Sea Spider (class Pycnogonida)

InvertebrateArthropodDeep sea

Overview

Sea spiders (class Pycnogonida) are among the strangest animals in the ocean: marine arthropods with such tiny, thread-thin bodies and such long, spindly legs that they look like little more than a set of walking legs. Their body is so slender that there isn't room inside for full-sized organs, so sea spiders have pushed parts of their digestive and reproductive systems out into the legs themselves. They range from tiny species a few millimetres across to deep-sea giants with leg spans of tens of centimetres.

Despite the name and a passing resemblance, sea spiders are not true spiders. They belong to their own ancient lineage of arthropods, and their unusual anatomy makes them a puzzle that scientists are still working to place on the tree of life.

Note: “sea spider” covers the whole class Pycnogonida; details here describe the group broadly. Treat general statements as approximate and verify against authoritative sources.

Habitat & Range

Sea spiders live only in the sea, and they are found throughout the world's oceans — from shallow tide pools, rocky shores, and reefs to the abyssal deep sea. They are especially diverse in cold waters, and the largest species live in the deep and in polar seas (an example of “polar gigantism”). They are bottom-dwellers, clambering slowly over the seabed, rocks, seaweed, and other animals.

Diet

Sea spiders are carnivores that feed mostly on soft-bodied, slow or stationary prey such as sea anemones, soft corals, hydroids, sponges, and sea slugs. They feed using a prominent tube-like mouthpart called a proboscis, which they use to suck up the soft tissues or fluids of their prey. Their slow, deliberate lifestyle suits feeding on animals that cannot escape.

Behavior

Almost everything about a sea spider is shaped by its extreme thinness. With too little room in the narrow trunk, the gut branches out into the legs to digest food, and the animals are thought to take in oxygen largely by diffusion across their long legs rather than through dedicated breathing organs. They move slowly over the bottom on their stilt-like legs, and many can swim feebly or drift. A striking feature is paternal care: in most sea spiders the male carries the developing eggs, holding clutches on a special pair of legs (the ovigers) until they hatch. Sea spiders are harmless to people and, because of their odd anatomy, are of great interest to biologists studying arthropod evolution.

Human Interaction & Conservation

Sea spiders are little-known to most people but are an important part of seabed communities and a fascinating subject for researchers, thanks to their bizarre body plan, leg-based organs, and the giant forms found in cold and deep waters. They are harmless, not exploited commercially, and most are not of particular conservation concern, though, like all marine life, they depend on healthy oceans. Consult authoritative sources for details.

More photos of the sea spider