Author: Massimo Boyer
What purpose does the mouth serve? Raise your hand if you answered "to speak." Ultimately, it's partly true for us: if our species has reached this level of evolution, it owes much of it to an organ capable of producing articulated sounds in the form of language. Without language, our culture would hardly have arisen; we would have no writing, no literature, no theater, no singing, no science; I couldn't tell you these things. For all this, we need a mouth with an articulated jaw, muscles capable of varied and precise movements, teeth, lips, and a tongue. But the more correct answer to the opening question would have been much more prosaic: the primary biological function of a mouth is, above all, eating.
Let's move on to another question: who invented the articulated mouth of vertebrates? Demonstrating that the primary function of the mouth opening is to introduce food and not to emit sounds, the mouth, as we see it today, originated with animals that are mute by definition: fish.
The first aquatic vertebrates possessed only a single, hooked sucker, useful for parasitic survival by sucking the blood of other animals or for sucking up decaying material. Lampreys still have similar mouths. But the change that allowed fish to become dominant in the seas, overtaking invertebrates, and embark on an evolutionary path that led them to conquer the land, was precisely the evolution and perfection of a mouth equipped with articulated jaws.
This invention led to a massive evolutionary radiation: efficient and versatile, with a series of small adjustments, fish mouths became tools for capturing and subduing fast, darting prey, for detaching and crushing animals protected by hard shells from rocks, and for gathering algae or soft invertebrates clinging to the seabed. Small adjustments allowed fish to specialize in exploiting all the food resources the sea offered. The result of this radiation, which we see today, is the presence in the sea of so many different fish species, and so many different mouth models.
If you don't look a gift horse in the mouth, looking a fish in the mouth will tell you a lot about it, its habits, its role in the ecosystem.
The mouthparts of fish are more than what we see from the outside: in addition to the lips and teeth, almost all fish have a second set of jaws, called pharyngeal jaws. They are located inside the mouth and are responsible for processing the food that the external jaws capture and convey.
This second set is very important in the nutrition of the so-called durophagi, that is, fish that feed on hard prey, protected by armor, shells, calcareous skeletons.
No braces for the triggerfish Balistoides viridescens. Its protruding, fearsome teeth are an excellent tool for removing stubbornly attached organisms from the bottom.
Left. Close-up of the four chisel-like teeth of the pufferfish Arothron mappa, which it uses to retrieve its prey from the seabed.
Right. The beak of the coral-crushing parrotfish Chlorurus microrhinos is formed by fused teeth.
Evolution is a race: as prey—molluscs, crustaceans, and echinoderms—evolve their arsenal of shells, armor, and spines, their predators respond with increasingly robust jaws, capable of shattering any protective covering.
The chisel-like teeth of triggerfish, pufferfish, and many wrasses are formidable tools for detaching tenaciously attached organisms (mollusks, crabs, sea urchins) from rocks. They can also chip away at the shell, but the task of crushing it is delegated to the pharyngeal jaws, which also separate the organic matter, which is conveyed to the throat, from the skeletal parts that are spat out or expelled through the gill slits.
Parrotfish eat away at coral. In fact, only a few species feed on living coral; most simply nibble on dead coral, which is home to tasty algae. The parrotfish's formidable teeth, fused into a beak, chip away small pieces of coral, but a single, large pharyngeal molar acts like a grinder, crushing the ingested material into fine sand and extracting organic matter.
The Moorish idol, Zanclus cornutus, uses its small, elongated mouth, equipped with toothbrush-like teeth, to feed on the soft, mucus-rich inner tissues of sponges. Here it is in action.
Close-up of the gaping mouth of the batfish Platax teira, a herbivore.
Left. Perfect fit! The tapered mouth of the Chaetodon baronessa butterflyfish enables it to sever the densely branched tentacles of Acropora coral polyps.
Right. An unusual close-up of the angelfish Pomacanthus imperator highlights its tiny teeth. When disturbed, the male makes a clicking sound with his jaw.
Many other fish, however, specialize in scavenging benthic animals or plants, feeding on small pieces of gelatinous, slimy, soft organisms that they must tear from the seabed to which they are attached. These include the herbivorous surgeonfish, rabbitfish, and batfish, as well as the butterflyfish that feed on coral polyps, and the angelfish and Moorish idolfish that eat sponges. All these fish have small, often prominent mouths to reach the most hidden crevices; they have comb- or brush-like teeth, ideal for scraping soft organisms.
In all major families of reef fish there are species that feed on plankton. All of these planktivores, while sharing the characteristics typical of each family, they will have some common features, such as a tube-shaped mouth that can be thrust forward. This sudden movement allows the fish to quickly bring its mouth towards its prey, making it difficult to escape. Furthermore, when combined with the opening of the gills, the movement creates a depression that sucks the prey in.
Left. The protrusible mouth of plankton-eating fish. This elongation facilitates prey capture. Female Pseudanthias hypselosoma.
Right. Although it doesn't use them to catch the plankton it feeds on, the redtooth triggerfish Odonus niger displays strange Dracula-like canines.
