One of the reasons why cichlids are so successful in terms of the number of species and their diversification is their evolutionary plasticity; a feature that has allowed them to take advantage of most of the potentially edible organisms in the habitat they dwell. The so called “trophic radiation”, where species evolve driven by the developing of morphological specializations necessary to take advantage of a particular source of food, is what give us the great variability and many species in the family cichlidae.
The evolution affects both the anterior part of the body of the organism, driven by the food it specializes on and the posterior part of the body, driven by the necessary propulsion or habitat adaptation necessary to obtain that particular food.
In the anterior part of the body the jaws apparatus of fishes are comprised by two separate sets of jaws: the first set is the visible mouth, which has evolved the form, size and lips suitable to better grab the food. Both upper and lower jaws are equipped with specialized teeth that help them in this chore. The second set of jaws is found inside the oral cavity at the base of the pharynx and is also comprised by lower and upper parts with teeth specialized to process the captured food and push it back into the esophagus, before digestion; they are known as the pharyngeal jaws and evolved from the system of arches of bone that support the gills. Cichlids are one of the two known families of fish that possess movable pharyngeal jaws (the other is moray eels) that allow them to pre-process the food before it is swallowed. The mobility of the pharyngeal jaws is believed to be the cause for cichlids to be one of the most diverse families of vertebrates (cichlids comprise more than five percent of all fish species); although this is still a debated topic.
To better illustrate these distinct pairs of jaws the beast, created for the science-fiction movie “Alien”, is regularly used as an example. Alien would open the mouth and reveal a second set of jaws that projected forward to kill its victim. The function of the second set of jaws in Alien is, however, better illustrated by moray eels than by cichlids, since cichlids do not project the pharyngeal jaw forward, but are still able to move upper and lower pharyngeal jaw independently.
When animals crawled out of the sea in the course of evolution, they evolved legs to be able to adapt to their new environment and humans and other mammals subsequently developed arms and hands. Those arms and hands serves us well to grab, hold, and tear apart the food that we eat. Fish rely entirely on their mouth to grab, hold, and tear apart the food they eat, and they manage this wonderfully.
The snout of fishes is more elaborated than those of mammals. In humans for example, the upper jaw, attached to the skull, is unmovable while the lower jaw moves up and down to cut and chew food, with some lateral movements to help in this feat, that’s it. Fish jaws on the other hand are both movable which is nicely demonstrated by the Central American cichlid Petenia splendida, which has a snout that allows it to both to ram forward and at the same time create a void which produces a strong vacuum that sucks in the approaching prey. This sucking-tube snout has independently evolved in several other cichlid species like in those species in the South American Caquetaia species (once believed to be closely related to Petenia because of this trait); other examples are the African Pterochromis congicus and in Lake Malawi the plankton-eating haplochromines, better known as utaka.
In my observations young adults of Petenia splendida ambush fry-guarding cichlid pairs at a distance and suddenly rush into the nest; by the time they are in front of the fry they extend their jaws forward and suck in several fry. I have witnessed adults in Lake Bacalar in Mexico keeping motionless in the water column among a passing shoal of the silvery characids of the genus Astyanax. I presume the silver color of Petenia allows them to better blend in, while all of a sudden they project their mouth towards a nearby passerby in an effort to suck it in.
In other cases a long snout is a useful adaptation for searching prey in a soft substrate, such as is found in species of the Central American cichlid genus Cribroheros. These cichlids possess a horizontal substratum-sifting cranial morphology that allows them to collect substrate and sift it through their gills (cribrum = sieve [Latin]). The well-known South American cichlids of the genus Satanoperca are another good example of this feeding behavior. In Africa there are numerous examples, like the species in the Lake Malawi genus Aulonocara. The species in this genus have an additional adaptation so that they do not pick up mouthfuls of dirt randomly. On the head they have large sensory pores that allow them to feel the tiniest pushes of water made by invertebrates hiding in the substrate. They just move slowly and very close to the soft bottom and after noticing some movement they dig into the substrate and capture their prey!The position and orientation of the mouth is also variable and part of the feeding strategy. Predators, including piscivores, that capture large prey have often a horizontal mouth facing forward and placed in the middle of the head; consider most of the many species in the South American genus Crenicichla, as well as those species with highly protrusile snouts like those mentioned earlier. This position of the mouth is named terminal, and sometimes has evolved a longer lower jaw (prognathous).
Fish that attack from below or feed near the surface have mouths placed in the upper part of the snout and often facing upwards, those are called superior or supra-terminal, like the mouths in the species in the Tanganyika genera Cyprichromis and Benthochromis.
Species that feed from rocks or from the substrate often have mouths placed in the lower part of the head and often facing downwards; those mouths are referred to as inferior or sub-terminal. They often help to feed in strong currents without having to lean the body forwards and hence would use more energy, like in the Paraneetroplus species of Central American. A highly specialized and fascinating looking mouth of this type is that of Gnathochromis permaxillaris, a Lake Tanganyika endemic species. This species feeds on tiny invertebrates which it vacuums off the bottom thanks to its wide-gaped mouth.
