Have you ever seen a fish swimming around with what looks like spikes or spines sticking out of its back? If you’ve spotted this strange sight in oceans, rivers, or lakes and wondered why some fish evolved this unusual feature, you’ve come to the right place.

If you’re short on time, here’s a quick answer: Fish develop spines, spikes, and other bony protrusions on their backs and fins for protection from predators. These spines can inflict pain and injury, making the fish less appealing as a meal.

In this comprehensive guide, we’ll explore the reasons behind various fish species’ spiked anatomy, which types of fish have prominent back spines, and how the spines benefit the fishes’ survival.

Defensive Adaptations: Why Fish Evolved Spines

Predator Protection

Fish with spines on their backs likely developed these features over time to help protect themselves against predators (amnh.org). Spines serve as armor that make the fish harder to swallow and deter potential attackers. Some spines are even loaded with venom for extra defense.

According to research, fish like lionfish and stonefish carry toxins in their spines that cause extreme pain and swelling when predators bite them (floridamuseum.ufl.edu). These venomous spikes provide an extremely effective shield against enemies.

Other fish like pufferfish take protection a step further by inflating their bodies to transform their spines into an unappetizing ball of prickles when threatened. By making themselves appear much larger and pricklier than normal, pufferfish send a clear “do not eat me” message that keeps most hunters at bay.

Spines as Armor and Weapons

The bony spikes protruding from fish like tetraodontidae serve multiple defensive purposes. They act as armor plating to protect the body from attacks. Their sharp, pointy structure also allows them to inflict injury on predators that persist in trying to eat them.

In essence, these spines double as defensive weapons that turn the fish’s body into a living fortress. According to marine biologists, over 320 fish species have evolved spiny armor over time for protection (sms.si.edu).

An interesting example is the crested spinefoot fish, which has dangerous spines on its fins, head, and upper body. One look at this fish tells potential predators “do not mess with me.” Its intimidating appearance makes it left alone, allowing it to go about its business safely.

Other spiny fish species likely rely on a similarly frightening look to avoid becoming a tasty meal for hungry hunters.

Fish Species With Distinctive Spiny Anatomy


Catfish get their name from the “whiskers” or barbels around their mouths that resemble a cat’s whiskers. However, some catfish like Corydoras or “cory cat” species have sharp defensive spines along their bodies and fins. These spines can easily stick humans while netting or transferring the fish.

Nearly all of the over 2,000 recognized catfish species have a spine on their dorsal and pectoral fins. The stout, serrated spines serve to deter predators but make catfish tricky captives in home aquaria.


One of the most infamous spiky fish is the pufferfish. When threatened, pufferfish can inflate themselves to appear larger, revealing intimidating inward-facing spikes across their bodies. These spines contain the deadly neurotoxin tetrodotoxin, making pufferfish a dangerous delicacy if improperly prepared for human consumption.

There are about 190 pufferfish species divided into 12 genera, most living in marine habitats and a few in freshwater. All possess a distinctive beak-like mouth and ability to inflate when agitated or attacked.

Lionfish and Scorpionfish

Lionfish and scorpionfish comprise the family Scorpaenidae and suborder Scorpaenoidei. They possess some of the most venomous fin spines in the ocean, which they use to ambush smaller prey or ward off threats. Over 400 species exist globally in this diverse group.

Well-known members include lionfish, stonefish, waspfish, and zebra turkeyfish. Their defining features are large, fan-like pectoral fins and a “scorpion-like” array of dorsal fin spines stretching over their heads and backs.

Handling these fish can be extremely dangerous due to their toxic spines and ability to deliver painful defensive stings.


Sturgeon are ancient fish dating back 200 million years to the Triassic Period. Their armored bodies retain five rows of bony plates called “scutes” along their backs and sides. These scutes come to a point forming protective spines that become more pronounced in certain species.

For example, the Siberian sturgeon has very large, sharp scutes helping it survive frigid Arctic waters. Overall, there are 27 species of sturgeon divided between two genera. Despite their prehistoric appearance, sturgeon remain important economically for their meat (caviar) and are considered threatened due to habitat loss and overfishing.


As their name implies, triggerfish have spines on their dorsal fins that can be unlocked or “triggered” to stand rigidly upright when the fish feels threatened. This creates an intimidating barrier against predators trying to swallow them.

Triggerfish spines are extremely tough and sharp, capable of inflicting painful wounds. There are about 40 species of triggerfish, generally found in tropical and subtropical oceans. Beyond their trademark spines, triggerfish also have powerful jaws used to crunch through shelled invertebrates like urchins, mollusks, and crustaceans.

Key Takeaways and Interesting Facts About Spiny Fish

Here are some key takeaways and fascinating facts about fish with spikes on their backs:

They Use Spines for Defense

The main function of the spines is for protection against predators. When threatened, spiny fish can erect their spines to make themselves harder to swallow. The spines are sharp and can deter potential attackers. Some spines are even venomous, like those of lionfish and stonefish.

Spines Help Them Blend into Their Environment

The spines allow spiny fish to camouflage themselves and avoid detection. Species like picasso triggerfish have color patterns that match the coral reefs they inhabit. Their spines break up their outline and make them harder to spot by predators.

There Are Many Types of Spiny Fish

Some common examples of spiny fish include:

  • Pufferfish
  • Porcupinefish
  • Triggerfish
  • Cowfish
  • Lionfish
  • Stonefish

These fish live in various marine habitats like coral reefs, kelp forests, and sandy or rocky bottoms. They can be found in tropical, subtropical, and temperate waters worldwide.

Their Spines Are Modified Scales

The spines are composed of modified protective scales called “ctenoid” or “platoid” scales. They differ from the thin, flexible scales that cover the rest of the fish’s body. The spines are thicker, bonier, and sharper.

Spines Can Be Raised or Lowered

Many spiny fish can voluntarily raise or lower their spines. Muscles at the base of each spine allow them to control the movement. When threatened, they erect all their spines simultaneously as a defense response.

The Spines Are Not Poisonous to Humans

While the spines can give a painful sting if you step on them, they are not venomous to humans in most species. However, lionfish and stonefish spines contain neurotoxins capable of causing extreme pain, paralysis, and even death in rare cases.

They Have Antipredator Adaptations

In addition to their spines, spiny fish have evolved other defensive adaptations such as:

  • Ability to inflate their bodies like pufferfish
  • Noxious skin secretions
  • Warning coloration like lionfish
  • Ability to deliver electric shocks like stargazers

Some Use Spines to Lock into Crevices

Spiny fish like scorpionfish and stonefish use their sturdy spines to wedge themselves into holes and crevices in reefs and rocks. This helps them avoid getting swept away by currents and waves.

Their Diet Varies by Species

Spiny fish feed on a wide range of prey including:

  • Crustaceans like shrimp, crabs and lobster
  • Mollusks such as snails, clams and squid
  • Other fish
  • Polyps of coral and anemones
  • Plankton
  • Algae

Some species also eat slow-moving invertebrates while others are specialized coral feeders.


As we’ve explored, those strange spikes protruding from many fish serve an important defensive purpose. By weaponizing parts of their endoskeletons into hard spines and spikes, fish gain protection from predators looking for an easy meal.

The next time you come across a pokey, prickly fish, take a closer look at its anatomy. You’ll gain appreciation for the innovative ways fish have adapted and evolved to survive in aquatic environments swarming with danger.

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