The Surprisingly Large Brains Of Some Fish Species

Fish are often thought of as primitive creatures lacking intelligence. However, some species of fish have remarkably large brains relative to their body size. In fact, certain fish have shown evidence of complex cognition and behavior once believed exclusive to land animals.

If you’re short on time, here’s a quick answer: Some fish species like mormyrid electric fish and shark species have brains as large or larger than other vertebrates, allowing advanced cognitive abilities.

In this article we’ll explore fish neuroanatomy, look at fish species with unusually large brains, discuss what cognitive abilities big brained fish may have, and the evolutionary reasons behind these big brains.

Fish Brain Anatomy

Telencephalon

The telencephalon is the anterior-most and largest part of the fish brain. It controls functions like learning, memory, and behavior. Some fish like mackerel and herring have surprisingly large telencephalons relative to their body size.

In fact, Atlantic mackerel have telencephalon to body mass ratios approaching those of some primates! The telencephalon receives sensory input from the eyes, nose, and other parts of the head. It integrates this information to coordinate the fish’s movements and behaviors.

Diencephalon

The diencephalon sits between the telencephalon and mesencephalon. It regulates important functions like vision, temperature control, sleep, and homeostasis. An interesting part of the diencephalon is the pineal gland, which secretes melatonin and controls circadian rhythms.

In some fish like salmon, the pineal gland is light-sensitive and allows the fish to orient itself using sunlight or moonlight.

Mesencephalon

The mesencephalon or midbrain connects the forebrain and hindbrain. It has important nerve tracts and nuclei that coordinate visual and auditory reflexes and reactions. For example, when a fish sees a predator, visual signals are processed in the mesencephalon and initiate an escape response without input from higher brain regions.

The mesencephalon also regulates swimming posture and balance.

Rhombencephalon

The rhombencephalon or hindbrain controls vital functions like respiration, circulation, and motor coordination. It consists of the metencephalon which regulates balance and movement, and the myelencephalon which controls cardiac and respiratory functions.

The vagus nerve, the longest cranial nerve, originates in the hindbrain and innervates the heart, lungs, and viscera. The rhombencephalon allows fish to perform innate behaviors like schooling, escape reactions, and spawning migrations largely independent of conscious thought.

Fish With Large Brains

Mormyrid Electric Fish

Mormyrid electric fish have unusually large brains for their body size. Their brain to body weight ratio rivals that of many primates (Fishes, 2022). These African freshwater fish generate and sense weak electric fields using specialized organs in their tails and heads, allowing them to navigate murky waters, communicate, and locate prey in the dark (Mormyridae, 2023).

Neuroscientists find mormyrids fascinating as their large brains possess exceptional learning and memory capabilities. For instance, a mormyrid species called the elephantnose fish can remember complex electrical signal patterns for over a year.

Researchers are studying these fish to understand the neural mechanisms behind their remarkable intelligence (Szabo, 2021).

Sharks and Rays

The brains of sharks and their relatives, rays and skates, are surprisingly complex. Though smaller in absolute size than those of mammals, shark brains are large for their body mass when compared to bony fishes.

Areas of the shark brain associated with learning and memory are also well developed (Yopak et al., 2021).

Scientists have demonstrated that some shark species use complex hunting strategies that involve elements of planning and cooperation. Port Jackson sharks, for example, exhibit intricate coordinated group behaviors when hunting for mollusks.

This suggests advanced cognitive capabilities and strong social bonds between individuals (Vail et al., 2013).

Groupers

Groupers are large predatory reef fish remarkable for their problem-solving abilities. In one study, groupers were presented with a clear pipe containing food. To reach the food, the groupers had to rapidly swim into one end so the flow would carry the morsel within reach.

Most groupers mastered this tactic within 164 attempts (Downes et al. 2021).

Researchers were surprised that groupers not only learned these abstract techniques, but would innovate new solutions without additional training. Such advanced reasoning skills were previously associated only with mammals and birds (Gruber et al. 2019).

Clearly, not all fish possess simple “three-second memories.”

Wrasses

Wrasses comprise a family of small, brightly-colored reef fish. Though seldom considered intellectuals of the sea, at least one wrasse species engages in remarkably intricate behaviors. Tubelip wrasses (Labroides dimidiatus) establish “cleaning stations” on coral heads where other fish visit to have parasites removed from their skin and gills.

Cleaner wrasses carefully court potential clients with ritualized dances before providing their services (Soares, 2017).

What’s fascinating is that cleaner wrasses appear to understand the concept of future consequences. If they accidentally bite a client fish instead of eating parasites, they will rapidly swim away to avoid retaliation, clearly anticipating the client’s likely response.

Such foresight further demonstrates the greater than expected mental capacities of some fishes (Vail et al., 2014).

Cognitive Abilities

Learning and Memory

Fish have demonstrated impressive learning and memory capabilities in scientific studies. For example, archerfish can learn to aim and shoot down targets with remarkable accuracy, remembering complex tasks for months or even years later (1).

Some fish like rainbow trout can learn to avoid harmful prey after just one bad experience through a process called conditioned taste aversion (2).

Social Behavior

Many fish species exhibit complex social behaviors. Some fish cooperate to hunt, defend territories, or care for young together. For example, some damselfish species tend exclusive algae gardens, chasing away intruders (3).

Others like the cleaner wrasse maintain cleaning stations that act as “fish barber shops” for parasites and dead skin (4).

Communication

From electric eels to singfish, fish have evolved diverse forms of communication. Over 800 fish species are known to produce sounds for courtship, warnings, claiming territory, and other social interactions (5).

While not a true language, recent studies on coral reef zebrafish communication recorded over 100 distinct sound patterns exchanged in different behavioral contexts (6).

Spatial Reasoning

Experiments reveal advanced spatial reasoning and mapping abilities in some fish as well. When collecting food items placed in complex maze configurations, archerfishes demonstrated mental representations of spaces beyond their direct line of sight – outperforming dogs in similar tests! (7).

Evolutionary Drivers

Foraging Demands

The impressive brain size of some fish species like mormyrids and elephantfish can be attributed to the complex foraging demands of their aquatic environment. These fish inhabit structurally complex habitats like coral reefs and must locate and extract hidden or hard-to-find prey.

This requires excellent spatial memory and ability to build complex 3D maps of their surroundings. Additionally, some species like archerfish use advanced physics calculations to spit jets of water to knock down aerial prey.

Research has shown mormyrid fish species with larger brain to body mass ratios tend to inhabit more densely vegetated foraging habitats (Kotrschal et al., 1998). The brainier archerfish also excel at judging distance, angles, and perspective to hit their targets (Schuster et al., 2004).

So the need to remember spatial relationships and perform complex hunting techniques seems to have driven the evolution of larger brains and expanded pallial areas in these fish.

Social Complexity

Many fish demonstrate surprising social sophistication, like long-term social bonds, cooperative hunting, and use of “names” to identify each other. The cleaner wrasse, for example, can recognize hundreds of “client” fish and modify behavior toward each (Bshary, 2011).

Fish also often form complex dominance hierarchies and alliances within social groups.

These behaviors likely impose heavy cognitive demands. Fish need good memories to track social relationships and appropriate responses for each individual they encounter. Making alliances also requires strategic thinking, analysis of costs and benefits, and understanding of third-party relationships.

So the complex social structures of fish could be a selective factor for larger brain evolution in species like wrasses and cichlids.

Predator Avoidance

Avoiding predators is clearly essential for survival and reproduction in fish. Fish have evolved a variety of innovative predator avoidance strategies like schooling behavior, camouflage, body armoring, and chemical defenses.

Many fish can also assess predation risk very effectively based on visual, chemical and social cues.

But detection and appropriate response to predators requires processing lots of sensory information and making complex decisions. Fish also benefit from good spatial memory and mapping abilities to evade predators in their habitat.

So dealing with extreme predation pressures could drive brain expansion and intelligence in fish species heavily targeted by predators.

Conclusion

While most fish may not be as smart as land animals, some species rival even mammals and birds in relative brain size and complexity. This allows advanced cognition helpful for survival in challenging aquatic environments.

Rather than primitive and simple, the big brains of mormyrid, sharks, groupers and other fish species reveal surprising capabilities. There is still much to uncover about how these unexpected big brained fish make use of their expansive neural power.