I have turned over a lot of rocks on this trip. Not one sea spider yet.
On our lap around Australia, the rock pools have been generous. At Red Cliff, we found a gloomy octopus so well camouflaged we nearly stepped past it. That is exactly how sea spiders hide. Most are smaller than a fingernail, pale or patterned like the algae they sit in, and they barely move.
They are here, though. Sea spiders live in Australian rock pools, on the Great Barrier Reef and down in the cold southern deep. So this guide is half science, half search plan.
Sea spiders are some of the strangest marine arthropods, and marine invertebrates, on Earth. They breathe through their legs. Their guts reach into their knees. And they have been doing this for at least 425 million years.
1. Sea Spiders Aren’t Actually Spiders
A sea spider is not a true spider (meaning not within arachnids), and it isn’t a crab either. Sea spiders form their own class, Pycnogonida, which is why scientists call them pycnogonids.
They are distant relatives of terrestrial spiders, scorpions and horseshoe crabs. Most genomic studies place them as the sister group to all other chelicerates. An older idea, the Cormogonida hypothesis, put sea spiders outside every other living arthropod, but the DNA hasn’t backed it.
No webs, no venom, no fangs. Just a misleading name.
2. They’re All Legs and Almost No Body
The first thing you notice is the enormous walking legs. The body between them is tiny, so tiny that biologists describe sea spiders as “all legs, no body.”
Most sea spider species walk on up to four pairs of stilt-like legs. A few carry up to two pairs more, for five or six pycnogonid leg pairs in total. The 12-legged Dodecolopoda lives in the Southern Ocean.
The head region is built from the four anterior segments of the body. It carries a few head appendages other animals lack. First comes a long proboscis for feeding. Then there are claws called chelifores, feelers called palps, and ovigers for carrying eggs.
3. Their Guts and Gonads Live in Their Legs
With so little body, the vital organs had to go somewhere. In sea spiders they went into the legs.
Branches of the gut run down each leg, almost to the tip. So do the gonads. In females, eggs develop inside the legs and are released through pores near the leg bases.
The nervous system is laid out as a segmented ladder-like structure. Each pair of legs, from the first leg pair to the last, has its own cluster of nerve cells. Under a microscope, it looks like a textbook drawing of an ancient arthropod.
4. They Breathe Through Their Legs, and Pump Blood With Their Guts
Sea spiders have no gills and no lungs. Oxygen simply soaks in through the thin, porous cuticle of their long legs.
Moving it around is the clever part. Their heart is too weak to push fluid all the way to the leg tips. In 2017, researchers found the gut does the job instead. Waves of contraction in the gut branches squeeze the hemolymph, their version of blood, up and down each leg.
It is the only known animal that uses its gut as its main pump.
5. They Live From Rock Pools to 7,000 Metres Deep
Sea spiders live in every ocean, from the north and south poles to tropical reefs. Most walk; a few use their legs as swimming legs, pulsing them like an umbrella. Most are found at relatively shallow depths, in rock pools, kelp forests and coral reefs. Some reach the deep sea, down to around 7,000 m. The giant sea spider waits at about 5,700 m, in the abyss, in my deep sea dive.
Antarctica is a hotspot. About one in five known sea spider species lives in the Southern Ocean.
In Australia, they turn up from the Great Barrier Reef to cooler southern waters. Look for them among hydroids, sea anemones, sponges and seaweed at low tide. Then look again, slowly.
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6. Polar Sea Spiders Grow as Wide as Dinner Plates
Sea spiders vary hugely in size. Most are small, with leg spans ranging from about 1 mm to a few centimetres. Head to the poles, and that changes.
The largest sea spider species, like Colossendeis in Antarctica, reach a leg span of around 70 cm. This is polar gigantism, the same pattern seen in Antarctic isopods and the colossal squid.
The usual explanation is that cold water holds more oxygen. A 2019 study added a twist. Large Antarctic pycnogonids were no more sensitive to warming than small ones. Their cuticle may get more porous as they grow, helping oxygen in. Both size groups, though, struggled as temperatures rose.
7. They Feed on Soft-Bodied Animals With a Straw
They feed on soft-bodied animals, and the menu includes sea anemones, hydroids, soft corals, sponges, bryozoans and worms.
So they don’t chew. The otherwise elongated proboscis ends in a typically Y-shaped mouth, lined inside with numerous sensory bristles. They pierce the prey and suck out its fluids.
Many prey animals survive the meal. Sea anemones survive being fed on, left wilted but alive, so some sea spiders behave more like parasites than predators. A few species even live inside clams.
8. Some Deep-Sea Sea Spiders Farm Their Own Food
This is the newest fact on the list, and my favourite.
In 2025, researchers described three new Sericosura species from methane seeps off California and Alaska. They live between about 400 and 2,000 m deep. These sea spiders grow colonies of methane-eating bacteria on their own exoskeletons, then feed on them.
Their eggs carry the same bacteria. So each generation inherits the farm.
9. Dads Carry the Eggs
Sea spiders have separate sexes. The only known hermaphrodite is Ascorhynchus corderoi.
When they mate, the female releases eggs while the male fertilises them outside her body. The male then gathers the eggs and glues them into clusters with cement from glands on his legs. He carries them on his ovigers. He guards them until they hatch.
Females often have smaller ovigers, or none at all. If you find a sea spider wearing what looks like a bunch of grapes, it’s a father.
10. Their Babies Hatch With Six Limbs and No Legs
Most sea spiders hatch as a distinct larval stage, the protonymphon. A typical protonymphon has only three pairs of cephalic appendages: chelifores, palps and ovigers. No walking legs at all.
