What is a Sea Spider? - Facts, Predators & Classification

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

Although not truly an arachnid, in this lesson we'll be learning about the so-called sea spider. We'll go over some interesting facts about this animal, its true classification, and its place in the ocean food chain.

What Are Sea Spiders?

In the cold depths of the Antarctic Ocean, life is tough. Temperatures can drop below freezing and cold, hard-packed ice covers the surface. Without much plant life, it can be difficult to make a home in these harsh conditions. But one animal that seems to thrive here is the sea spider. Antarctic sea spiders can grow over two feet wide. Imagine a spider crawling out from under your bed that's wider than a folding chair!

Luckily for arachnophobes, the sea spider is not actually a spider, but rather a distant relative in the phylum Arthropoda, called Pycnogonida. Pycnogonids don't just live in Antarctica, although they do grow the biggest there. Sea spiders can be found all over the globe, from shallow tropical seas, to freezing oceans, to the deepest parts of the ocean over four miles below the surface. Sea spiders in warmer climates, such as shallow tropical seas, tend to be smaller than their cold-water cousins.

A sea spider found at the base of Davidson Seamount off of the coast of California
sea spider


Sea spiders can be identified by their long legs and highly-reduced body, which is why they were originally called sea 'spiders'. Although there is a clear concentration of sensory organs towards the front of their body, there isn't a true head in these creatures. Instead the sea-spider has a head-like region called the cephalosoma. These strange creatures have tiny, thin bodies, with most of their mass in long legs.

Most sea spiders have four pairs of legs sprouting from a body with four segments, all of which follows the cephalosoma. However, other species have more segments, and thus more legs.

Although these creatures are probably starting to sound mighty strange, and possibly a bit frightening, they do know how to dress in style. There are many species of sea spiders, and some have beautiful, bright colors coating their legs and body. The yellow-kneed sea spider, for example, has bright yellow stripes contrasting a red body. Other sea spiders have purple stripes, bright yellow solid bodies, or black and white stripes, depending on the species.

Sea spiders come in many colors such as purple and white stripes
sea spider

Although as humans we like to sit down to eat a meal with our mouth, the sea spider prefers an all-liquid diet that they can eat standing up. They insert a long appendage called a proboscis into their prey, sucking up the nutrients like a seafood smoothie. Their diet consists of soft bodied animals like hydras, sea anemones, and other bryozoans.

If its enormous size and strange diet wasn't enough for you, there's more that makes this animal stand out. Unlike many species that require females to care for their offspring primarily, male sea spiders use special appendages called ovigers to carry the eggs. Special glands called cement glands wrap the eggs to the ovigers. Sometimes, males even carry the eggs after they hatch as larvae, keeping them save in the competition of the open ocean.


All living things are organized into groups, with successive groups sharing more characteristics in common. In one of the largest categories, kingdom, sea spiders are classified in Animalia. Many people are surprised to find out what scientists consider an animal! Although we usually think of dogs and cats as animals, any multicellular organism that is mobile and eats to get food is considered an animal in the scientific world.

Classification of living things

Animals are divided into smaller groups based on shared characteristics. Sea spiders are a member of the phylum Arthropoda. This means they are related to insects, like ants or bees, as well as the arachnid group for which they are named, which contains spiders and centipedes. However, their name is a misnomer. Sea spiders are not actually spiders. They fall in a different subgroup of Arthropoda, called Pycnogonida.

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