The beating heart is often seen as the very symbol of life in animals. But do fish, those creatures of the watery depths, also possess this vital organ? As it turns out, fish do indeed have hearts that pump blood throughout their bodies.

However, the structure and function of fish hearts can differ greatly from mammalian hearts.

If you’re short on time, here’s a quick answer: Yes, fish do have hearts. A fish heart consists of two or more chambers and pumps oxygenated blood obtained from the fish’s gills out to the rest of its body. The specific structure and operation of a fish heart depends on the species.

In this roughly 3000 word article, we’ll take a deep look at the circulatory systems and hearts of various fish species. We’ll examine how fish hearts differ from mammalian hearts in terms of anatomy and physiology.

And we’ll learn about the evolutionary reasons behind the development of different heart types in fish occupying various aquatic environments and habitats.

An Overview of Fish Circulatory Systems

Single vs. Double Circulation

Fish have a single circulatory system, meaning blood flows through the heart only once per circuit. Deoxygenated blood enters the heart, where it is pumped to the gills to become oxygenated, before circulating throughout the rest of the body to deliver nutrients and remove waste.

This differs from humans and other mammals with a double circulatory system, which features separate pulmonary and systemic circuits to optimize oxygen delivery.

The single loop system of fish works efficiently because oxygen is absorbed directly from the water by the gills, rather than requiring specialized lungs. Having one circuit also reduces cardiac workload. However, it limits active fish from achieving as high aerobic scopes as fast-moving mammals.

For example, tuna swim fast but must constantly keep moving to pass water over their gills, unlike whales which can hold their breath for long dives.

Variations Between Fish Species

While all fish utilize a single circulatory loop, the structure of the heart can differ substantially between fish types. For instance, many bony marine fish like cods have a simplistic two-chambered heart to pump blood, while sharks with higher metabolism rates boast up to five heart chambers to separate venous and arterial blood flows (American Museum of Natural History).

Cardiovascular adaptations also occur by habitat. Fast pelagic fish often have larger, more muscular ventricles to handle greater blood volumes. Demersal fish ventricles pump at very high pressures to circulate blood throughout an elongated body shape.

Deep sea species may even lack true gas bladders, using fatty organs instead for buoyancy aided by high blood pressure pumping.

Fish Heart Anatomy and Structure

Number and Type of Chambers

Most fish have a two-chambered heart consisting of one atrium and one ventricle. The atrium collects blood that returns from the body, while the ventricle pumps blood to the gills or directly to the body.

This two-chambered design is efficient for gas exchange but limits the ability to regulate systemic blood pressure.

Some fish, like tuna and sharks, have partial divisions in their ventricles to improve pumping efficiency. A few fish also have accessory chambers that may aid venous return to the heart. Overall, the two-chambered design in most fish hearts provides simple but effective circulatory function.

Accessory Chambers

While most fish have a straightforward two-chambered heart, some species evolved with additional cardiac chambers or sinuses that serve accessory functions:

  • Sinus venosus – Collects venous blood returning to the heart. Found in some cartilaginous and ray-finned fish.
  • Conus arteriosus – Smoothes out the pulsatile flow from the ventricle. Present in some sharks and ray-finned fish.
  • Bulbus arteriosus – Dampens the pressure of blood ejected from the ventricle. Occurs in some teleost fish.

These extra chambers allow the heart to cope with higher blood pressures or tailor blood flow for different swimming speeds. While not critical components, they highlight how fish cardiovascular systems adapted to unique demands.

Valves

To prevent backflow of blood, fish hearts contain valves at the junctions between chambers:

  • Atrioventricular valve – Between the atrium and ventricle; consists of flaps of connective tissue.
  • Bulboventricular valve – Between the bulbus arteriosus and ventricle when present.
  • Semilunar valves – At the exits of the bulbus arteriosus or conus arteriosus when present.

In the absence of accessory chambers, most fish have just the AV valve. These valves open and close passively in response to pressure changes, ensuring one-way flow of blood through the heart. This valvular system is simple compared to four-chambered hearts but effectively isolates oxygenated and deoxygenated blood.

How Fish Hearts Function and Circulate Blood

Systemic Circulation

Fish have a closed circulatory system where blood flows through vessels to deliver nutrients and oxygen throughout the body. The main components are the heart, arteries, veins and capillaries. Systemic circulation refers to the flow from the heart through the gills where blood receives oxygen, and then transports oxygenated blood to organs and tissues before returning to the heart.

The primary role of systemic circulation in fish is delivering oxygen and nutrients absorbed from food in the digestive system to body cells, and collecting carbon dioxide and waste products. The oxygenated blood flows from the gills into heart chambers that pump it under high pressure into thick-walled arteries and smaller arterioles.

The blood then flows through tiny capillaries surrounding tissue and cell structures to facilitate gas and nutrient exchange.

Pulmonary Circulation

Pulmonary circulation is the portion of circulation in which deoxygenated blood leaves the heart, flows to the lungs, and returns oxygenated to the heart. Most fish do not have lungs and therefore lack a pulmonary circulation. Only lungfish and a few other primitive species have both gills and lungs.

In these fish, some deoxygenated blood does flow to the lungs before returning to the heart. But since most fish breathe through gills, pulmonary circulation plays a minor role.