Many fish feed on other fish, that is, they are piscivoresFor a piscivore, the main requirement is to have a fast mouth capable of capturing fast, wriggling prey, covered in a layer of mucus that makes it slimy and elusive.
While sharks can tear any prey to pieces before swallowing it with a series of sharp teeth, bony fish typically swallow their prey whole. Their teeth are a deadly arc of hooks, sinking into the prey's flesh and holding it in place, while their pharyngeal jaws eventually work to subdue it and push it down the esophagus.
The large mouths of fish-eaters allow them to catch large prey. The red grouper, Cephalopholis sonnerati.
Left. The unusual pigmentation of the redmouth grouper Aethaloperca rogaa.
Right. In ambush predators, such as the scorpionfish Scorpaenopsis oxycephala, a large mouth allows for profitable catches.
Left. The lizardfish Saurida nebulosa displays a grin that is not very reassuring to its prey.
Right. Like daggers, the teeth of the moray eel Gymnothorax meleagris, also located on the roof of its mouth, pierce its prey and hold it, with little chance of escape.
Left. Almost invisible, the giant frogfish Antennarius commersoni displays a rather unreassuring mouth.
Right. Frogfishes are able to dilate their mouths incredibly to catch prey nearly their own size. The gaping mouth of the hairy frogfish Antennarius striatus.
Antennarius striatus
Left. The elongated mouth of the hawkfish Oxycirrhites typus closes with extraordinary speed on small fish.
Right. A peculiar predator. The false cleaner wrasse (Aspidontus taeniatus) approaches its prey by pretending to be a cleaner and then snaps it, breaking off small pieces of scales or fins.
How does it feel after being attacked by a lizardfish? This Pseudanthias squamipinnis escaped but suffered devastating injuries, making it easy prey.
Large mouths are especially characteristic of less mobile predators, those that hunt by ambush, such as frogfish, scorpionfish, and lizardfish. The difficulty of capture is balanced by the ability to capture very large prey, thus maximizing the yield.
Extreme specializations are found in long-beaked butterflyfish (genera Chelmon, Forcipiger): their elongated mouthparts enable them to pinch prey in the most inaccessible crevices. The mouth of syngnathiforms (seahorses, trumpetfish, pipefish, ghostfish) is an elongated tube closed at the end. The opening of the gills and the expansion of the tube create a vacuum that, when the tilting jaw opens, creates a powerful suction: deadly for small prey such as crustaceans and small fish.
Left. The elongated mouth of the butterflyfish Chelmon rostratus, adapted for catching small invertebrates among corals.
Right. Like all syngnathiforms, the ghostfish Solenostomus paradoxus uses its tube-shaped mouth to catch small prey.
The extraordinary tube-like mouth of the trumpet fish Aulostomus chinensis, a masterpiece of high engineering.
In limbless animals, such as fish, the mouth is often a valuable tool for other activities as well. For example, the reproductionJawfish and cardinalfish use it as an incubator: the male guards the eggs there until they hatch, practically suspending feeding.
Nest-building fish (Aqva no. 220) use their mouths to clear the soil on which the female will lay her eggs, and they will have teeth for scraping, tearing, and brushing, even if their diet may not include any of these activities. The male of these fish usually uses his mouth to care for the eggs, cleaning them and transferring important substances for their immunity through his saliva.
Left. The enormous mouth of the jawfish Opistognathus sp. is also used by the male to incubate its eggs.
Right. Cheilodipterus macrodon is a large piscivorous cardinalfish. Behind the set of teeth, the eggs can be seen, with the eyes of the young already visible.
Left. Determined for coral: the planktivorous golden damselfish Amblyglyphidodon aureus displays enviable incisors. The male uses them to tear away the soft tissue of black corals and expose the skeleton where the female will lay her eggs.
Right. Tender with his young: Amblyglyphidodon aureus uses its mouth to clean its eggs and pass antibiotics to them through its saliva. Here's dad's first kiss.
A note on theoral hygieneWe've clarified how precious the mouth is to fish. It's obvious that such an important organ needs thorough cleaning. As always, since they can't do it themselves, fish are forced to rely on cleaners, other fish, or shrimp, for cleaning. During their stay with the cleaner, opening their mouths to remove food debris from their teeth is always an important and prolonged process.
Finally, a final note. Among the secondary functions of the mouth, few fish emit sounds. Not all are mute; some clownfish, for example, and some angelfish emit clicking sounds by rapidly opening and closing their jaws to threaten intruders who come too close. In short, the use of the mouth for communication had already been discovered by fish in some way.
Left. Fish oral hygiene requires the use of a cleaner. Labroides dimidiatus in the mouth of the porcupinefish Diodon hystrix.
Right. The sweetlip Plectorhinchus polytaenia opens its jaws wide to allow the cleaner wrasse Labroides dimidiatus to enter. Cleaning is an important part of every fish's day.
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© Massimo Boyer – www.kudalaut.eu [article published in Aqva n. 227]
