The Streamlined Success: Why Fish are Fish-Shaped

Have you ever wondered why fish, despite their vast diversity, share a common body plan? From the speedy sailfish to the gliding flying fish, their shapes are no accident. The familiar form of a smooth, elongated body, fins, and a tail is a testament to the power of evolution, perfectly adapted for life in the water.

The Two Main Groups of Fish Motion

Fish can be broadly categorized into two groups based on their primary mode of propulsion:

• Body and Caudal Fin Driven Motion: This group, comprising about 85% of fish species, relies on the body and tail for propulsion. Fins primarily serve as stabilizers and steering mechanisms. This method is ideal for open-water species requiring speed, thrust, and control for efficient swimming.
• Median and Paired Fin Motion: This group utilizes fins as the primary means of propulsion, enabling fine-tuned movements at slower speeds. This is commonly found in fish navigating complex habitats.

Body and Caudal Fin Motion: A Spectrum of Styles

Within the body and caudal fin driven group, there's a fascinating range of swimming styles:

• Anguilliform: Eels exemplify this style, using their entire bodies to undulate and generate a wave-like motion.
• Subcarangiform: Species like salmon and trout utilize about two-thirds of their body mass for motion.
• Carangiform: Mackerel employ only about a third of their body mass. Generally, the less mass a fish uses, the more streamlined its shape.
• Ostraciiform & Thunniform: At the other extreme, boxfish and tuna rely almost exclusively on their tails (caudal fins) for propulsion. Tuna, with their powerful tails attached to multiple muscles, can reach impressive speeds.

Median and Paired Fin Motion: Navigating Complexity

Fish in this group use their fins for precise movements, crucial for navigating intricate environments:

• Bottom-dwelling fish, such as rays, use their pectoral fins to lift off the sea floor, allowing them to inhabit shallow waters without being tossed around by waves.
• Shallow-water flatfish use their entire bodies as a single fin to move across the sand.
• Ocean sunfish, lacking tails, beat their median fins to move slowly.
• Reef species like angelfish, surgeonfish, and wrasse also rely on fin-based propulsion, allowing their bodies to evolve into diverse and unique shapes.

The Art of Disguise: Outliers and Adaptations

Even among these groups, some fish appear to be exceptions. However, closer inspection reveals that their common traits are often cleverly disguised:

• Seahorses, though unconventional in appearance, use their dorsal fins as makeshift tails.
• Pufferfish retract their spines for rapid swimming when needed.
• Handfish use modified fins to