The Midas Cichlids Complex is highly variable: in addition to the normal black and gray striped variants, there are yellow to orange morphs and large variations in body shape. These species and forms colonize not only in the great lakes of Nicaragua, but also in the volcanic crater lakes, which obviously contributes to the speciation of fishes.
The sparsely populated Nicaraguan lowlands are dominated by two large lakes, Lake Managua and Lake Nicaragua. Managua is more than half the size of Lake Okeechobee, Florida’s largest freshwater lake, and Nicaragua is over four times bigger than Okeechobee. Despite their enormous sizes, these lakes are very shallow, with an average depth of 30 feet (9 m) for Managua and 39 feet (12 m) for Nicaragua, so they are constantly agitated by the wind, reducing underwater visibility to a few centimeters. Useful underwater shots are often impossible to get.
Nevertheless, the great lakes of Nicaragua are aquaristically very interesting: when local fishermen bring in their nets, they discover cichlid species from the complex around Amphilophus citrinellus, a species that has long enjoyed great popularity among cichlid fans.
Yellow color morphs
These appropriately named Midas cichlids are the most important food fish for the local population and dominate the ichthyofauna of Lakes Nicaragua and Managua, where they occur in large numbers. The English name is based on the Greek myth of King Midas, who turned everything that he touched into gold. In addition to the normal black and gray striped variants, the Midas cichlids include very attractive yellowish-orange variants.
The so-called “gold morph” is relatively rare and occurs in about 10 percent of natural populations. In a few populations, no gold morphs have been detected so far. The expression of this trait is genetically predisposed. Particularly interesting is the fact that all fry initially have an unspectacular dark body color. Most, but not all, of the genetically golden animals gradually lose the black pigment in their body cells, so that yellow and orange pigments dominate. Occasionally there are absolutely white animals that have no dark or orange pigments.
The color change usually begins on the fin bases and expands to different parts of the body flanks and fin tips. Animals that are in the process of transformation often show bizarre color patterns that resemble a red, black, and white patchwork.
Midas cichlid variety
Midas cichlids are quite variable in coloration and body shape. This was memorably highlighted in 1907 by the American ichthyologist Meek, the experienced curator of the Natural History Museum in Chicago. He was not sure whether he was encountering one or a number of species among the Midas cichlids:
“Of all the species (of) fishes in these lakes, this one is by far the most variable. I made many repeated efforts to divide this material…in from two to a half-dozen or more species, but in all cases I was unable to find any tangible constant characters to define them. To regard them as more than one species meant only to limit the number by material at hand, and so I have lumped them all in one”.
When my colleagues and I visited the fish market in Granada on Lake Nicaragua in the early morning to find specimens of rare forms, we were able to confirm Meek’s view.
The most common species kept in aquariums are certainly Amphilophus citrinellus and A. labiatus; the latter is found much less frequently, at least at the fish market. Amphilophus labiatus has a more pointed head and thick, fleshy lips that it probably uses as a kind of suction tool to capture crustaceans and insect larvae from the crevices and interstices of the rugged rocky habitats in which the species occurs (Barlow & Munsey, 1976). These lips could also function as a type of sensing probe. Over generations, aquarium strains usually lose a large part of the lip volume, and only a small swelling remains. This suggests that the expression of enlarged lips has a genetic background that is modifiable by the environment.
Species laboratories: crater lakes
Besides the large Nicaraguan lakes, every traveler immediately notices the number of active and inactive volcanoes that belong to the Pacific Ring of Fire and dot the country near the Pacific coast from north to south. In some of the extinct and isolated volcanic craters, lakes have been formed by the accumulation of rainwater and groundwater. In stark contrast to the shallow Lakes Managua and Nicaragua, these crater lakes are up to 650 feet (200 m) deep and characterized by steep, sloping shorelines and extensive open-water areas.
Some of these crater lakes were colonized long ago by fishes, including the Midas cichlids. In some, if not all, of these still largely unspoiled crater lakes, endemic species occur that are found nowhere else in the world.
Based on evolutionary time scales, the emergence of new species has progressed rapidly and continues to do so. In Lake Apoyo, which is estimated to be 24,000 years old, there are at least six endemic species. Genetic research has shown that the founder population of Midas cichlids that originated from large Nicaraguan lakes populated Lake Apoyo about 10,000 years ago and then diverged (Barluenga, 2006). Lake Xiloa, which is at most 6,100 years old, has at least three endemic species.
The history of the colonization and the emergence of biological diversity in these and some other crater lakes is the subject of current research. Our studies have already shown that Midas cichlids in each of the lakes are morphologically unique (Elmer, Kusche, 2010).
Today, quite a few crater lakes contain species derived from the same ancestral Midas cichlids, so these lakes are natural laboratories for the study of evolution.
However, human interference is already evident. African Tilapia have been released in some lakes. This invasive species probably has a strong influence on species composition. For example, in Lake Apoyo the population collapse of the filamentous Chara sp. green algae was apparently due to the plants being eaten by Tilapia. Chara plays an important role in the food web of the volcanic crater lakes and also provides breeding grounds for some of the endemic species.
What follows is a brief overview of the diversity of Midas cichlids in Nicaraguan crater lakes.
Elongated open-water species: In Lake Apoyo there is a Midas cichlid that appears particularly well adapted for life in the open water of the crater lake: the elongated Arrow Cichlid Amphilophus zaliosus (Barlow & Munsey, 1976). Elongated species also occur in Lake Xiloa and probably in Asososca Managua. Most likely, these forms developed in parallel and independently from each other. These elongated Midas cichlids might be the only neotropical cichlid species of the approximately 600 species known that live in open water.
Thick-lipped forms: The thick-lipped forms of Amphilophus cf. citrinellus occur more frequently in some of the crater lakes (Lake Masaya and Lake Apoyeque). The fish with the impressive thick lips probably have a different-diet than their “normal” relatives and therefore occupy a different ecological niche. It is not clear whether these lips originated from Amphilophus labiatus or evolved independently in the crater lakes. It is especially interesting that at least the Midas cichlid population of Lake Apoyeque appears to be diverging into two new species, one with enlarged lips and one without (Elmer, Lehtonen, 2010).
Color morphs: The above-mentioned color morphs occur not only in the large Nicaraguan lakes, but also in many crater lakes. The orange variant is genetically dominant, but it still occurs relatively rarely. Perhaps the selection pressure on the conspicuous gold morph is much stronger than it is on the well-camouflaged gray-black Midas cichlids. Apparently, speciation based on this color polymorphism takes place in some of the crater lakes.
At least the gold morphs preferentially mate with each other and only rarely with the “normal” variants (Elmer, 2009). It is also conceivable that there are intraspecific “communication problems.” The gold morph lacks the vertical bars on the flanks, which represent an important mood indicator and communication signal.