29.2 Fishes

Jawless and Jawed Fishes

One of the most important distinctions in fish evolution is whether a species has jaws. This single trait shaped how fishes feed, compete, and diversify across aquatic environments.

Jawless fishes (Agnatha) are the most primitive living vertebrates. They lack jaws and paired fins, and their skeleton is made of cartilage. Unlike jawed fishes, the notochord persists throughout life and is never replaced by a true vertebral column. They also lack scales. The only surviving jawless fishes are hagfishes and lampreys.

• Hagfishes are scavengers that feed on dead or dying organisms on the ocean floor
• Lampreys are often parasitic, attaching to host fish with a sucker-like mouth and feeding on blood and body fluids

Jawed fishes (Gnathostomata) represent a major evolutionary leap. The evolution of jaws, likely from modified gill arches, opened up a huge range of feeding strategies. Jawed fishes also have paired fins, and their notochord is replaced by a vertebral column during development. Their skeletons can be either cartilaginous or bony, and most species have scales.

Sharks, Rays, and Bony Fishes

Characteristics of sharks and rays

Sharks and rays belong to class Chondrichthyes, meaning their entire skeleton is made of cartilage rather than bone. Their skin is covered in placoid scales, which are small, tooth-like structures that reduce drag and protect against parasites.

What makes sharks and rays especially effective predators is their sensory toolkit:

• Ampullae of Lorenzini detect weak electrical fields generated by the muscle contractions of nearby prey. This is useful for finding prey hidden in sand or murky water.
• The lateral line system runs along the body and detects changes in water pressure and movement, helping with navigation and locating prey.
• A keen sense of smell allows sharks to track prey over long distances by following chemical gradients in the water.

Reproduction in sharks and rays involves internal fertilization, but development varies by species:

• Oviparity: eggs are laid and develop externally (skates and some sharks)
• Viviparity: embryos develop internally and are born live, nourished by a placenta-like structure (most sharks)
• Ovoviviparity: eggs hatch inside the mother's body, and young are born live but without placental nourishment (some sharks and rays)

Compared to bony fishes, sharks and rays have low fecundity (few offspring), slow maturation, and long gestation periods. This makes their populations especially vulnerable to overfishing.

Adaptations of bony fishes

Bony fishes (Osteichthyes) have skeletons made of bone, which distinguishes them from the cartilaginous sharks and rays. They are by far the most diverse group of fishes.

A key adaptation in many bony fishes is the swim bladder, a gas-filled internal organ that allows the fish to control its buoyancy without constant swimming. By adjusting the amount of gas in the swim bladder, a fish can hover at a given depth.

• Some bottom-dwelling species, like flatfishes and certain eels, have lost the swim bladder entirely since they don't need to maintain neutral buoyancy.
• In lungfishes and gouramis, the swim bladder has been modified to function as an accessory respiratory organ, allowing them to breathe air.

Bony fishes split into two major lineages:

Ray-finned fishes (Actinopterygii) have fins supported by thin, bony rays. This is the largest group of vertebrates, including familiar species like salmon, tuna, and perch.

Lobe-finned fishes (Sarcopterygii) have fleshy, muscular fins supported by bones that articulate with the body. Only a few species survive today:

• Coelacanths were thought to be extinct for millions of years until a living specimen was discovered off the coast of South Africa in 1938.
• Lungfishes possess both gills and a highly vascularized swim bladder that functions as a lung, allowing them to survive in oxygen-poor water or even out of water for short periods.

Lobe-finned fishes are evolutionarily significant because the bony structure within their fins is homologous to the limb bones of tetrapods. They represent the lineage that gave rise to the first four-legged vertebrates to transition onto land.

Fish Physiology and Adaptations

Respiratory and circulatory systems

Fish extract dissolved oxygen from water using gills. Each gill is composed of rows of filaments, and each filament is lined with thin plates called lamellae. This arrangement dramatically increases the surface area available for gas exchange.

The key to gill efficiency is countercurrent exchange. Here's how it works:

1. Water flows over the lamellae in one direction.
2. Blood flows through capillaries inside the lamellae in the opposite direction.
3. Because the two flows run counter to each other, there is always a concentration gradient favoring oxygen diffusion from water into blood, even as the blood becomes increasingly oxygen-rich.

This system extracts up to 80-90% of the dissolved oxygen from water, far more efficient than if blood and water flowed in the same direction.

Fish have a single-loop circulatory system with a two-chambered heart (one atrium, one ventricle). Blood is pumped from the heart to the gills, picks up oxygen, then travels directly to the body tissues before returning to the heart. This differs from the double-loop circulation found in terrestrial vertebrates.

Osmoregulation and excretion

Because fish live in water, they constantly face osmotic challenges. The strategy depends on whether the fish lives in freshwater or saltwater:

• Freshwater fish live in a hypotonic environment (the surrounding water has a lower solute concentration than their body fluids). Water constantly flows into their bodies by osmosis. To compensate, they actively absorb ions through specialized cells in their gills and produce large volumes of very dilute urine.
• Marine fish face the opposite problem: they live in a hypertonic environment and lose water to their surroundings. To compensate, they drink seawater, absorb the water in their gut, and actively excrete excess salt through chloride cells in their gills. Their kidneys produce small volumes of concentrated urine.

Thermoregulation

Most fish are ectothermic, meaning their body temperature matches the surrounding water. They cannot generate significant internal heat.

A notable exception is regional endothermy, found in some tunas, lamnid sharks (like great whites), and swordfish. These species use a countercurrent heat exchange system called a rete mirabile in their blood vessels. This retains metabolic heat in specific muscles or organs (like swimming muscles, the brain, or the eyes), keeping those tissues warmer than the surrounding water. The result is faster muscle contraction, sharper vision, and improved performance in cold water.

External features and locomotion

The body of a typical fish is streamlined to reduce drag in water. Most species are covered in scales, which provide protection and further reduce friction during swimming.

Fish use several types of fins for different functions:

• Caudal (tail) fin: primary source of thrust for forward propulsion
• Pectoral and pelvic fins (paired fins): used for steering, braking, and stability
• Dorsal and anal fins: prevent rolling and help with stability

The shape of the caudal fin often reflects a fish's lifestyle. Fast, open-water swimmers like tuna have a deeply forked or lunate tail, while fish that need quick bursts of speed in tight spaces tend to have rounded tails.