Betta Fish Anatomy: Interactive 3D Model and Labeled Diagrams

A betta is a small, slender bony fish, up to about 6.5 cm long, with an upturned mouth, one dorsal fin set far back, a very long anal fin, long thin pelvic (ventral) fins, and a pair of labyrinth organs above the gills that let it breathe air. Its body plan is otherwise that of a typical spiny-rayed fish: gills under a bony gill cover, a swim bladder, a four-part heart, and a short gut suited to eating insects and small animals.

Wild bettas have short fins. Wild Betta splendens live in still, shallow water in the Mekong basin of Southeast Asia: floodplains, canals, rice paddies and ponds (FishBase; Watson et al.). They are dark and fairly short-finned. Bettas were bred for fighting in Thailand for centuries, with reports going back to the 14th century (Kwon et al. 2022), and ornamental breeders later turned these short-finned fish into the long-finned veiltails, halfmoons and crowntails sold today. Wild B. splendens are listed as Vulnerable by the IUCN.

These labeled diagrams match the 3D model. The first shows the external anatomy of a male with every fin named; the second shows the internal organs with the body wall removed.

A betta has seven fins: three single fins on the midline (dorsal, caudal and anal) and two pairs (pectoral and pelvic). Hobbyists often call the pelvic fins ventral fins. Each fin is a thin membrane held up by jointed bony fin rays.

Ray counts: Tan & Ng (2005), wild-caught B. splendens.

• Mouth (superior, upturned): Picks food from just under and at the water surface, and gulps air for the labyrinth organ. Males also use it to blow mucus-coated bubbles for the nest and to carry eggs.
• Nostrils (nares): Smell only. They do not connect to the mouth and are not used for breathing.
• Eye: Colour vision for spotting prey and rivals. Fish have no eyelids.
• Operculum (gill cover): Protects the gills and pumps water over them. During a display a betta 'flares': it swings both gill covers out sideways to look bigger.
• Branchiostegal membrane (the “beard”): Seals the gill chamber from below during breathing. When a betta flares, the membrane is pushed out and down and shows as a dark ruff under the head.
• Scales: Flexible armour that protects the body while letting it bend.
• Caudal peduncle: Transfers the pull of the trunk muscles to the tail fin.
• Anus (vent): Releases faeces.
• Egg spot (ovipositor): The opening through which the female releases her eggs.

Betta "types" are named mostly by the shape of the tail. All are the same species; they differ in fin length and webbing, which breeders have selected for. Genetic studies show that several fin traits are controlled by single major genes, and that different fins can be changed independently.

Very long fins come at a cost: they add drag and tear easily, which can make swimming harder for show bettas than for short-finned plakats.

The betta labyrinth organ is a pair of air-breathing organs, one on each side of the head, above the gills and under the gill cover. Each sits in an air space called the suprabranchial chamber. The organ itself is a set of thin, folded bony plates that grow from the first epibranchial (the upper bone of the first gill arch), covered with thin, blood-rich respiratory epithelium. The folds give a large surface for taking oxygen from a bubble of air (Tate et al. 2017).

How a breath works. The betta swims to the surface and gulps air. On the next breath, water pumped into the chamber pushes the old air forward into the mouth, where it leaves as a bubble; fresh air is then pushed up into the chamber, and any surplus escapes through the gill openings (Peters 1978; Graham 1997). Blood from the first gill arches flows through the labyrinth and picks up oxygen before returning to the heart.

Do bettas have to breathe air? Bettas breathe air routinely, but studies class B. splendens as a facultative air breather: in well-oxygenated water it can meet its needs with its gills, unlike the obligate air-breathing blue gourami (Mendez-Sanchez & Burggren 2014, 2019). The popular claim that a betta always "drowns" without air is too simple. In practice, warm, still aquarium water is often low in oxygen, and displaying males use much more oxygen, so a betta must always be able to reach the surface easily. Bettas are not born with a working labyrinth; it develops during the first weeks of life.

Betta blood also binds oxygen extremely tightly (half-saturated at only about 2.9 mmHg), a trait of fishes that live in low-oxygen water (Mendez-Sanchez & Burggren 2019).

• Labyrinth organ: Takes oxygen from gulped air. The bony plates grow from the first epibranchial (the upper bone of the first gill arch), and their folds give a large surface of thin respiratory epithelium close to blood capillaries.
• Suprabranchial chamber: Holds the gulped air bubble against the labyrinth. On the next breath, water pumped into the chamber pushes the old air out into the mouth; it leaves as a bubble. Fresh air is then gulped, and surplus air escapes through the gill openings (Peters 1978; Graham 1997).

Betta gills are like those of other bony fish: four gill arches on each side, each with two rows of red gill filaments and short gill rakers. Because the labyrinth chamber takes up space above them, labyrinth fishes have relatively small gills compared with fishes that breathe only water. Among labyrinth fishes, though, bettas have fairly large gills: young B. splendens have about three times the gill area of the blue gourami at the same stage, which fits their ability to live on water-breathing alone (Mendez-Sanchez & Burggren 2019).

• Gill arches: Support the gill filaments and the blood vessels that serve them.
• Gill filaments: Take oxygen from water and release carbon dioxide and ammonia.
• Gill rakers: Keep food from passing out through the gills.

