Sea Spiders [Some Fascinating Facts]

Sea Spider

Sea Spiders, also known as Pycnogonids, are marine animals that belong to the order Pantopoda and the class Pycnogonida.

They are found in oceans all around the world, with over 1300 known species.

Their leg sizes can range from 1 mm (0.04 in) to over 70 cm (2.3 ft), and they can be quite large in Antarctic and deep waters.

Although they are called Sea Spiders, they are not true spiders or even arachnids.

Their traditional classification as chelicerates would place them closer to true spiders than other arthropod groups like insects or crustaceans.

However, this is disputed, as genetic evidence suggests they may be a sister group to all other living arthropods.

How Do Sea Spiders Look?

Body Structure

Sea Spiders have long legs and small bodies. They usually have eight walking legs (four pairs), but some species may have five or six pairs of legs.

Their body is made up of a cephalothorax (head and thorax) and a much smaller, unsegmented abdomen.

They have simple eyes on their non-calcareous exoskeleton, but some deep-ocean species may not have eyes at all.

Respiration and Circulation

Sea Spiders don’t have a traditional respiratory system. Instead, they absorb gases through their legs and transfer them through the body by diffusion.

Their open circulatory system and nervous system help transport oxygen and nutrients throughout their body.

The Sea Spider’s heart beats vigorously, creating substantial blood pressure and helping to drive circulation in the trunk and part of the legs.

Feeding and Digestion

Sea Spiders have a proboscis that allows them to suck nutrients from soft-bodied invertebrates.

Their digestive tract has branches that extend into the legs.

Some species have well-developed and flexible proboscises, often equipped with sensory bristles and strong rasping ridges around the mouth.

Reproduction and Development

Sea Spiders have separate genders, with females having ovaries and males having testes.

They reproduce through external fertilization, and only the males care for the eggs and young.

The larvae have a blind gut and only consist of a head and three pairs of cephalic appendages.

As they grow, the abdomen and thoracic appendages develop later.

There are four types of larvae for Sea Spiders: the typical protonymphon larva, the encysted larva, the atypical protonymphon larva, and the attaching larva.

The typical protonymphon larva is the most common and gradually turns into an adult.

Distribution and Ecology

Sea Spiders are found in many different parts of the world, including Australia, New Zealand, the Pacific coast of the United States, the Mediterranean Sea, the Caribbean Sea, and the north and south poles.

They are most common in shallow waters but are also found at depths of up to 7000 meters (23000 ft).

They live in both marine and estuarine habitats and are well camouflaged among rocks and algae along shorelines.

Sea Spiders either walk along the bottom with their stilt-like legs or swim just above it using an umbrella-pulsing motion.

They are mostly carnivorous predators or scavengers, feeding on cnidarians, sponges, polychaetes, and bryozoans.


The class Pycnogonida comprises over 1300 species, which are usually split into 86 genera.

The taxonomy within the group is uncertain, and there is no agreed list of orders. All families are considered part of the single order Pantopoda.

Sea Spiders have long been thought to belong to the Chelicerata group, along with horseshoe crabs and the Arachnida.

However, some research suggests that they may belong to their own lineage, distinct from chelicerates, crustaceans, myriapods, or insects.

This would mean that Sea Spiders are the last surviving members of an ancient stem group of arthropods that lived in Cambrian oceans.

More recent research, though, supports the idea that sea spiders are the sister group to the remaining Chelicerata.


Reef Safe?No
Maximum Size:0.2–50 cm (0.1–19.7 in)

Scientific Classification

Also Known As:Pycnogonids
Conservation Status:Unknown

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