Marine Biology

Physical Characteristics of Sea Spiders

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"Physical Characteristics of Sea Spiders"
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Sea spiders (scientific name Pycnogonids) are sometimes also known by the name 'Pantopoda' which means 'All Legs.' This is a good descriptor for the sea spider, as it is little more than a collection of legs attached to a comparatively small body. 

A sea spider has from 4 to 6 pairs of legs, which are long and clawed. They have hardly any abdomen, and their cephalothorax region consists mainly of a proboscis. The proboscis is used to suck the nutrients out of the soft invertebrates like sponges and anemones that make up the sea spider's prey.  Three or four tiny appendages not used for walking can be found around the proboscis. These are used by sea spiders to clean themselves, in courtship, and in caring for their young.

There is so little abdomen on a sea spider that their digestive tract extends down into their legs, with an anus located on the spider's last body segment. Their muscles are made up of one single cell and connective tissue.  

Because a sea spider is so slender, they can take in the oxygen they need directly through their exoskeleton without the need for gills to process it. A sea spider's heart is long and thin, beating at 90-180 beats per minute to power an open circulatory system. Their brain is connected to two nerve cords, which are themselves connected to nerves throughout the spider's body.

Most types of sea spiders have separate sexes, with the females having ovaries and the males having testes. Little is known about the courtship ritual of these creatures, but they use an external fertilization method. Still, it is the male sea spider who cares for the eggs and young.

There are 1,300 varieties of sea spider, and they can be found in almost every ocean in the world. Though many subspecies range from 1 to 10 millimeters (0.039 to 0.39 in), deep sea versions of the sea  spider can grow to over 90 cm (over 3 feet).  However big or small they are, all sea spiders move in the same fashion. They walk along the ocean floor using their legs like stilts, or swim right above it folding and unfolding their legs like an umbrella opening to gain locomotion.

Though some sea spiders are scavengers, most walk or swim along the seafloor seeking prey. An unsuspecting anemone or sponge is their most likely target. The sea spider inserts its long proboscis into its prey and sucks out nourishment. Because most sea spiders are so small, the anemone or sponge (comparatively large) usually survives this attack. 

With its wide range, efficient body, and unique dining habits, the sea spider is a interesting ocean creature to study. Scientists are unsure whether it belongs in the same order as horseshoe crabs, scorpions and ticks; or whether it should have its very own order. But whether you call it a Pycnogonids or a Pantopoda; a true spider or something other entirely;  the sea spider is a fascinating part of ocean life.

More about this author: Laura Cushing

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