Wondernose: What animal has most of its stomach in its legs?

by Rebecca Martin | Aug 1, 2023 | 0 comments

Sea spider
Sea spider. Photo © Francesco Ricciardi/iStock.com.

That’s almost too weird to be true, isn’t it, Wondernose? But we don’t make up these stories. Let me tell you why this animal has its stomach (and a few other organs as well) in its legs—because that’s practically all this creature is, legs! If you’ve ever heard a gangly boy being described as “all legs,” well, in the case of our mystery animal it comes close to being true.

There are nearly 1500 known species of this animal. Most of them have four pairs of walking legs; a few species have six, eight, or more pairs. And what legs they are—long and jointed! I don’t know how many segments there are in each leg. As for size, the different species measure anywhere from a fraction of an inch to more than 2 feet (60 cm)
across. Can you picture a two-foot animal that’s mostly legs, Wondernose?

Our mystery animal is found all over the world, even in the polar regions. But you’ve never seen one because they live under water. So you were going to ask whether this is some kind of a spider? Then I spoiled it by saying they live in the water. Yet your guess was correct, Wondernose. These are called sea spiders. You may think of spiders as being arachnids. However, scientists call these spiders pycnogonids. So, in the technical sense, these creatures are not spiders.

Some live in shallow water along the shorelines, but others are found in all depths of the ocean, right down to the floor.

Obviously, being mostly legs, the sea spider’s actual body isn’t very big. Usually it consists of four sections, with one pair of legs to each section. The mouth, which looks rather like a beak and is made for sucking, is nearly as big as the rest of the body! As for the stomach, it’s tubular, and part of it is actually in the abdomen. But tubes also run from the main stomach into the legs. So you see, it really is accurate to say that part of the stomach is in the legs.

What do these pycnogonids eat? Think about those sucking mouth parts, Wondernose. Yes, sea spiders often live right on bottom-dwelling sea animals and suck juice out of them. Huge sea spiders have been caught in nets; they’re found clinging to the larger animals that were caught.

The sea spider is known by a few other names too. Sometimes they’re called sea scorpions or whip scorpions. Interestingly, they can cling to or walk over dangerous stinging animals that dwell on the sea floor—animals like the sea anemone and the sea fir, which act like plants and are rooted to the bottom. They have stinging cells with which they capture other animals going by. But not the sea spiders! Apparently they’re immune to those stinging cells. Their legs must be well-armored.

You wonder whether a sea spider’s eyes are also on its legs. As a matter of fact, some of the species don’t even have eyes. But those that do, have them on a little turret that grows between the head and the next segment of the body.

After the female sea spider has laid eggs, the male cares for them. He has organs on his legs that give out a kind of glue. With this the eggs are cemented into a mass, and then he carries them around using still another pair of special legs.

Once the eggs hatch, they turn into a sort of larvae known as protonymphons. At this stage they have no legs at all. They simply cling with their mouths to juicy creatures like sea cucumbers, sucking away. Later they grow three pairs of legs. They’re not considered adults until they’re complete with four pairs.

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