 1994. Alternative perspectives on some controversial aspects of cichlid fish speciation Arch. Hydrobiol. Limnol. 44: 101-125
Ribbink AJ, Marsh BA, Marsh AC, Ribbink AC, Sharp BJ. 1983. A preliminary survey of the cichlid fishes of rocky habitats in Lake Malawi. S. Afr. J .Zool. 18: 149-310
Seehausen 0 . 1999. Explosive speciation rates and unusual species richness in Haplochromine Cichlids - effects of sexual selection. Adv. Ecol. Res. 30: 325-371
Seehausen 0 , Mayhew PJ, Van Alphen JJM. 1999. Evolution of colour patterns in East African cichlid fish. Evol. Biol. 125: 14-534
Snoeks, J. 2000. How well known is the ichthyodiversity of large East African lakes? In: Rossiter A, Kawanabe H, eds. Ancient Lakes: Biodiversity, Ecology and Evolution, vol. 3 1. Boston: Academic Press. p.17-38
Sturmbauer C, Meyer A. 1992. Genetic divergence, speciation and morphological stasis in a lineage of African cichlid fishes. Nature 358578-581
Trewavas E. 1935. A synopsis of the cichlid fishes of Lake Nyasa Ann. Mag. Nut. Hist. 10: 65-118
Turner GF. 1994. Speciation mechanisms in Lake Malawi cichlids: a critical review. Arch. Hydrobiol. Beih. Limnol. 44: 139-160
Turner GF. 2000. The nature of species in ancient lakes: perspectives from the fishes of Lake Malawi. In: Rossiter A, Kawanabe H, eds. Ancient Lakes: Biodiversity, Ecology and Evolution, vol. 31. Boston: Academic Press. p. 39-60
Turner GF, Burrows MT. 1995. A model of sympatric speciation by sexual selection. Proc. R. Soc. Lond. B 260: 287-292
Van Oppen MJH, Turner GF, Rico C, Robinson R, Deutch J, et al. 1998. Asortative mating among rock-dwelling cichlid fishes supports high estimates of species richness from Lake Malawi. Mol. Ecol. 7: 991-1001

 

Some images of some of my favourite Malawi Cichlid species (Please note these are not my pictures):

 

Here is a link to my video ------> Malawi Cichlid Tank 150 gallon/600 liter - YouTube

At the end you will see my Cichlids eating Bloodworms Very cool!

 

+1 on increasing my knowledge on Malawi Cichlids. A well written and easy to understand article. Love the video & the pics of some of the more colourful species.

Why did you have to do the article?

 

Thanks @Skye01 I had to do it for my 2nd year subject: Evolution. I chose Malawi Cichlids because I thought it would be interesting, and it was!