If you’ve ever handled a fish, you may have noticed small sharp protrusions along their fins or bodies. These pointy structures can seem like thorns, but are fish actually covered in thorns like roses or blackberry bushes? The quick answer is no, fish do not have true thorns like plants.
However, many species do have hardened spines and rays that serve a similar protective purpose.
In this approximately 3000 word article, we’ll take a close look at the anatomy behind those pokey fins and investigate why fish evolved these thorn-like structures. We’ll overview the different types of fin rays and spiny projections found on fish, explain how they work to deter predators, and highlight some fish species well-known for their dramatic fins and spines.
Anatomical Terminology: Spines vs. Rays
Fin rays are slender, flexible, segmented structures that compose the bulk of a fish’s fins. They provide structural support for the fins but are also flexible, allowing the fins to undulate and propel the fish through the water. Fin rays are composed of two parallel half-rays joined by a membrane.
Each half-ray consists of a stack of thin bony segments, rather like the vertebrae that make up the spine in mammals. Fish can often actively control the movement and stiffness of each fin ray, allowing for fine control of swimming motions and maneuvers.
There are two main types of fin rays: soft rays and spiny rays. Soft rays tend to be flexible along their entire length. They are unbranched, with each segment extending the length of the ray. Soft rays are found in fins like the dorsal and anal fins.
Spiny rays are stiffer, usually only flexible at the base, where they attach to the rest of the fin. Each segment branches into two, giving a feathery, thorny appearance. Most spiny rays are found in the anterior portion of the dorsal fins and the pectoral fins.
Key Differences Between Fin Rays and Spines
| Fin Rays | Spines |
| – Slender segmented structures | – Rigid, thorny protrusions |
| – Provide flexibility | – Provide stiff support |
| – Usually branched | – Usually unbranched |
| – Composed of soft bone | – Composed of hard bone |
The flexible fin rays allow fins to undulate, propelling the fish forward. The mix of fin rays and spines in fins like the dorsal fin allows for both flexible propulsion and rigid structural support. By actively controlling the fin rays, fish can make minor adjustments to steering and propulsion.
Fish spines are hard, rigid structures embedded in the flesh of fish for defensive purposes. They are composed of dense bone and usually have a sharp, pointed tip. Unlike the flexible fin rays that make up most fins, spines are completely stiff and unmoving.
Spines are found along the dorsal, pelvic and anal fins of many fish species. They can lock into an erect position when the fish is threatened, making the fish harder to swallow.
Fish spines evolve as a defence mechanism against predators. If a predator tries to swallow a fish sideways, the erect spines can stick in the predator’s throat. Most spines are mildly venomous as well, secreting mucus that causes irritation, pain, or even paralysis.
The venom makes sure the predator remembers not to swallow that fish again! Some notable examples of fish with venomous spines are lionfish, stonefish, and catfish.
While all fish spines can be painful if stepped on, the most dangerous are those that can pierce through shoes and human flesh. Stingrays, scorpionfish, and weever fish all have spines that can inflict severe wounds on humans. On the other hand, fish like perch and walleye have small, relatively harmless spines.
Fish use their spines both for protection and communication. During mating, male fish will erect their spines to signal dominance and claim territory. Spines also help identify different species – the small spines on walleye and perch make them distinguishable from their spineless relatives!
Functions and Adaptive Advantages
Defense Against Predators
Fish spines and fin rays serve an important defensive function against predators (source). Their spiky protrusions make them difficult to swallow and deter attacks. When threatened, fish can flare their spines and rays outward as a warning.
Some spines are even venomous or release chemicals to repel predators. For example, catfish have pectoral and dorsal spines that can inject toxins.
In particular, the perch family has especially sharp and stiff spines on their fins. They use quick sideways sweeps and jabs as defense tactics. Other fish like lionfish and stonefish have delicately beautiful but poisonous spines. Getting pricked can cause excruciating pain.
Clearly, spines and rays act as highly effective predator deterrents for many fish species.
Maneuverability and Hydrodynamics
Fish fins with rays provide essential maneuverability and control while swimming (source). Different fins serve specialized functions – pectoral fins act as brakes, pelvic fins assist with acceleration, anal/dorsal fins aid in stability.
The fin rays offer both flexibility and structure to alter direction and trajectory precisely.
The hydrodynamic design of fin rays, arranged in sequential joints and segments, also improves swimming efficiency. As the rays bend, the joints maintain smooth continual contact with the water to reduce drag. So fish can glide effortlessly using minimal energy expenditure.
This combination of agility and streamlining makes fish adept swimmers and fierce hunters.
Fish Species With Notable Fins and Spines
Catfish
Catfish have unique flexible spines on their fins that can lock into place when threatened. These sharp, serrated defensive structures help protect them from predators looking for an easy meal. While small catfish spines may only cause a sting, larger species can inflict deep wounds with their larger fin bones.
There are reports of fishermen needing surgery to remove embedded spines over an inch long!
Pufferfish
Be careful around pufferfish – they can inflate themselves to appear much larger by gulping water or air into a unique elastic stomach. Along with this quirky defense mechanism, many puffer species also have poisonous spines on their fins and skin that contain tetrodotoxin, an extreme neurotoxin.
So while they may look silly all blown up like a balloon, their spines can mean serious business!
Triggerfish
Triggerfish have stout, large fins with tough, weaponized spines used to deter enemies. Their first dorsal spine can be locked in place by a small secondary spine behind it, akin to a triggered gun. When confronted, they erect the sharp first spine and swing their fins to slash opponents with their bony blades.
Some species even have grooves on the spines to inflict deeper gashes!
Lionfish
Few fish can compete with the intimidating appearance of lionfish. Their expansive fan-like pectoral fins and swirling stripes give them a fierce persona. But it’s the venomous fin rays that make them truly dangerous.
Thirteen sharp spines deliver extremely painful stings that can cause vomiting, breathing difficulties, paralysis and even death in rare cases. Sadly, invasive lionfish are now a major threat to coral reef ecosystems in the Atlantic Ocean and Caribbean Sea.
Conclusion
While fish fins may seem covered in thorns, those sharp projections are actually hardened fin rays and spines composed of bone, cartilage, or keratin. These structures serve a defensive purpose in many species and also aid in locomotion.
Over the course of evolution, fin shapes and compositions have adapted to meet the lifestyle needs of different fish.
The next time you come across a pokey-finned fish, take a closer look at its anatomy. Those fin rays and spines tell a story about the species’ evolution and habitat – without the need for true botanical thorns!
