Fins and flippers

One of the characteristic features of aquatic vertebrate animals is the presence of single and paired appendages used for locomotion. In fishes, these structures are known as fins, and in marine mammals, they are known as flippers or flukes (on the tail). Although these structures bear superficial resemblances to each other, there are significant differences in their anatomies, attesting to their different evolutionary histories. However, at a deeper level, they are related structures, having evolved from the same basic structures found in the earliest vertebrates. This is a classic example of evolutionary parallelism: Finlike structures have evolved independently in both fishes and marine mammals from a common ancestral structure.

The Evolution of Fins

The earliest known vertebrates lived between 500 and 600 million years ago. The fossil record indicates these were elongated aquatic animals that lacked a vertebral column similar to modern-day hagfish, and their locomotion was probably accomplished by an eel-like undulation of the body. The efficiency of this form of locomotion is decreased by unwanted motion, resulting in an up-and-down (pitch) or side-to-side (yaw) seesawing and rolling around the long axis of the body. Fins first evolved as stabilizers to resist these motions and increase swimming efficiency. Since their appearance, however, they have also evolved other functions.

There are two theories as to how fins first evolved. The fin-fold theory suggests that the early vertebrates had two paired folds of tissue extending along the side of the body. These folds fused just behind the anus to form a single fin which extended around the tail and up onto the midline of the upper body (dorsal) surface. The theory states that several regions of these fin folds have persisted, resulting in the paired and unpaired fins in modern fishes, whereas the rest of the early fin folds have been lost. As evidence for this theory, the sand launce, Brachiostoma (amphioxus), is used as an example. Amphioxus is an animal that is closely related to the vertebrates, and the earliest protovertebrates are believed to have resembled it. It has extensive folds that closely resemble the theoretical fin-folds of the earliest vertebrates. A different theory, the body-spine theory, states that the earliest vertebrates possessed two or more pairs of spines extending from both sides of the lower portion of the body. Fins were then formed when membranes extended from the tip of the spine to the side of the body, rather like a sail extending from the mast of a boat. Internal support structures within the fin (the endoskeleton) developed later. There is no evidence to conclusively support one theory or the other.

Further evolutionary changes emerged over time as modern species evolved and fossils were discovered. For example, the Remo flounder, a deep-water dwelling flat fish, evolved to have taste buds on the front part of its dorsal fins, which evolved to look like a small finger. While other flatfish species have tastebuds on their dorsal fins, the Remo flounder is the only species to gain an appendage that evolved from its fins with the sense of taste. Also providing evidence of the evolution of senses, the fins of the round goby fish have the same sense of touch as primitive human fingertips. The round goby uses these fins to touch surfaces in a similar way that humans use their hands, implying the possibility of a common ancestor. This observation is further supported by a study conducted by researchers from the University of Chicago and the Andalusian Center for Developmental Biology in Spain, who found genetic evidence of the similarity between fins and limbs—the Sonic hedgehog (Shh) gene and the gene mutation gli3 are shared in vertebrate appendages and fish fins.

The Uses of Fins

Fins in fishes are either paired, meaning there are equivalent fins on either side of the body trunk, or unpaired, meaning that there is a single fin located on the midline of the upper or lower body. The paired fins are the pectoral fins, generally located just behind the gills, and the pelvic fins, which are located behind and below the pectoral fins. The unpaired fins comprise one or more dorsal fins, located on the midline of the back, the anal fin, located on the midline of the bottom (ventral) surface of the body, behind the anus, and the caudal fin, or tail.

In fishes, the tail, or caudal fin, provides a large portion of the forward thrust required for moving through the water and has evolved as a specialized portion of the rearward end of the body trunk and spine. The spine itself is composed of repeating skeletal structures, the vertebrae, which together form a flexible column extending through the long axis of the animal. Sharks, sturgeon, and paddlefish possess a heterocercal tail, in which the vertebral column extends into the upper portion, or lobe, of the tail, giving it a distinctly asymmetric and sharklike appearance. Most bony fishes possess a homocercal tail, in which the spine ends at the base of the tail and in which the tail is composed of equal-sized upper and lower lobes. A few extinct species of fishes possessed hypocercal tails, in which the spine extended into the ventral lobe of the tail. These animals, therefore, possessed caudal fins in which the lower lobe was larger than the upper, the opposite of what is seen today in sharks.

The paired and unpaired fins of sharks are solid, broad-based, and relatively inflexible. Like all fins, they possess an internal skeleton, which provides structural support as well as attachment for muscles that allow the fin to be moved. These consist of a series of cartilages, known as basals and radials, located in the base of the fins. Long rods of cartilage called ceratotrichia extend from the radials out to the edges of the fin, providing support. The principal role of these fins is to resist the yawing, pitching, and rolling motions generated during swimming. Because of the asymmetric heterocercal tail, the thrust generated in forward swimming extends downward from the upper lobe of the tail through the shark’s center of gravity, pushing the animal downward. An important function of the large pectoral fins in sharks is to generate lift, in much the same manner as an airplane’s wing. The lift generated by the pectoral fins counters the downward thrust and moves the animal forward through the water. The pelvic fins are specialized in male sharks and rays to form claspers, which serve as the male organs of reproduction in internal fertilization.

The fins of bony fishes are distinctly different from those seen in sharks, although the endoskeleton of the fins also consists of basal and radial bones. The fins of most bony fishes, particularly the paired fins, are much narrower at their base, more flexible, and may play a more direct role in locomotion in addition to stabilizing the fish during swimming. The radials of the endoskeleton are reduced to very small structures located within the muscles at the base of the fin. These articulate with modified scales called lepidotrichia, or fin rays, which extend out to form the main structural elements of the fin. Membranous connective tissue extends between the fin rays, giving them their typical weblike appearance. Most bony fishes possess this type of fin structure, and they are known collectively as the ray-finned fishes, or Actinopterygii.

