Octopus vs. Spider: Are They Distantly Related?

Introduction

As bipeds and storytelling animals, we humans are curious about animals with different numbers of limbs. We look at spiders and octopuses with their eight limbs and wonder, “Are these creatures distantly related?” (Note: “Octopi” is grammatically incorrect because it assumes that the word has Latin roots.) Let’s look at these two animals a little more closely.

Octopus

Octopuses are ocean-dwelling, soft-bodied, eight-limbed mollusks of the order Octopoda, which has over 300 species. This order is within the class Cephalopoda, along with squids, cuttlefish, and nautiloids. Octopuses are symmetrical, with two eyes and beaked mouths which can tear their prey. They can drastically alter their shape and have siphons to breathe and propel themselves through the water. Their eight appendages are actually two back legs that can push them over rocks and six arms used for feeding, swimming, and moving along the ocean floor. They are venomous, capable of expelling ink, and many can even camouflage themselves.

Spider

Spiders are air-breathing arachnids of the class Arthropoda. They have eight legs, fangs, chelicerae (jaws or pincers that can inject venom), and spinnerets that can spin silk. These creatures have two body segments: a cephalothorax and an opisthosoma (abdomen), which are joined by a pedicel. Additionally, they lack antennae. They extend their limbs using hydraulic pressure and digest their food externally, using enzymes to liquefy it before ingesting it.

Scientific Classifications

So, are they related? Well, while all life on Earth is “distantly related,” let’s examine the taxonomy of these two orders of animals.

Octopuses

Biologists classify octopuses in the phylum Mollusca and the class Cephalopoda. The order Octopoda includes some 300 species, including the common octopus.

• Kingdom — Animalia
• Phylum — Mollusca
• Class — Cephalopoda
• Order — Octopoda

Spiders

Spiders are classified in the phylum Arthropoda and the class Arachnida.

• Kingdom – Animalia
• Phylum – Arthropoda
• Class – Arachnida
• Order – Araneae (Spider)

Conclusion

As you can see, the two are no more than distantly related taxonomically in that they are members of the Animalia kingdom.

So, why do they both have eight legs if they’re unrelated? Many scientists believe that this is a case of convergent evolution. Convergent evolution is a process in which two unrelated species independently evolve similar traits due to adapting to similar environments or ecological pressures. In this case, the two species have developed eight legs because it is an efficient way to move in all directions and interact with their environment.

This is just a case of eight legs being helpful to get around and manipulate their environments in the same way that elephants and dogs benefit by having four legs.

Up Next

• Octopus Lifespans: How Long do Octopuses Live
• Types of Octopuses
• Black Widow Spider vs Brown Recluse Spider: 5 Differences

The photo featured at the top of this post is © DiveIvanov/Shutterstock.com