A betta's internal organs are packed into a short body cavity behind the head, because the vent lies far forward, at about 40–45% of the body length. From front to back you find the heart under the throat, the liver, a small stomach, the looped intestine, the gonad, and above them the swim bladder and the dark red kidney against the backbone. The brain sits in the skull close to the labyrinth chambers.

• Stomach: Stores and starts digesting prey.
• Intestine: Digests food and absorbs nutrients. A short gut suits a diet of insects and small animals.
• Liver: Makes bile, stores energy and processes nutrients and toxins.
• Gall bladder: Stores bile for fat digestion.
• Spleen: Makes and stores blood cells; part of the immune system.
• Swim bladder: Controls buoyancy, so the fish can hover without effort.
• Kidney: Makes large amounts of dilute urine to remove water that enters through the gills in fresh water.

Betta stomach size. A betta's stomach is a small sac. You may read that it is "the size of its eye"; that is a feeding rule of thumb, not a measured fact. The useful point stands: a betta is a small fish, and a few small food items once or twice a day is enough.

Heart. Like other fish, a betta has a single circulation and a heart with four parts in series: sinus venosus, atrium, ventricle and bulbus arteriosus. It pumps oxygen-poor blood to the gills and labyrinth.

• Sinus venosus: First of the heart's four parts in series.
• Atrium: Fills the ventricle.
• Ventricle: Pumps blood forward to the gills and labyrinth.
• Bulbus arteriosus: Smooths each pulse of blood.
• Dorsal aorta: Carries oxygenated blood to the organs and muscles.

Male and female bettas differ most in their fins, colour and beard. Use the Male/Female switch on the model to compare them. Sex becomes easy to tell at about 3–4 months of age (Watson et al.). In ornamental bettas, sex is mainly set by the dmrt1 gene, with males as the XY sex (Kwon et al. 2022).

The egg spot is a useful clue but not perfect: it can be hard to see on pale fish, and young males can show a similar spot.

The betta beard is the branchiostegal membrane, a skin fold under the gill covers held up by thin branchiostegal rays. When a betta flares, it swings both gill covers out sideways, pushes the beard down and spreads its fins to look larger. Males flare at rival males and even at their own reflection.

Flaring is costly. With the gill covers held open, water does not flow well over the gills, so the fish briefly puts itself into a low-oxygen state. Displaying males therefore breathe air more often; the oxygen they take in per breath stays the same, so they make more trips to the surface (reviewed by Tate et al. 2017).

Male bettas build bubble nests: they gulp air at the surface, mix it with mucus in the mouth, and blow sticky bubbles that float together, often among plants (Tate et al. 2017). The female releases eggs during the spawning embrace, when the male wraps his body around her and fertilises the eggs as they are released (Watson et al.). The male then places the eggs in the nest and guards the eggs and young. The nest keeps eggs at the surface, where oxygen exchange is best in warm, still water. Larvae use up their yolk sac about 3 days after hatching.

• Ovary: Produces eggs, released in batches during the spawning embrace.
• Testes: Produce sperm.
• Egg spot (ovipositor): The opening through which the female releases her eggs.

Betta colour comes from chromatophores (colour cells) in the skin. Black-brown melanophores hold melanin, yellow-red cells hold pteridine and carotenoid pigments, and iridophores contain stacks of reflective crystals that produce blue, green and metallic sheens. Genetic studies of pet bettas found strong selection on genes for melanin (mitfa), red pigments (xdh, bco1l) and iridophore development (alkal2l), and an albino form caused by a change near mitfa (Wang et al. 2021; Kwon et al. 2022).

Bettas have thin, overlapping scales, with 29–31 in a row along the side of the body (Tan & Ng 2005). Wild males have red parallel bars on the gill cover.

Knowing the anatomy helps explain common pet problems. These notes are general; for diagnosis and treatment, see a veterinarian who treats fish.

Swim bladder problems. The swim bladder controls buoyancy. A betta that floats, sinks or swims tilted is often said to have "swim bladder disease", but that describes a sign, not a cause. In fish, buoyancy problems can come from the gut (for example, a swollen stomach or intestine pressing on the bladder), from infection or fluid inside the swim bladder, from deformity or from injury (Sirri et al. 2020; Merck Veterinary Manual). A vet can use X-rays to see the swim bladder and tell these apart.

Fin damage and fin rot. Long betta fins are thin membranes over fragile rays, so they tear easily and are slow to heal. Fin damage can come from biting by tank mates or from sharp decorations. "Fin rot" is the name for fins that fray and recede from the edges; bacteria such as Aeromonas, Pseudomonas and Flavobacterium columnare are common in diseased aquarium fish, and outbreaks are strongly linked to poor water quality (Merck Veterinary Manual). Fin rays can regrow if the base of the fin is healthy.

Breathing. Because bettas take air at the surface, they need open surface water.

Bettas are pets, not lab specimens, and are far too small for a class dissection. To learn fish dissection, use the step-by-step virtual perch dissection; the organs you find in a perch match those of a betta, apart from the labyrinth organ. Use the 3D model above to explore the betta's internal organs without harming an animal.

Compared with a generic bony fish such as the yellow perch on our fish anatomy page, a betta has the same basic organs but several special features: