Have you ever noticed a fish still wiggling around even after being pulled from the water? The unsettling movements of a dead fish can look like a sign of life, but in reality, the motions have everything to do with biology and little to do with the fish still being alive.
If you’re short on time, here’s a quick answer to your question: Fish continue to move after death due to involuntary muscle contractions triggered by the release of stored energy in their muscle cells. These movements have nothing to do with the fish still being alive.
In this article, we’ll explore why fish continue to exhibit motion after dying. We’ll cover topics like rigor mortis, ATP energy stores, and involuntary muscle spasms to shed light on this phenomenon.
Rigor Mortis Causes Muscle Stiffness
Definition of Rigor Mortis
Rigor mortis is the stiffening of muscles that occurs after death. It is caused by chemical changes in the muscles after the heart stops beating. Here’s an overview of how it works:
- When we are alive, our cells constantly produce adenosine triphosphate (ATP) which provides energy for muscle contraction. After death, ATP production stops.
- Without ATP, another chemical called adenosine diphosphate (ADP) builds up. ADP causes the muscles to contract and become rigid.
- The muscle stiffness usually begins within 2-6 hours after death. Maximum stiffness is reached around 12 hours after death.
- Rigor mortis starts to dissipate around 24-84 hours after death as the muscles start to decompose.
Rigor Mortis Process in Fish
The rigor mortis process in fish has some unique attributes compared to land animals:
- It starts sooner – within 30 minutes after death due to the lower muscle mass and body temperature of fish.
- It also resolves sooner – within 24 hours, as the fish body decomposes more quickly.
- Only partial rigor mortis may occur since the cold aquatic temperatures can inhibit ATP breakdown.
- The muscles stiffen in a caudal to cranial direction – starting from the tail and moving toward the head.
According to a study published in the Journal of Food Science, the rigor mortis pattern in fish depends on the glucose reserves and antioxidant concentrations in their muscles right after death. Fish with higher reserves exhibit faster onset and longer duration of rigor mortis.
The muscle stiffness in post-mortem fish has a direct impact on the texture and quality of the meat. Fish that enter rigor mortis pre-slaughter yields tough, gaping fillets. This makes rigor mortis an important aspect of quality control in the fish processing industry.
Lingering ATP Stores Fuel Motion
ATP Role in Muscle Contraction
Adenosine triphosphate (ATP) is an essential energy source that allows muscles to contract. ATP releases energy when its phosphate bonds are broken, powering muscle fibers to slide past each other and move body parts.
As long as ATP is present, muscles can continue contracting even after an animal dies.
Fish rely heavily on red slow-twitch muscles for swimming. These muscle fibers have abundant capillaries and mitochondria to produce ATP for sustained activity. So when a fish dies, its robust red muscles still contain significant ATP reserves to drive post-mortem motion.
ATP Breakdown After Death
Once blood circulation halts after death, cells quickly become depleted of oxygen and switch to anaerobic respiration. This less efficient ATP production pathway causes lactic acid buildup, reducing pH.
But while anaerobic ATP synthesis only persists for minutes after death, preexisting ATP stores take longer to degrade.
Enzymes continue breaking down ATP into ADP and phosphate ions post-mortem. However, the cool temperatures of water slow these enzymatic reactions. So in fish, ATP levels may be sufficient to fuel muscle contractions for hours before finally becoming exhausted.
According to a study published on CiteSeerX, the post-mortem movement of rainbow trout depended greatly on water temperature. Trout muscles contracted up to 147 minutes after death when kept at 46°F. But at temperatures above 68°F, motion declined significantly due to accelerated ATP breakdown.
Nerve Impulses Trigger Involuntary Spasms
Nervous System Activity After Death
Even after an animal dies, its nervous system can still send signals that cause muscles to contract and relax. This explains why the bodies of freshly dead animals often twitch or spasm involuntarily. The nerves and muscle fibers remain functional for a period of time after the heart stops beating and oxygenated blood stops flowing to the brain and other organs.
Without oxygen, the cells in the brain and nervous system quickly become damaged and lose function. However, it takes time for all the cells to die off completely. In the meantime, the remaining functional nerves may fire off random signals that stimulate muscles to move.
Studies show that nerve and muscle cells can remain active for several hours after death in many animals. For example, researchers monitoring neural activity in decapitated rats found that brainwaves persisted for about 4 seconds after decapitation.
Other experiments also detected brief electrical activity in the brain stems of decapitated rats for up to 1 minute after death. Additionally, muscles may contract for up to 12 hours after slaughter in chickens due to residual nerve impulses.
Overall, the window of time that nerves and muscles remain functional post-mortem depends on the animal species and conditions at death.
Random Nerve Signals Cause Twitching
The involuntary muscle spasms observed after death are caused by random electrical nerve impulses rather than coordinated signals from the brain. Without regulation from the brain, the remaining live nerves may fire haphazardly, leading to unintentional and reflexive muscle twitches.
