Are sea spiders really spiders?

Sea spiders are not actually spiders.

During the 2017 Discovering the Deep: Exploring Remote Pacific MPAs expedition, this sea spider was seen feeding on a cup coral along a steep slope on the southern ridge of Baker Island. Video courtesy of the NOAA Office of Ocean Exploration and Research, Discovering the Deep: Exploring Remote Pacific MPAs. Download larger version (mp4, 27.9 MB).

Sea spiders are more formally known as pycnogonids because they belong to the class Pycnogonida within the phylum Arthropoda. The look-alike land-dwelling spiders after which they are named are also arthropods, but they belong to the class Arachnida. So, although they are related, sea spiders and spiders are not the same creature.

There are about 1,500 known species of sea spiders. They are widespread throughout the ocean, but are particularly abundant in the polar regions. They can be found in tide pools, where they are so small they can be hard to see, and on the floor of the deep sea, where they can grow to over 50 centimeters (20 inches), an important adaptation that helps them cope with the extreme conditions.

Classified in the order Pantopoda, which means “all legs,” sea spiders are also known as pantopods. While not quite all legs, they are mostly legs (usually eight), and they use those legs to walk along the seafloor and even swim, or tread water, above it. Also, their trunks are so small that some of their organs extend into their legs. Additional appendages are used for cleaning and courtship and by the males to carry developing eggs and sometimes their young.

Sea spiders don’t have lungs, they get oxygen through their exoskeleton. In the deep sea, they may also be lacking eyes.

With a diet that includes soft-bodied animals like anemones, bryozoans, hydroids, worms, and corals, most sea spiders are carnivorous (some are known to also dine on algae). They use their proboscis, a tube-like mouth that is often longer and larger than their body, to suck bodily fluids out of their prey.

Despite their presence throughout all regions and depths of the ocean, sea spiders are understudied. Much remains to be learned about these fascinating marine arthropods.