Evolution and systematics

The families Cichlidae (cichlids) and Embiotocidae (surf-perches) have traditionally been grouped in the suborder Labroidei, along with Labridae (wrasses), Scaridae (parrot-fishes), Pomacentridae (damselfishes), and Odacidae (western Pacific butterfishes). These groups were first suggested to share a common ancestry by Müller in 1843. All possess a pharyngeal jaw apparatus (a second set of jaws in the throat) that aids in the processing of food, and the morphological characters supporting their close relationship are all a part of this functional complex. Characters used to unite labroids include fused fifth ceratobranchials that bear teeth, a muscle sling suspending the lower pharyngeal jaw from the skull, an articulation between the upper pharyngeal jaws and the base of the skull without intervening muscle, and an undivided sheet of sphincter esophagi muscle. The reliability of these characters has been questioned, and the reality of the Labroidei has been challenged for several reasons. One is that none of the characters are unique to labroids (in fact, a whole suite of pharyngeal jaw features are found in the quite distantly related Beloniformes); another is that not all labroids present all the characters. In addition, characters supporting the Labroidei that are independent of the pharyngeal jaw apparatus, and perhaps less strongly influenced by selective forces related to food processing, have eluded detection. Given these challenges, it isn't surprising that recent molecular studies have failed to uphold the Labroidei as a natural group.

Nonetheless, this chapter considers two of the traditional labroid families, the cichlids and surfperches. Surfperches are temperate, almost entirely marine fishes found only in the northern Pacific, and are represented by 13 genera and 24 species. Cichlids are mostly tropical freshwater fishes, with a distribution on fragments of the supercontinent of Gond-wana. There are about 105 genera of cichlids and 1,405 described species, although some estimates place the total number of cichlid species to be in excess of 2,500 if a full ac counting is made of the exceptional species diversity in the three African Great Lakes. Different geologic processes and time scales have shaped these three lakes and their species flocks. Two of the lakes, Malawi and Tanganyika, formed in a rift created by plate movements that are slowly ripping apart the African continent. Lake Malawi is roughly 700,000 years old and contains 500-1,000 cichlid species; Lake Tanganyika is 9-12 million years old and contains about 200 cichlid species. Lake Victoria, formed by processes of mountain building that blocked the flow of rivers draining eastern Kenya, was probably entirely dry as recently as 12,400 years ago, leaving precious little time for the subsequent evolution of its 500 or so haplochromine cichlid species from a single ancestral species.

Evolutionary biologists have been intrigued by the question of how so many cichlid species evolved together in African lakes. Clearly, the ability of cichlids to finely partition resources provides some explanation, but the lakes themselves also play a role. Lake Victoria is relatively shallow and turbid, with few rocky microhabitats along its shoreline, but the two rift lakes are clear and deep, with abundant patches of rocky microhabitat scattered along their shores. Within Tanganyika, there are three distinct communities: the pelagic, containing six fish species; the benthic, with 80 fish species; and most diverse, the littoral, with 207 species of fishes. Most of the species diversity is restricted to inshore regions. Malawi's piscine endowment is similarly biased; so species poor are the pelagic waters in comparison with the littoral rocky communities that two zooplanktivorous species of the genus Diplotaxodon make up 71% of the open-water fish biomass. In Malawi and Tanganyika, populations in rocky littoral habitats separated by sandy bottom and deepwater barriers wider than 1.2 mi (2 km) exhibit little or no gene flow. Thus, a model of microal-lopatric speciation may be appropriate for the rift lakes, whereby the numerous and fragmented littoral rocky habitats, coupled with attributes of some of the cichlids, such as

Kribensis pair (Pelvicachromis pulcher) with young. (Photo by Mark Smith/Photo Researchers, Inc. Reproduced by permission.)

resource specialization and adherence to circumscribed territories, have fostered explosive speciation. Another speciation mechanism, sexual selection, has been considered most important in Victoria due to its paucity of isolated microhabitats, and may also be important in the rift lakes.

Age differences between the lakes are also manifest in the cichlid species flocks. The two younger lakes, Malawi and Victoria, contain relatively more homogenous assemblages of strictly mouth-brooding cichlids. In contrast, Lake Tanganyika contains a very diverse assemblage of both mouth brooders and substrate spawners descended from numerous colonization events. In a 1998 paper, Sturmbauer contrasted the relatively young lake Victoria species flock, which lacks extreme morphotypes and has many intermediate species, with the ancient Lake Tanganyika flock, characterized by few intermediates and high morphological distances between species, and proposed that the two lakes represent opposite ends of a species flock evolutionary continuum. He suggested that in older lakes with mature flocks (as in Tanganyika), very diverse communities sharing most of their species will predominate, because dispersal events (facilitated by lake level fluctuations) have, over long stretches of time, mixed together species that arose in isolation. Consequently, lesser competitors in shared niches will have alternately diverged or perished.

Despite the present diversity of cichlids, both cichlids and surfperches are poorly represented in the fossil record. Fossil surfperches are first known from the upper Miocene epoch (5-10 million years ago), from the Monterey Formation in California. Only a few species of fossil cichlids are known, the earliest of which are 45 million years old (Eocene epoch) from Tanzania, East Africa. Cichlid material from Maranhao, Brazil, may also date from the Eocene, and putatively cichlid material of similar age has been found in Vicenza, Italy. If, as many researchers suggest, the present geographic distribution of cichlids is an artifact of their being widely distributed on the supercontinent of Gondwana, then cichlids must be at least 130 million years old, leaving an inexplicable gap of 85 million years in the fossil record. This has led some to argue that cichlids are actually a younger group that has dispersed across marine barriers to achieve their present distribution, an argument supported by the ability of several cichlid species to survive in sea water and the fact that cichlid distribution is not strictly Gondwanan.

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