9 Unexpected Facts About Sea Spiders

Sea spiders are long-legged marine arthropods that inhabit oceans around the world, from the frigid waters of the Southern Ocean to the balmy Caribbean. There are over 1,000 species of sea spiders, and their diversity is extraordinary, from their striking array of color combinations to their vast size differences.

Sea spiders are fascinating creatures, and scientists still have much to learn about them. Here are nine of the most intriguing facts about sea spiders.

1. They Live in Extremely Shallow and Deep Waters

There are sea spiders found in tide pools around the world, but unlike many other shallow-water animals, they aren't limited to coastal areas. In fact, sea spiders have been found more than three miles below the surface in the very deepest parts of the ocean.

2. Sea Spiders Are Not True Spiders

Sea spiders do not spin webs, and they are not arachnids like tarantulas or house spiders. However, they are not entirely unrelated. Like true spiders, sea spiders are members of the phylum Arthropoda and the subphylum Chelicerata. The difference is at the class level: True spiders are arachnids, whereas sea spiders are members of the class Pycnogonida. That means that, in terms of classification, sea spiders are closer to true spiders than to other arthropods like crustaceans and insects.

The resemblance is undeniable, and scientists consider the genetic relationship "enigmatic."

3. The Smallest Sea Spiders Are Nearly Invisible

It's quite possible that you overlooked sea spiders in a tidal pool, especially if you were in a warm-water lagoon. That's because sea spiders living in those locations can be tiny: just a millimeter or so across. Some are so small that their muscles consist of only one cell. These nearly invisible creatures are surprisingly common, so chances are you've run across them without knowing it.

4. Giant Sea Spiders Live in Extreme Cold

While most sea spiders are extremely small, those that live in the depths of the polar seas are relatively enormous, with leg spans greater than 20 inches. Their gigantism is an adaptation that helps them live comfortably in extreme conditions. Larger animals have a lower surface area to volume ratio and thus radiate less body heat, which allows them to stay warmer in very cold water.

5. Male Sea Spiders Carry the Eggs

Sea spiders have a special pair of egg-carrying legs, which are called ovigers. After the female lays her eggs, the male fertilizes them and attaches them to his ovigers, where he carries them until they hatch. and the male carries the eggs until they hatch.

6. They Suck the Life Out of Their Prey

Sea spiders don't have the ability to spin webs; instead, they use their tubelike proboscis (nose-like structure) to suck the life out of their prey. The tip of the proboscis has three lips; some even have teeth. Once inside the proboscis, the juices are mixed with enzymes for digestion. Sea spiders eat sponges, jellyfish, sea anemones, and other prey. One group of scientists even observed a sea spider clipping off a sea anemone's tentacles in order to suck out the juices later.

7. They Use Their Digestive Systems to Breathe

Sea spiders have no lungs or gills, nor do they have a respiratory system. Instead, the oxygen they need passes through their exoskeleton and into their tissues. The oxygen circulates through their bodies when their digestive systems contract, which moves blood through the animals' entire circulatory system. This unique process, called gut peristalsis, has only been observed in sea spiders.

8. Sea Spiders Are Incredibly Resilient

Sea spiders have lived on Earth for almost 500 million years. Like very few other multi-cellular animals, they've survived multiple mass extinctions, extreme changes in climate, and even asteroid strikes. One potential explanation for their incredible resilience is the fact that they don't rely on calcified exoskeletons, but scientists believe there's still more to be uncovered about the sea spider's incredible survival.

9. Their Guts Are in Their Legs

Sea spiders' bodies consist almost entirely of long legs (four, five, or six pairs) and a proboscis. This leaves very little room for digestive organs—but that's not a problem. Sea spiders keep their guts in their legs. The organs consist of tube-shaped "intestines" which chemically reduce food to nutrients and then contract to send the nutrients around the rest of the spider. The contractions also aid in the circulation of oxygen.