For example, a nerve that previously controlled leg muscles may suddenly fire and cause the dead animal’s leg to jerk without warning. The muscles contract in response to the stimulatory nerve signals even though the brain did not actually command the movement.
The arbitrary nerve signals originate from nerves that are either still functioning or deteriorating after death. As cells die, they may release internal chemicals that can stimulate nearby nerve fibers. Additionally, reductions in oxygen can cause nerves to become hyperactive and fire spontaneously.
With no control from the brain, these random electrical impulses get transmitted to the muscles via motor neurons, triggering involuntary reflex reactions. Overall, the apparently purposeful muscle movements are just accidental after-effects of uncontrolled nerve cell activity after the animal’s death.
Cadaveric Spasms Create Brief Convulsions
Cadaveric Spasms Overview
Cadaveric spasms, also known as post-mortem spasms, are brief and irregular muscle contractions that occur in deceased animals and humans after death. These involuntary spasms are caused by a sudden loss of adenosine triphosphate (ATP), which provides energy for muscle function.
Without ATP, calcium builds up in muscle fibers and triggers brief contractions as the muscles of the deceased briefly activate in a random, sporadic manner.
Though disturbing to witness, cadaveric spasms are a normal part of the process of corpse decomposition. The spasms usually begin within minutes or a few hours after death and may affect isolated groups of muscles or the entire body.
For example, the arms or legs may twitch or jerk briefly, facial muscles may spasm, or the head may tilt or turn to one side. The spasms are entirely involuntary and have no effect on the consciousness of the deceased, as all brain function has ceased.
Cadaveric spasms occur more frequently in cases of sudden death, such as from accidents, murder, or suicide. When the muscles are depleted of ATP at a faster rate, the incidence of post-mortem contractions increases.
The spasms persist for a few hours until the muscles fully exhaust their ATP reserves. After that point, the muscles permanently relax and the spasms cease.
Incidence in Fish and Other Animals
The involuntary movements of fish after death are also caused by cadaveric spasms. Shortly after a fish dies, the muscles may begin to contract in brief convulsions. This causes the body of the fish to jerk or flop around sporadically after being pulled from the water.
Like humans, the random muscle movements are due to a rapid loss of ATP following death.
These movements can persist for several hours after death in fish before the energy reserves in the muscle tissue are depleted. Smaller fish may exhibit more pronounced and longer-lasting post-mortem spasms compared to larger fish species.
However, the duration and intensity of the involuntary movements depend on the specific manner of death.
Cadaveric spasms are not isolated to fish and humans. Many other animals, including amphibians, reptiles, birds, and mammals can experience these random muscle contractions for a period of time after death.
For example, the legs of a dead frog may twitch or the tail of a lifeless lizard may move temporarily after dying. The wide occurrence of cadaveric spasms across animal species highlights the shared mechanisms of muscle activation and energy depletion after death.
Other Contributing Factors
Water Temperature
Water temperature plays a key role in the movement of fish after death. Generally, fish are cold-blooded animals, meaning their body temperature matches that of the surrounding water. In cold water, a dead fish’s metabolism and enzymatic reactions slow down dramatically, delaying the onset of rigor mortis.
Studies show that fish in near-freezing water can continue to exhibit reflex motion and muscle twitches for up to 8 hours after death.
In contrast, warm water accelerates the fish’s decomposition process. At higher temperatures, their cells run out of oxygen more quickly. This rapid onset of rigor mortis explains why fish in warmer water tend to stop moving much sooner after expiration.
For most fish species, muscles stiffen approximately 2-4 hours post-mortem in 70-90°F water. Tropical species like betta fish may cease motion within 30-60 minutes of death in 80°F+ aquariums.
Cause of Death
How the fish died also influences its postmortem movement. If a fish endures a prolonged illness or passes slowly from old age, its body is often starved of oxygen before death. This anaerobic state causes lactic acid to build up in the muscles, accelerating rigor mortis.
Fish euthanized quickly and humanely with an overdose of anesthetics like clove oil, however, tend to exhibit more pronounced final twitches and reflexes.
Physical trauma can also stimulate involuntary motions. For instance, a fish severely injured by a boat propeller may exhibit uncontrolled tremors or writhing after death as damaged nerve cells fire. Cause of death likewise relates to water temperature’s effects; sick or injured fish in warm water tend to succumb to rigor mortis most rapidly.
Conclusion
In summary, the unsettling postmortem movements of fish have biological explanations. Rigor mortis, residual ATP energy, involuntary nerve impulses, and cadaveric spasms can all contribute to the phenomenon.
While a dead fish thrashing around looks like a sign of life, rest assured it is simply an involuntary process within the fish’s decaying body. Understanding the science behind these postmortem movements can help dispel the myth that a fish is still alive.
So next time you reel in a fish that continues to kick its legs well past its death, remember it is only biology in motion. The fish has definitely passed, even if its body has yet to get the memo.
