Meet the sea spider

Meet the sea spider

Sea spiders swim and crawl along sandy seafloors around the world. They might be as small as a grain of sand or as long as a housecat. When a sea spider discovers a soft-bodied animal to snack on, it thrusts its straw-like proboscis into the animal’s flesh, then sucks out its insides like a smoothie. Learn more in this website‘s article update.


Animals in the class Pycnogonida are considered sea spiders, and are called pycnogonids. There are more than 1,300 species that belong to this class!

Pycnogonids are marine arthropods, which means they’re ocean-dwelling animals related to crabs and spiders. Like a spider, a sea spider usually has eight legs. And like a crab, it has a hard exoskeleton.

Though they share an ancestor with crabs and spiders, sea spiders have been evolving on their own for hundreds of millions of years (scientists have discovered a 500 million-year-old sea spider larva!). In all this time, these animals have developed their own unique characteristics.


Sea spiders range in size from .03 inches (1mm) to 20 inches (50 cm) in leg span — that’s about as long as a housecat!

In a phenomenon known as “polar gigantism,” the largest sea spider species live close to the North Pole and the South Pole. Scientists continue to investigate this mysterious phenomenon.

Meet the sea spider


A sea spider has a head with a long tubular mouthpart (called a proboscis), several simple eyes, and three to four sets of appendages including a pair of claws and a pair of ovigers which are used for grooming and egg-carrying.

The sea spider’s defining characteristic is its sets of long, multi-jointed legs. These spindly stilts allow the sea spider to swim and crawl along the sandy bottom without getting stuck in the seafloor silt.

The legs also house many of the animal’s other major functions. A sea spider’s legs contain its vital organs. For example, its intestines have pouches that extend all the way to the end of its legs.

The sea spider also uses its legs to breathe. Oxygen passes along the legs’ large surface area, and the intestinal pouches in the legs diffuse the oxygen throughout its tissues.


Sea spiders live in the ocean worldwide. They live in tropical, temperate, and polar oceans — from shallow water to abyssal depths. About 20 percent of the world’s known species are found in Antarctica.


A sea spider feeds on soft-bodied animals like anemones, worms, jellies, sponges, soft corals, and nudibranchs.

Feeding strategies

A sea spider uses its proboscis — a long, tubular mouthpart — to suck the guts of its soft-bodied prey. It punctures its prey’s skin with the proboscis, then sucks out the contents of the animal’s body. Because of this feeding strategy, sea spiders are often called a suctorial predator.

Sometimes sea spiders do not kill their prey, and can be considered a parasite rather than a predator. For example, if a sea spider sucks the guts from a larger animal, it will likely weaken, but not kill, that animal. In 2009 MBARI (Monterey Bay Aquarium Research Institute) researchers found giant sea spiders next to pom-pom anemones. It appears the sea spiders had been sucking the juices from the anemones’ tentacles and left them wilted-looking — but still alive.


Sea spiders mate by using the genital pores in their legs. The male sea spider climbs onto the female, and the pair adjusts until the pores are aligned. The female releases her eggs, then the male combines them with his sperm. He carries the eggs in his egg-carrying organs, called ovigers, until the eggs hatch.


Sea stars, shorebirds, rays, crabs, and fishes eat sea spiders. Some species of sea spiders may be able to hide from their predators since their whitish color blends in with the sandy seafloor.


Sea spiders live in sandy seafloor habitats, which can take a long time to recover from trawl nets (a fishing technique in which nets are dragged over the seafloor).

Sea spider species in the cold waters of the North and South Poles tend to be larger than their tropical counterparts. This could impact their ability to survive in warming oceans, though recent studies found no evidence that size impacts a sea spider’s ability to survive warmer oceans.

Cool facts

  • Sea spiders can move forward, backward, or sideways without turning their trunk.
  • The sea spider’s egg-carrying organs, called ovigers, are also used for grooming!
  • Female sea spiders are often larger than their male counterparts.
    One sea spider species, Ascorhynchus corderoi, has been described as hermaphroditic, meaning it has both ovaries and testes.
  • All sea spiders molt as they grow larger.
  • The sea spider’s mouthpart is called a proboscis — but many other animals have a proboscis, too. Bees and butterflies use a proboscis to gather nectar from flowers. The proboscis of anteaters, elephants and aardvarks is an elongated nose or snout.