Cuttlefish Rem Sleep: Understanding Their Unique Dreaming Patterns

Cuttlefish are marine mollusks known for their sophisticated camouflage and kaleidoscopic displays. They are close relatives of squids and octopuses and achieve their colourful displays via millions of chromatophores – tiny sacs of pigment under the skin that are attached to muscles. A recent study has observed cuttlefish displaying behaviour that resembles sleep, including cycles of rapid eye movement (REM).

REM sleep is a sleep phase that may involve eye movements and loss of muscle tone; in humans and some other mammals, it is often linked to dreaming. While it is not yet clear whether cuttlefish experience the same kind of sleep that we do, their active phase during rest is similar to the REM sleep observed in humans and other vertebrates.

Cuttlefish display behaviour that resembles sleep

Cuttlefish do display behaviour that resembles sleep. They exhibit frequent quiescent periods that are homeostatically regulated, satisfying two criteria for sleep. They also display a quiescent state with rapid eye movements, changes in body colouration, and twitching of the arms, that is possibly analogous to REM sleep.

During periods of inactivity, cuttlefish rest against or bury themselves in the substrate at the bottom of their aquarium. This state is rapidly reversible, as the animals quickly move from the bottom and begin swimming when disturbed. There is considerable variability in the amount of rest across different cuttlefish.

In a study by Teresa Iglesias and colleagues, cuttlefish were filmed in laboratory tanks for 24 to 48 hours at a time. While resting, the animals demonstrated REM-like behaviour for periods lasting between two and three minutes. They made sporadic arm and eye movements, and the chromatophores around their eyes darkened. Such phenomena were not observed during waking activity or inactive parts of their sleep cycles.

The findings of the study suggest that cephalopods, which are molluscs with elaborate brains and complex behaviour, possess a sleep-like state that resembles the REM sleep state of vertebrates. However, the exact nature and mechanism of this form of sleep may differ from that of vertebrates.

They show signs of rapid eye movement (REM) sleep

Cuttlefish show signs of rapid eye movement (REM) sleep. They exhibit periods of quiescence, during which they meet two of the three criteria for sleep: a rapid reversal to an alert state and rebound sleep. They also show sporadic arm and eye movements, and their chromatophores get darker. These phenomena are not observed during waking activity or inactive parts of their sleep cycles.

REM sleep is a sleep phase that may involve eye movements and loss of muscle tone; in humans and some other mammals, it is often linked to dreaming. While it is challenging to determine whether cuttlefish experience the same kind of sleep as humans, their active phase during rest resembles the REM sleep observed in humans and other vertebrates.

The lineage that includes cuttlefish diverged from vertebrates around 500 million years ago, but the presence of REM-like sleep in cuttlefish hints at a common evolutionary origin for sleep. Further research is needed to understand the exact nature and mechanism of this form of sleep in cuttlefish and its potential relationship to dreaming.

They have sophisticated camouflage abilities

Cuttlefish are known for their sophisticated camouflage abilities. They are able to blend into their environment by contracting and relaxing the millions of chromatophores (tiny sacs of pigment under the skin) using the muscles attached to them. This pushes different colours to the surface of their skin.

Cuttlefish have been observed to change their skin colour and pattern during their REM sleep-like state. This is thought to be for a variety of reasons, including practicing their camouflage for when they are awake, maintaining the health of their pigment cells, or even remembering or re-learning moments from their conscious periods – perhaps indicating a dream state.

The path that cuttlefish take to reach their final pattern is not always the same, even when exposed to the same background. They have been observed to "overshoot their target skin pattern, pause, and then come back", indicating that they continuously receive feedback about their skin pattern and adjust their camouflage accordingly.

The sophistication of their camouflage abilities is further demonstrated by the fact that cuttlefish use their chromatophores for more than just camouflage. They also use their sophisticated colour-changing abilities for mating and mesmerising prey.

Cuttlefish are related to squid and octopuses

While cuttlefish, squid, and octopuses share many similarities, there are also several distinct differences between them. One of the most noticeable differences is their general shape. Squid are longer and more cylindrical, cuttlefish are smaller and spoon-shaped, and octopuses have round heads and larger tentacles.

Another difference is their shell structure. Squid have an internal structure called a gladius or quill, which is mostly an evolutionary leftover. Cuttlefish, on the other hand, have an interior cuttlebone made of aragonite, which helps with their buoyancy. Octopuses are unique among the three as they do not have any internal shell or bone, making them highly flexible and able to fit through small openings.

Cuttlefish, squid, and octopuses also vary in their intelligence. Octopuses are considered the smartest among the three due to their ability to escape and camouflage. They also have nine brains—a mini-brain in each of their eight arms and the largest in their head. Cuttlefish and squid, on the other hand, have one brain each.

In terms of culinary applications, all three species are eaten worldwide, but their meat differs slightly in texture and taste. Squid has the firmest meat, followed by cuttlefish, while octopus has the softest meat.

Researchers have observed cuttlefish in a laboratory setting

In one study, researchers filmed cuttlefish in tanks for 24 to 48 hours and observed them exhibiting REM-like behaviour for periods lasting between two and three minutes. During these periods, the cuttlefish made sporadic arm and eye movements, and the chromatophores around their eyes darkened. This behaviour was not observed during waking activity or inactive parts of their sleep cycles.

Another study observed five adult/senescent cuttlefish and found that they exhibited periods of quiescence where they rested against or buried themselves in the substrate at the bottom of the tank. Three of the five cuttlefish also displayed a sleep-like state with rapid eye movements, changes in body coloration, and twitching of the arms. This state lasted for an average of 135 seconds and only occurred once during the recording period.

A third study observed four juvenile cuttlefish and found that they exhibited clear diurnal organisation in rest and activity, with activity highest at night. They also found that 48 hours of rest deprivation resulted in compensatory increases in rest time during the recovery period, indicating homeostatic regulation of sleep.

Overall, these studies suggest that cuttlefish exhibit behaviour that resembles sleep, including periods of quiescence and REM-like behaviour. However, more research is needed to fully understand the sleep patterns of cuttlefish and their potential evolutionary origins.

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