Cichlid lips, unlike those of mammals, have no muscles and hence don’t move, but they however have developed adaptations in some species where they became fleshy to help them get their food. It is believed that fleshy lips work as gaskets helping the fish seal small crevices and suck out their food, but it can also be that the fleshy lips serve to absorb the shock when fish quickly seals the crevices with the mouth. Those lip modifications have evolved independently in most geographic areas where cichlids are found: Examples in South America are Bujurquina labiosa, Crenicichla tuca, and Gymnogeophagus labiatus; in Central America: Amphilophus labiatus and Wajpamheros nourissati. In Africa we have several examples as well: Lake Edward has Haplochromis labiatus, Lake Benin in Cameroon: Coptodon bakossiorum: Lake Tanganyika: Lobochilotes labiatus: Lake Malawi: Abactochromis labrosus, Chilotilapia euchilus, Protomelas ornatus, and Placidochromis milomo. There are a few more, although apparently not so many species have developed this trait. Most of the species that have fleshy lips have names that reflect this trait; labia meaning lips in Latin and cheilos the same in Greek.
I have personally observed two species of cichlids making use of their fleshy lips: Wajpamheros nourissati in the Chocolja River in Chiapas [Mexico], where adults face perpendicularly to the rocks and use its fleshy lips to seal crevices and/or absorb shock and suck out what I assumed were invertebrates. In Lake Malawi I was able to observe and photograph Chilotilapia euchilus, although in this case it would strike directly on a rock where the fish had apparently detected a prey, in both cases the lips would both seal the area and absorb the shock caused against an abrasive surface.
Behinds the lips cichlids possess several rows of teeth, with the first row (outer row) holding the larger teeth, and those in the front normally being the larger of all. Teeth have different shapes depending on what the fish eats. The most generalist shape of teeth is conical, used normally for predators as they serve them well to hold prey they capture, the fangs on the jaws of the powerful Central American predatory cichlid Parachromis dovii are a perfect example. In some cichlid species teeth in the outer row have become incisor-shaped, those serve well to herbivorous cichlids that either cut pieces of fruits, plants, chunks of periphyton or algae. In the latter case I can think for example in three Central American cichlid species that harvest algae (and its related fauna) attached to the rocks: Neetroplus nematopus in the San Juan River drainage; Paraneetroplus nebuliferus in the Coatzacoalcos drainage in Mexico and Herichthys tamasopoensis in the Panuco River drainage.
Those flattened teeth sometimes have cusps on the top, they could be two or three, and make them act as a comb, being able to filter out small particles, being diatoms, detritus, small organisms or lose threads of algae from a matrix of algae without having to cut off and ruin the supporting media. In Lake Malawi many of the rocky reef dwelling species known as mbuna have this type of teeth, like those in the genus Metriaclima with bicuspid teeth. This particular genus feeds at almost perpendicular angles to the substrate and is able to align the teeth of both upper and lower jaws in the same plane by abducting its jaws to a 180° angle opening! In Lake Tanganyika the genus Petrochromis acts similarly but they have several rows of teeth with three cusps.
In fishes, lost teeth are individually replaced and have some mobility, being able to face forward at one point (like when preparing to cut algae) and then backwards at another (like when actually cutting it or swallowing it).
The second set of jaws, the pharyngeal jaws, also have teeth on them, sometimes small fine teeth useful for tearing and transporting food, referred to as papilliform (resembling a papilla), or large molar shaped teeth useful for crushing, referred to as mollariform.
A fascinating example that illustrates the shape of teeth in the pharyngeal jaw is that of the Cuatro Cienegas cichlid: Herichthys minckleyi. H. minckleyi is known to be a polymorphic species, which means the species has several different morphs that in the case of H. minckleyi are three and each specialize in different types of food present in the environment. In this species, the ancestral form possess papilliform teeth on the lower pharyngeal jaw, this form mainly eats detritus; a second morph has developed the shape and behavior of a piscivore and also has papilliform teeth, useful for tearing meat. The third form has adapted to eating snails, for this end the teeth in the lower pharyngeal jaw have evolved into bulky molars, and serve well as a mill to break the shells of their prey. Pharyngeal jaws possess powerful muscles, but in those species where the jaws are used as mills the muscles are massive.
The combination of all the mentioned elements makes cichlids such a successful family. In this article I intend to give you a general overview of the structure and function of the food capturing apparatus in cichlids, for a more in-depth explanation, like in other articles, I highly recommend George Barlow’s wonderful book “The Cichlid Fishes: Nature’s grand experiment in evolution”, in particular chapter 2.
- Barlow, George W.. 2002. "The Cichlid Fishes (Nature's Grand Experiment in Evolution)". Perseus Publishing. pp. 352 pp. ISBN: 9780738203768 (crc03927) (riassunto)
© Copyright 2020 Juan Miguel Artigas Azas, all rights reserved
Artigas Azas, Juan Miguel. (agosto 24, 2020). "Feeding in cichlids – The feeding apparatus". Cichlid Room Companion. URL consultato in data aprile 12, 2021, da: https://cichlidae.com/section.php?id=310&lang=it.