Some are attaching larvae that cling to their father’s ovigers. Others live on or inside hosts like hydroids and clams. As the larva moults, walking leg buds appear. New leg-bearing segments develop one after another, starting with the first walking leg. The small and young juvenile then looks like a miniature adult. Many scientists think this typical protonymphon is possibly an ancestral larva for all arthropods.
The whole life cycle can take months. In some polar species it probably takes years.
11. They Can Regrow Their Back End
Most moulting animals can regrow a leg. Almost none can regrow part of their body.
In 2023, scientists cut young Pycnogonum litorale sea spiders across the body. They regrew the missing segments, the hindgut and even the anus. Some ended up with six legs instead of eight and still reproduced.
It was the first clear case of a moulting animal regrowing its main body axis.
12. They’re Older Than Dinosaurs, and the Fossils Are Rare
The oldest possible sea spider fossils are tiny larvae from Late Cambrian rocks in Sweden, about 500 million years old. That species, Cambropycnogon, is known from only its protonymphon larvae, so not every scientist accepts it.
I studied marine biology in Sweden on exchange. I never saw those fossils, but I like knowing the story may start there.
The first undeniable adults come from 425-million-year-old Silurian rocks in Herefordshire, England. Haliestes dasos already had the chelifores and long legs of extant sea spiders. Devonian slates in Germany and Jurassic rocks in France add a few more.
Only a few fossil species are known, some from fragmental body segments. An enormous fossil gap separates them. Their soft, thin bodies rarely fossilise.
Where Sea Spiders Fit in the Tree of Life
Scientists have argued about sea spiders for 150 years. The chelifores were the puzzle.
In 2006, a study tracked Hox genes in a developing sea spider. It found the chelifores grow from the same body segment as a spider’s fangs. The paper’s title says it plainly: “Homology of arthropod anterior appendages revealed by Hox gene expression in a sea spider”. It helped settle where they belong.
Today most genetic trees put them as the closest relatives of all other chelicerates. You can see them on the phylum explorer, alongside the spiders, scorpions and mites that share their ancestry.
Are Sea Spiders Under Threat?
No sea spider is formally listed as threatened. Most have never been assessed.
Antarctic species are the ones to watch. The 2019 warming study showed even giant polar sea spiders lose performance as water heats up. Deep-sea trawling and mining also damage the seafloor habitats where many of them live.
Common Questions About Sea Spiders
No. Sea spiders have no venom and no fangs, and they can’t bite people. Their proboscis is built to suck fluid from soft-bodied invertebrates, not to pierce skin.
Yes. Australia has many sea spider species, from tropical reefs to temperate rock pools and the deep sea. Most are small and well camouflaged, so you need to search hydroids, sponges and seaweed slowly at low tide.
The largest sea spider species live in Antarctic waters. Giants like Colossendeis reach a leg span of around 70 cm, about the width of a dinner plate. Most species have leg spans of under 1 cm.
Through their legs. Oxygen diffuses in through the thin, porous cuticle. Contractions of the gut branches inside each leg then pump hemolymph around the body.
Yes. More than 1,300 extant sea spider species live in oceans worldwide, from rock pools to the deep sea. New species are still described every year.
Look Slowly in the Rock Pool
Sea spiders don’t reward speed. They reward patience.
Next time you’re rockpooling, pick one patch of hydroids or seaweed and watch it for five minutes. If something thin and spindly moves, you might be looking at an animal whose body plan is 425 million years old. I’m still looking for my first one on the lap. If you find one, tell me where.
Want more tiny, overlooked animals? Meet the tardigrade, the nudibranchs that share sea spiders’ rock pools, and the giants of the deep sea.
Wildlife Nomads Challenge
The All-Legs Challenge 🕸️
Ten questions on leg-breathing, gut-pumping, egg-carrying dads and methane farmers. Instant feedback after every answer.
References:
Brenneis, G., Frankowski, K., Maaß, L., & Scholtz, G. (2023). The sea spider Pycnogonum litorale overturns the paradigm of the absence of axial regeneration in molting animals. Proceedings of the National Academy of Sciences, 120(5), e2217272120. https://doi.org/10.1073/pnas.2217272120
Dal Bó, B., Guo, Y., Mayr, M. J., Pereira, O. S., Levin, L. A., Orphan, V. J., & Goffredi, S. K. (2025). Methane-powered sea spiders: Diverse, epibiotic methanotrophs serve as a source of nutrition for deep-sea methane seep Sericosura. Proceedings of the National Academy of Sciences, 122(26), e2501422122. https://doi.org/10.1073/pnas.2501422122
Jager, M., Murienne, J., Clabaut, C., Deutsch, J., Le Guyader, H., & Manuel, M. (2006). Homology of arthropod anterior appendages revealed by Hox gene expression in a sea spider. Nature, 441(7092), 506–508. https://doi.org/10.1038/nature04591
Shishido, C. M., Woods, H. A., Lane, S. J., Toh, M. W. A., Tobalske, B. W., & Moran, A. L. (2019). Polar gigantism and the oxygen–temperature hypothesis: A test of upper thermal limits to body size in Antarctic pycnogonids. Proceedings of the Royal Society B: Biological Sciences, 286(1900), 20190124. https://doi.org/10.1098/rspb.2019.0124
Woods, H. A., Lane, S. J., Shishido, C., Tobalske, B. W., Arango, C. P., & Moran, A. L. (2017). Respiratory gut peristalsis by sea spiders. Current Biology, 27(13), R638–R639. https://doi.org/10.1016/j.cub.2017.05.062
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