Book SummaryWhat a Fish Knows, by Jonathan Balcombe

1-Page Summary1-Page Book Summary of What a Fish Knows

Fishes' Sensory and Perceptual Abilities

This section of the book will explore the remarkable senses and the perception abilities fishes employ to navigate their environments and meet their daily needs. Contrary to the common stereotype of fish being unfeeling creatures on autopilot, Balcombe reveals a world of exquisite sensory perception and complex social interactions, guided by an array of senses, some of which extend beyond our own human experience.

Fishes Have Diverse, Advanced Visual Capabilities

Balcombe overturns the common misconception that fish are poor-sighted. Fish actually possess a powerful visual system that rivals that of most land mammals, and in certain instances even extends beyond our human capacities. The author challenges readers to look beyond our tendency to interpret other creatures' abilities from a human-centered perspective.

Vision Adaptations in Water-Dwelling Life Resemble Human Eyes Underwater

Balcombe explains that aquatic creatures generally see very well in their watery world, using eyes adapted for efficient functioning in a denser, more viscous environment than the air through which land-dwelling animals see. Like most of their vertebrate brethren, fishes use eye muscles and ligaments to swivel their eyeballs in their sockets and focus light on the retina. Balcombe describes their spherical lens, made of transparent tissue with a high refractive index, as allowing them to see as well underwater as we can in air, no doubt thanks to the innovative design first laid down by their distant ancestors when all vertebrates lived in the seas.

Certain fish species use eye muscles to enhance their visual abilities beyond our own. Some deep-sea anglerfishes have solved the problem of finding mates in the vast, dark depths by having males cling to the first female they meet. Once they find her, they hang on literally for dear life, sometimes becoming little else than a sex organ, relying on her blood supply. Seahorses, blennies, gobies, and flounders have each independently developed eye musculature that enables them to rotate each eye independently, letting them focus on two visual fields simultaneously.

Context

• At greater depths, pressure increases and light decreases. Some aquatic species have developed eyes that can withstand high pressure and detect minimal light, aiding in survival in deep-sea environments.
• Ligaments in the eye help maintain the shape of the eyeball and support the lens. They play a role in focusing by adjusting the position of the lens to ensure that light is properly directed onto the retina.
• The development of a spherical lens in fish is an evolutionary adaptation that dates back to early vertebrates, highlighting the long history of adaptation to aquatic environments. This adaptation is crucial for survival, aiding in navigation, hunting, and avoiding predators.
• Fish that hunt or navigate complex underwater landscapes may have enhanced depth perception, allowing them to judge distances more accurately than humans can underwater.
• In many deep-sea anglerfish species, males are significantly smaller than females. This size difference is an adaptation to their unique reproductive strategy.
• Once a male anglerfish finds a female, he bites into her skin and releases an enzyme that digests the skin of his mouth and her body, fusing them together. This process allows the male to become a permanent appendage.
• The neurological control required for independent eye movement is complex, involving specialized brain regions that coordinate the movement of each eye separately.
• Similar to chameleons on land, these fish can track multiple objects at once, which is beneficial for both hunting and avoiding predators.

Fishes See Ultraviolet and More Colors

Balcombe describes how fishes have evolved visual capabilities that surpass our own. For instance, most bony fishes are tetrachromatic, meaning they have four cone types for perceiving color. By contrast, we trichromats are limited to a more restricted color palette since we rely on just three cone types. Certain fish can perceive near-ultraviolet light, which includes electromagnetic wavelengths shorter than the visible spectrum for humans.

Highlighting the importance of visual signaling in fish behavior, the author explains how the color patterns visible on fish bodies convey a range of social information, including species identification, gender, age, social status, and mood. Some species of fish rely on these visual cues to avoid being eaten, while others enhance them to attract a possible partner. For instance, two species of damselfishes in the western Pacific that look identical to humans actually have subtly different facial patterns that can only be seen in the ultraviolet light spectrum that fishes see with. When scientists used ultraviolet filters to eliminate these visual cues, the fish could no longer distinguish each other.

Context

• Studies on fish vision often involve examining the genetic and physiological mechanisms that enable tetrachromacy, providing insights into how these systems evolved and function.
• Each fish species may have unique color patterns that are crucial for species recognition, helping to prevent interbreeding and maintain genetic diversity.
• The ability to detect and interpret visual cues can be a matter of life and death. Predators may evolve to better detect prey, while prey species develop more sophisticated ways to avoid detection, leading to an evolutionary arms race.
• They can be used for communication among the same species while remaining hidden from predators that cannot see UV light, effectively acting as a form of camouflage.
• Researchers use ultraviolet filters in experiments to study how fish perceive their environment. By blocking ultraviolet light, scientists can observe changes in fish behavior and understand the importance of these visual cues.

Fishes'...

What a Fish Knows Summary Fishes' Emotional, Cognitive, and Communal Capacities

This section of the book further challenges the common prejudice that fish are merely reflexive, unemotional automatons by examining the science of fish cognition, emotion, and behavior. Balcombe explores the growing evidence of fish not only being able to think and feel, but having personalities, memories, and well-developed social lives that exhibit elements of cooperation, virtue, and even deception.

The Feelings of Fish: Anxiety, Stress, Joy, Curiosity

Balcombe tackles a particularly vexing and widely debated issue regarding fishes' mental experiences and moral status: Can they feel pain? Building on the previous section by describing how fishes are exquisitely equipped to sense and respond to stimuli in their environments that could be dangerous, the author explains the distinction between nociception—a reflexive response to a nasty stimulus, independent of conscious awareness—and true pain, which involves higher-order processing by the brain. Drawing on decades of research on diverse species, including humans, Balcombe explores the evidence that supports fishes' capacity for experiencing emotions like fear, stress, pleasure, joy, playfulness, and curiosity.

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What a Fish Knows Summary Reproductive Behaviors and Tactics of Fishes

Balcombe reveals the creative and groundbreaking ways that aquatic creatures reproduce, as well as strategies they employ to maximize their reproductive success in their diverse watery environments.

Fishes Exhibit Diverse Mating Systems and Reproductive Strategies

Balcombe examines the remarkable range of fish's sexual proclivities and challenges readers to expand their understanding of sex and gender beyond human social norms. He explains that fishes display the full gamut of sexual expression: most are gonochoristic, remaining male or female throughout life; but others regularly change sex according to the needs of the situation; and some can even fertilize themselves. Balcombe notes that fish have an unmatched degree of sexual adaptability compared to other vertebrates, quoting the ichthyologist Thavamani J. Pandian.

Fishes Often Shift Sex, Demonstrating Sexual Plasticity

Balcombe overturns the common misconception that every fish is sexually “boring.” He points out that, while most remain male or female throughout their lives, a surprising number are gender switchers, performing as males one year, as females the next, or changing sex as needed. This sort of...