What Kind of Symmetry Does a Starfish Have?

Starfish with a radial symmetry close up

Do you find the starfish symmetry intriguing? What kind of symmetry do they have? This natural phenomenon in sea stars is an excellent example of how life forms have developed specific structures to thrive in their environments.

In this article, we’ll share everything we know about starfish symmetry and the vital roles they play in the sea star’s survival and adaptation. Let’s begin.

What Kind of Symmetry Does a Starfish Have?

Radial symmetry in starfish

A typical starfish showcases five-sided radial symmetry, which means it doesn’t have a front or back and can move in any direction, led by any of its arms. 

This starkly contrasts with humans, who have bilateral symmetry — our right half mirrors our left and we possess an obvious head.

Despite their radial symmetry, research by Chengcheng Ji and Liang Wu from the China Agricultural University reveals that sea stars might still carry a hint of bilateral symmetry, especially under stress.

Through experiments involving over a thousand starfish, they discovered that situations such as being flipped over or exposed to irritants make starfish lead with the fifth arm. 

This behavior suggests an underlying bilateral tendency, which could be a remnant from their larval days. It further indicates that even sea stars might have a “favorite side,” much like how humans are right or left-handed. 

Moreover, researchers speculate this might help sea stars make quicker decisions in dangerous situations. They’re now exploring if this symmetry extends to the starfish’s nervous system.

Fun Fact: While most starfish have five arms, the ocean’s diversity means you can find species with six, seven, or even over 50 arms! The Antarctic sun starfish, for example, starts with five but can grow up to 50 arms as it matures.

How Do Starfish Get Their Symmetry?

Two starfish underwater

Starfish get their symmetry from being echinoderms. They’re from a group of marine animals that evolved their unique form of symmetry from a bilateral stage during their early life. 

Basically, they start life looking one way, with a left and right side like us, but then change dramatically as they go through metamorphosis.

While the most common pattern is five sides or pentagon, it’s not the same for every starfish. Some might end up with more sides.

In a conversation I had with a marine biologist during a recent visit to an oceanarium, I discovered that the sea star’s evolution in body symmetry isn’t just for aesthetics; it’s also super useful. 

The ability to move arms in any direction aids in the starfish’s quest for food and serves as a critical defense mechanism. It also allows them to bounce back from injuries by growing back their arms.

The Symmetry of Starfish vs. Other Marine Animals

Symmetry of starfish vs bilateral symmetry

Starfish are quite unique because they’re built differently from most sea creatures. 

While animals like fish and dolphins have a clear front and back, making it easy for them to zoom through the water, sea stars don’t play by those rules. They can move any way they want, thanks to their star-shaped body.

Fish, dolphins, and even sea turtles share a common design where one side mirrors the other, known as bilateral symmetry. This design is perfect for slicing through water quickly and efficiently. 

Meanwhile, the starfish have radial symmetry, so they can head in any direction without turning around. It’s a unique trait that gives them special freedom underwater.

Then there are creatures like crabs and lobsters, which also have bilateral symmetry but add their own flair with hard shells and the ability to scuttle sideways. 

Despite these cool features, they still cannot beat the uniqueness of the starfish because the latter can heal themselves by regrowing lost arms.

Check out this video of a sea star regenerating arms: 

Starfish Limb Regeneration

Why Is Starfish Symmetry Important?

The unique symmetry of a starfish isn’t just a cool design; it plays an essential role in their survival in the ocean. Here’s a list of reasons why sea star symmetry is so important:

  • Efficient Movement: Having radial symmetry means a starfish can move in any direction without turning its body. This is super helpful for quickly responding to threats or moving toward food.
  • Sensory Reception: With no front or back, starfish can detect food, predators, and other important signals from any direction, making them highly aware of their surroundings.
  • Regeneration: This symmetry is key to their ability to regenerate lost arms. It’s not just about growing back an arm; the symmetry helps ensure the new limb grows correctly and functions well.
  • Adaptation to Environment: Starfish’s radial symmetry allows them to adapt to various environments like rocky shores and sandy bottoms. It makes them more versatile inhabitants of the ocean.

The radial symmetry of the starfish isn’t just a quirky trait. It also provides them with incredible adaptability and resilience that are useful in the ocean’s diverse habitats. 

At What Stage Do Starfish Develop Their Symmetry?

Two orange sea stars at the beach

Starfish begin to develop their unique symmetry during a critical phase of their life called metamorphosis. This is when they transition from their larval stage to their adult form with radial symmetry.

After hatching, starfish larvae swim freely in the ocean, looking nothing like the adults they will become. They have a simple, bilateral body plan, which is essential for their initial growth and development.

The metamorphosis process is where they gradually reshape their bodies from a bilateral to a radial form. This transformation involves not just a physical reshaping but also a reorganization of their internal systems to suit their new body plan.

By the end of metamorphosis, the starfish has fully developed its radial symmetry, complete with the arms and central disk we recognize. 

We hope you find this guide on starfish symmetry helpful and interesting! Leave your comments below and ask us questions, too! We’d be happy to hear from you. 

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