The Fins of Bony Fishes

A key development in the evolution of bony fishes was the appearance of a swim bladder, a gas-filled sac in the abdominal cavity that counters the fish’s tendency to sink in the water and thus provides it with neutral buoyancy. With the evolution of this structure, it was no longer necessary for the pectoral fins to generate lift, and they could then be employed as brakes to stop forward motion. The pectoral fins of most bony fishes have a large surface area and a narrow base that is inserted into the body wall almost vertically, as opposed to horizontally in sharks. They are, therefore, admirably suited to act as brakes both singly and together, and greatly increase the ability of the fish to stop or change direction rapidly. The pelvic fins act to counterbalance any pitching or rolling motion generated by the pectorals. The combined activity of the paired pectoral and pelvic fins, often called the four-fin system, provides enhanced maneuverability and control. This system is best observed in many types of coral reef butterflyfishes (Chaetodontidae), in which the pelvic fins are inserted almost directly below the pectoral fins. These fishes are highly maneuverable, able to move, turn, and stop quickly and accurately within the complex and constrained multidimensional environment of the coral reef.

A relatively small group of bony fishes possess paired fins of a different sort. This group of fish, which includes the lungfishes and the coelacanth, is characterized by pelvic and pectoral fins that possess fleshy lobes at their base. The lobes contain muscles and skeletal elements; however, only a single basal bone articulates with the rest of the skeleton of the fish. These lobe-finned fishes (the Crossopterygii) are important because they represent descendants of the lineage that moved from the aquatic environment onto land. The pectoral and pelvic fins of these fishes gave rise to the forelimbs and hindlimbs of terrestrial vertebrates, and the single bone articulating with the body skeleton is the forebear of the humerus, the bone of the upper arm.

In all fishes, the paired fins are connected to the rest of the body’s support framework, the skeleton, via structures known as the pectoral and pelvic girdles. The pectoral girdle in sharks is relatively simple and consists of a large bar of cartilage that extends across between the two pectoral fins, known as the coracoid bar. Scapular processes extend above the base of the pectoral fin and connect the pectoral girdle to the skeleton. The pelvic girdle of sharks has fused into a single bar of cartilage, the puboischiac bar.

The pectoral girdle of bony fishes contains many small bones including the cleithrum, supracleithrum, and the clavicle. A posttemporal bone attaches the pectoral girdle to the rear of the skull. The pelvic girdle in most bony fishes is composed of a pair of bones that act as extensions of the basal bones of the fins.

Flippers

During the Tertiary period (which began approximately sixty-six million years ago and ended around 2.6 million years ago), several groups of land-dwelling mammals returned partly or completely to the aquatic environment. The best known of these are the cetaceans (whales and dolphins), but other groups include the Sirenidae (manatees) and the pinnipeds (seals and sea lions). These animals faced the same problems of locomotion in water as did their fish ancestors, and during evolution, they have evolved finlike structures called flippers and flukes. These perform the same stabilizing functions as in fishes and bear a superficial resemblance to fins. However, closer examination of whales and dolphins demonstrates that the pectoral flippers of cetaceans are, in fact, modified mammalian forelimbs and contain the same bones present in the forelimbs (arms) of terrestrial mammals. Thus, the bones of the pectoral fins, which gave rise to the forelimbs of terrestrial vertebrates, have evolved back into finlike structures in these mammals as an adaptation to an aquatic lifestyle. The hindlimbs, along with their bones and supporting pelvic girdle (the hip bones in land mammals), have been lost in modern dolphins and whales, although they are still present in fossil forms such as the extinct fossil whale Basilosaurus. These mammals also possess fleshy dorsal fins to help stabilize them during swimming and large horizontal flukes on the tail, which provide the main forward propulsive thrust to these animals as they move through the water.

Principal Terms

Fin-Fold Theory: The theory that fins initially evolved as long folds of tissue extending around the body

Flipper: Finlike structures of marine mammals that have evolved from the forelimbs of their terrestrial ancestors

Four-Fin System: The combined activity of paired fins in some bony fishes that makes them highly maneuverable

Heterocercal: A tail in which the spine extends into the upper lobe, giving a distinctly sharklike impression

Homocercal: A type of tail at which the spine ends at the base of the tail, which consists of two equal lobes

Lepidotrichia: Modified scales that form the supporting rays of the fins of bony fishes

Pectoral and Pelvic Girdles: Skeletal structures that form a structural base for attachment of the paired fins in fishes, connecting them to the rest of the body’s skeleton

Bibliography

Bailey, Jill. Animal Life: Form and Function in the Animal Kingdom. Oxford UP, 1994.

Bone, Q., et al. Biology of Fishes. 3rd ed., Taylor & Francis, 2020.

Cottingham, Katie. "These Fish Transformed Their Dorsal Fins into Taste Buds." Smithsonian Magazine, 9 Sept. 2024, www.smithsonianmag.com/smithsonian-institution/these-fish-transformed-their-dorsal-fins-into-taste-buds-180985044. Accessed 10 Sept. 2024.

Moyle, Peter B., and Joseph J. Cech. Fishes: An Introduction to Ichthyology. 5th ed., Prentice Hall, 2015.

Riedman, Marianne. The Pinnipeds: Seals, Sea Lions, and Walruses. Reprint. U of California P, 2020.