A strange group of entirely marine animals that are known as ‘Sea Spiders’. While they are in the same phylum as scorpions. ticks, mites and spiders they are not arachnids at all but in their own class called Pycnogonida which means ‘knobby knees’. The body is so reduced that reproductive organs are located in the legs. While most species walk about on 4 pairs of legs like spiders, some forms have 10 legs (5 pairs) Decalopda sp. Or even 12 (6 pairs) Dodecalopoda sp., and the larvae start off with only 3 pairs !!They are mostly small slow moving creatures which live on their food source of seaweeds or other invertebrates, but a few giants are known from great depths with leg spans up to 70 cm.!!

• Small thorax with side projections bearing 3-6 pairs of jointed
• Tiny rear abdomen
• Prominent sucking proboscis surrounded by various paired mouthparts
• Single four part eye on top of a stalked tubercle
• Sex organs in long joints of legs

Sea spiders

Small bodied, long legged marine arthropods. Most are very small found moving slowly over seaweeds, corals , sponges etc. on 4 pairs of knobbly legs. Usually only a few mm , the body is long and narrow, the proboscis is sometimes as long as the body. Deep water pycnogonids tend to be larger and more active. Some even swim using leg motions similar to walking. Others are known to ‘hang’ from the water’s surface using light body weight and surface tension. Colour is usually pale or camouflaged. Larvae have massive pincerlike chelicerae and only 3 pairs of legs.

Pycnogonids are found worldwide but seem to be most abundant in colder waters. Mainly bottom dwellers they range from intertidal zones to depths over 6000m. One species has been seen in the hydrothermal vent communities at great depths in the Galapagos Rift. Larvae are often internal parasites of hydroids or corals but adults are solitary and free living. Young of a few species attach to planktonic medusae which would disperse them over large areas.

A few feed on algae but most are carnivorous, using 3 cuticular teeth at tip of proboscis to pierce host’s skin before sucking out body fluids of hydroids, sea anemones, soft corals, sponges. One group specialises in eating bryozoans, inserting the proboscis into each individual chamber. Little is known of feeding habits of deepwater species. Larvae live as parasites on hydroids and other cnidarians sometimes causing a sort of gall.

Not much to eat here; small bodied animals with a hard exoskeleton and long skinny legs.

Sexes are separate and females lay 1000’s of eggs which the male gathers and glues together into a ball which he carries , sometimes with a special extra pair of legs, during brooding period. Eggs hatch into larvae with only 3 pairs of legs and huge pincers which then attaches to a cnidarian host.

Adults are solitary but often live in close association with invertebrate food hosts or seaweeds

Rarely observed as small and cryptic, or hidden amongst other organisms.

The largest pycnogonids Collosendeis sp. has a leg span of 70 cm. And lives in the cold deep waters of the South Atlantic. They have been photographed from submersibles patrolling the muddy bottom at 1900m depth on 4 pairs of twiggy legs extending from a tiny thin body with long proboscis. Males have an extra pair of limbs for carrying eggs but little is known of their life history.