Sea Spider
A marine arthropod so thin its body has no room for organs — so most of them moved into the legs
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Superpower
Sea spiders (Pycnogonida) have bodies so small and thin that their digestive system, reproductive organs, and other tissues have extended into their legs to fit. Their legs contain branches of the gut that absorb and transport nutrients. The heart has no enclosed circulatory system — it pumps fluid through the open body cavity, and oxygen diffuses directly through tissue rather than through blood vessels. In the cold, oxygen-rich polar waters where they grow largest, this diffusion works across leg spans of 70cm.
Overview
Sea spiders are marine arthropods found in all oceans from the intertidal zone to 6,000m depth. They're not true spiders but belong to their own class (Pycnogonida), possibly one of the earliest arthropod lineages. In polar regions they exhibit gigantism — growing to 70cm leg spans far exceeding their tropical relatives. They're predators and parasites of cnidarians, bryozoans, and other soft-bodied invertebrates, feeding by pushing a proboscis into prey and sucking out the tissue.
Found in
All oceans worldwide from the intertidal zone to 6,000m depth. Particularly diverse and large in polar regions. Most common on rocky substrates, coral, and algal beds.
Things worth knowing
- 1
Male sea spiders carry the eggs — females deposit eggs onto a specialized leg on the male called an oviger, and the male carries the developing eggs until they hatch, sometimes carrying multiple clutches from different females simultaneously.
- 2
Sea spider legs can regenerate — individuals that have lost legs to predators or damage can regrow them during molting, a capability rare in arthropods of their complexity.
- 3
Polar sea spiders have been found growing at the extreme slow rate required by frigid Antarctic waters — reaching ages potentially measured in decades for the largest individuals.
- 4
Sea spider bodies are so reduced that even their brains are partially housed in their front leg segments rather than in the cephalothorax alone — the nervous system is distributed into the appendages along with the other organs.
- 5
Some sea spider species are kleptoparasites — they steal food from other sea spiders rather than hunting independently, following larger individuals and consuming their prey after it's been partially liquefied.