Kissing gouramis, also known as kissing fish or kissers (Helostoma temminckii), are medium-sized tropical freshwater fish comprising the monotypic labyrinth fish family Helostomatidae (from the Greek elos [stud, nail], stoma [mouth]). The kissing gourami is a popular aquarium fish.
Two colour morphs are known: greenish-silvery, which have lengthwise spotty stripes on the body and opaque, dark brown fins; and pink, which have a rose to orangey-pink body and silvery scales with transparent pinkish fins. While it has been claimed that the former fish originate from Thailand and the latter fish originate from Java,
Jaw and mouth
It is important to emphasize the particularity that this species has in its mouth: it has an additional joint between its jaw and the rest of the joints, known as intramandibular joint. This type of joint is also present in other species of fish that feed on nutrients attached to the substrate. By increasing the angle of the opening of the jaw, this joint allows kissing gourami access to these nutrients. H. temminckii, however, is the only species known that uses this articulation in this way.
The intramandibular joint divides the jaw into two independent parts, giving the mouth a greater degree of movement and increasing the potential number of ways of obtaining food. This articulation, although its mechanism and anatomy are still being studied, results in an adaptation of H. temminckii and its feeding.
Habitat and ecology
Shallow, slow-moving, and thickly vegetated backwaters are the kissing gourami's natural habitat. The fish use their toothed lips to rasp algae from stones and other surfaces. This rasping action, which looks superficially like kissing, is also used by males to challenge the dominancy of conspecifics.
Intraspecific behaviour
thumb|Video of two kissers in an aquarium kissing.
The kissing gourami display a unique behaviour associated with their characteristic jaw and mouth. Two individuals approach with their jaws fully extended and press their mouths together for a few seconds. This "kissing" behaviour has given H. temminckii the common name of kissing gourami. This has been considered an intraspecific aggressive behavior, also known as "mouth fighting" due to the contraction of the jaw muscles. However, it is not completely confirmed that it is an aggressive behavior and is rather understood as a ritualized form of aggression.
Ecosystem roles
Helostoma temminckii is a host of some parasitic algal species. These algae are able to survive under kissing gourami skin and appear as dark spots. Some hypotheses affirm that these algae communities receive nutrients required for photosynthesis from H. temminckii. Individuals possessing algae parasites in their bodies are commonly found to be less healthy than non-afflicted individuals.
H. temminckii are able to communicate with each other using their complex inner ear. It contains a suprabranchial air-breathing chamber capable of modulating sounds through air bubbles inside of it. Kissing gourami can also make sounds used for communication by moving their teeth.
Reproduction
Helostoma temminckii are oviparous, dioecious, and mate through external fertilization. Spawning occurs from May to October in Thailand at the beginning of the rainy season. Adults migrate through rivers to shallow lagoons or flooded forests to spawn. Kissing gouramis are open-water egg scatterers; spawning is initiated by the female and takes place under cover of floating vegetation. The female lays an average of 1,000 spherical buoyant eggs, which the adults do not guard. Initial development is rapid; the eggs hatch after one day, and the fry are free-swimming two days thereafter. The kissing gourami does not care for its young.
Feeding
The kissing gourami is an omnivorous microphagic filtering fish. The mouth, teeth, gills, and intramandibular joint make the kissing gourami well-adapted for scraping nutrients from algae-covered surfaces. Its diet consists of a wide variety of food sources, such as insects, algae, and larvae of other species attached to rocks and substrates.