Why Are Fish Tails Different Shapes? It Depends What the Fish Does All Day
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The shape of a fish's tail is a direct readout of how it lives, because the trade between acceleration and sustained speed cannot be avoided and every shape sits somewhere on it.
What the tail is doing
Most fish swim by passing a wave of bending down the body from head to tail, and the tail is the last and largest surface to push against the water. Its area determines how much water it moves, and its shape determines how efficiently it does so. A large area accelerates a lot of water slowly, which produces strong thrust from a standing start and considerable drag while cruising. A narrow high-aspect shape accelerates a small amount of water quickly, which is efficient at speed and poor at accelerating. That trade is unavoidable and every shape represents a position on it.
The shapes and what they indicate
The categories map onto ways of living:
- •Rounded, giving strong acceleration and poor cruising, in ambush hunters
- •Truncated and squared off, a general purpose compromise
- •Forked, faster and more efficient, in fish that cruise and chase
- •Crescent shaped and narrow, for sustained high speed, in tuna and sharks
- •Pointed, in fish that manoeuvre in tight spaces
- •Unequal lobes in sharks, related to their lack of a gas bladder
Why sharks are lopsided
The upper lobe of a shark's tail is longer than the lower in most species, which looks like a curiosity and follows from a problem. Sharks have no gas-filled bladder and are denser than water, so they sink when they stop swimming, and they generate lift from their pectoral fins and from the flattened underside of the head exactly as a wing does. A tail with a longer upper lobe pushes water downward and backward, which produces lift at the rear to balance the lift generated at the front, keeping the animal level. Fast open-water sharks have nearly symmetrical tails, because they swim fast enough for the fins alone to supply the lift.
The fastest ones
The extreme end of the range is occupied by a small group of fish that have converged on nearly identical solutions, and the whole body is involved rather than the tail alone. Tuna, marlin and mackerel sharks all carry a narrow crescent tail on a slender stalk that is stiffened sideways and flattened top to bottom, which lets the tail beat without the stalk itself dragging. The body is rigid and only the rear portion moves, so the fish holds a streamlined shape rather than undulating. Grooves accept the fins so they can be folded flush. Several also keep their muscles warmer than the surrounding water, which makes those muscles work faster.
What tail shape does not tell you
Reading the shape is informative and has limits worth stating. Many fish do not swim by bending the body at all, using their pectoral or dorsal fins instead, and in those the tail is small and says little about performance. Deep-bodied fish in structured habitats prioritise manoeuvring over speed and their tails reflect that rather than any lack of ability. And the same shape can be reached from different starting points, since tuna, mackerel sharks and extinct marine reptiles converged on nearly identical crescent tails from quite different ancestry, which is a standard illustration of how physical constraints force the same answer.
The takeaway
A large rounded tail accelerates a lot of water slowly, which suits a standing start and drags at cruise, while a narrow crescent accelerates a little water fast, which suits sustained speed and poor acceleration, and every shape sits between those. A shark's longer upper lobe generates lift to balance the lift from its fins, since it has no gas bladder and sinks when it stops.