The key to fish respiration is the large surface area of capillaries in the gills exchanging gases with water, not lungs and pulmonary circulation. Researchers believe the evolution of lungs and pulmonary circulation occurred as primitive fish moved to low-oxygen waters and needed supplementary breathing mechanisms.

Modern bony fish generally utilize only gill respiration.

Regulation of Heart Rate

Fish heart rates vary by species and range from 12 beats per minute in largemouth bass to 240 beats per minute in the mako shark. The muscles of the hearts are stimulated by nerves connected to pacemaker cells that establish the rhythm.

Heart rate is regulated by controlling nerve signals to the cardiac muscles in response to activity levels and external stressors through complex neurochemical and hormonal mechanisms. This allows sufficient oxygenated blood circulation for different levels of exertion.

Fish Type Typical Heart Rate (bpm)
Salmon 55
Trout 72
Tuna 90

When fish engage muscles for swimming or reacting, coordinated signals from the brain and nervous system activate the release of adrenaline and other hormones. These horomones bind to receptors in pacemaker cells and adjust heart rate to match metabolic demands.

An increase pumps more oxygenated blood to muscles and organs.

External stressors like changes in temperature, oxygen levels and pollutants also stimulate receptors that alter pacemaker signals and heart rate to compensate. These control mechanisms allow fish cardiovascular systems to efficiently circulate blood and meet variable body needs.

Evolutionary Adaptations of Fish Cardiovascular Systems

Environmental Pressures

Fish live in aquatic environments that can vary greatly in temperature, oxygen levels, salinity, and pressure. These environmental factors have led to adaptations in the cardiovascular systems of fish over evolutionary time (Graham, 1997).

For example, fish that live in extremely cold waters like Antarctic icefishes have antifreeze proteins and fats in their blood to prevent freezing. Tropical fish tend to have fewer red blood cells and greater blood volume to improve oxygen dispersal.

Deep sea fish have adapted to high hydrostatic pressure through higher hematocrits and specialized proteins that function under pressure.

Activity Levels and Metabolism

Fish cardiovascular systems must match the oxygen and energy demands of different activity levels and metabolic rates (Icardo, 2022). For instance, fast-moving predatory fish like tuna have high aerobic scope thanks to larger hearts, greater stroke volume, and higher hematocrit than sluggish bottom dwellers.

Cardiovascular features also correlate with metabolic rate, a measure of oxygen use; fish with higher standard metabolic rates tend to have larger hearts and gills to deliver more oxygen.

Developmental Changes

The cardiovascular systems of fish change dramatically during embryonic development and growth (Burggren, 2022). In early embryos, circulation relies on a simple tubular heart that pumps blood through a capillary network. Later the chambers differentiate and valves form to improve flow.

The relative size of the heart shrinks as fish grow larger, since cardiac output depends more on increased stroke volume than heart rate. Hematocrit and blood oxygen carrying capacity also increase from larval to adult stages.

Unique Fish Heart Adaptations

Electric Eel

The electric eel has a very unique heart adaptation that allows it to produce powerful electric shocks. Its cardiac system is made up of four elongated heart chambers that contract sequentially to generate electricity.

Specialized cells called electrocytes are stacked together to form an electric organ that makes up 80% of the eel’s body. When the eel wants to emit an electric discharge, its brain sends signals that cause the electrocytes to open their channels and release positively charged ions.

This ion flow generates an electrical current of up to 600 volts! Truly an amazing example of a unique fish cardiovascular system.

Sharks and Rays

Sharks and rays have cardiac adaptations that enable their active, predatory lifestyles. Their hearts are composed of four chambers like mammals, but with differences that improve performance. For example, the spongy myocardium has improved blood and oxygen supply.

There is also a special vein called the sinus venosus that improves filling of the atria. These adaptations allow the shark heart to pump blood more efficiently when swimming fast. Some sharks can raise their metabolic rate by 50% during swimming!

Sharks and rays also have very high blood volumes compared to other fish, which helps supply oxygen while moving. Their unique hearts are clearly tailored for speed, strength and stamina.

Tuna

Tunas are among the fastest and most powerful swimmers in the ocean, thanks in part to cardiovascular adaptations. Their hearts are proportionately larger than most other fishes, with compact myocardium that can deliver high cardiac output.

Tunas also have blood with elevated hemoglobin levels for increased oxygen carrying capacity. Some tunas like mackerel even have special blood vessels called retia mirabilia that help keep their swimming muscles warm. They also have sophisticated countercurrent heat exchangers to prevent overheating.

These cardiovascular adaptations allow tunas to cruise at fast sustained speeds and make long-distance migrations across the seas.

Conclusion

As we’ve seen, fish have evolved a remarkable diversity of cardiovascular systems and heart structures to thrive in aquatic environments. While all fish hearts pump oxygenated blood to the body, the specific anatomy and physiology can vary greatly depending on the species’ habitat, activity levels, metabolism, and more.

One unifying theme across all fish, however, is that their hearts are generally simpler, have fewer chambers, and operate at lower pressures compared to mammalian hearts. These adaptations allow the fish heart to efficiently deliver oxygen while handling the high densities and viscosities of water.

Understanding the form and function of the piscine circulatory system not only sheds light on the evolutionary pressures faced by our cold-blooded cousins – it also gives us a new appreciation for the beating heart that gives all vertebrates, even fish, their vitality.

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