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Facts about Sea Spiders

Reshma Jirage
Sea spiders are marine arthropods belonging to the class 'Pycnogonida' and phylum 'Arthropoda'. This story furnishes some interesting facts about these marine creatures.

The Giant from Antarctica!

In 2008, a 9.8-inch-long giant sea spider was located during a marine census trip in southern Antarctica. The reasons that favored its gargantuan size are said to be cold temperatures, less predators, and high oxygen levels.
Sea spiders are referred to by other names such as pycnogonids or Pantopoda. Pantopoda means 'all legs' in Greek. This is due to the fact that sea spiders have extremely small bodies and very long legs. Their legs house most of the organs.

Confusion in Classification

There has been quite a lot of confusion regarding the classification of sea spiders. There are around 1,350 species, which are divided into 8 families and 86 genera.
However, it is believed that these creatures should belong to the subphylum Chelicerata along with the horseshoe crab, or the class Arachnida along with spiders, mites, scorpions, and ticks.
There is another theory suggesting that they belong to a different class of their own. Despite all this confusion, as of now, they belong to the order Pantopoda.


Sea spiders possess long legs and a small body. They have 8 walking legs (four pairs). However, there are some species with five and six pairs of legs. They walk along the bottom of the water or swim using an umbrella-pulsing motion. They vary in size ranging from a few millimeters of leg span to the giant-sized spider with a leg span of about 70 centimeters.

Internal Organs

Due to their small and slender body and long legs, they have no respiratory system. An exchange of gases is made through a process of direct diffusion. The body is composed of two sections: an abdomen and a cephalothorax. The abdomen is very small in size. Their digestive system consists of diverticula that extends into the legs. They suck the nutrients from the soft-bodied invertebrates through their proboscis.
The pycnogonids have a small, thin, and long heart that beats at 90-180 beats per minute, creating substantial blood pressure. They have an open circulatory system and nervous system, including a brain. The brain is connected to the two ventral nerve cords, which, in turn, are connected to certain nerves. These creatures are very small in size, and their tiny muscles contain only a single cell surrounded by connective tissue.


The proboscis is a unique feature of the pycnogonids. The size and shape of the proboscis is variable from species to species. Its degree of movement is also variable. However, it has a restricted range of the dorsoventral (up and down) as well as lateral (sideways) movement. The internal structure of the proboscis and the entire alimentary canal appear trifoliate in its cross-section.

Outer Body

The body is supported by a non-calcareous exoskeleton. It is made up of a thick, hardened ectocuticle, a thin epicuticle, and a soft, more flexible endocuticle. The joints are movable as they consist of a flexible endocuticle. There is an eye turret, known as an ocular tubercle, on the dorsal side of the cephalothorax. It supports 4 simple eyes, which offer good all-round light detection. The cuticle also consists of hair and pits, which act as chemo-sensory and tactile sense organs.
The anterior end (head) of the body has two pairs of non-walking appendages and proboscis. These appendages are known as chelifores, and there are palps behind them. In some species, the palps and chelifores are reduced or absent. In such species, the proboscis is well-developed and more flexible and mobile. They are provided with a number of sensory bristles and strong rasping ridges around the mouth. The last segment contains the anus and tubercle that projects dorsally.


The ovigerous legs are a peculiar feature of the Pycnogonida. In many species, they are present only in the males, while in some genera such as Pycnogonum, they are present in the females too. They consist of 5 - 10 segments along with a claw at the distal end. In females, they are used for grooming and cleaning the carapace.
During breeding season, the males use them for courtship and then to carry the eggs until they hatch. The remaining appendages are the walking legs, also known as the ambulatory legs. They contain a femur, 3 coxae, a tarsus, a propodus, two tibias, and a claw. The legs consist of the muscles and nerves, which allow them to move, as well as the extensions of the gut and reproductive organs.

Reproduction and Development

These creatures are gonochoristic, i.e. they have two distinct genders. However, there is an exception of one species known as schorhynchus corderoi, which is a hermaphrodite. Each spider is both male and female. The reproductive system of sea spiders consists of organs such as a pair of testis or a pair of ovaries lying above the digestive tract.
The reproductive organs extend into the leg cavities as far as the femur. In most species, the eggs ripen in the legs, and genital openings are on the legs, usually on the second coxae. Reproduction is by means of external fertilization after a brief courtship. Males take care of the laid eggs and young ones.
The larva possesses a blind gut and its body consists of a head and three pairs of cephalic appendages like ovigers, palps, and chelifores. The abdomen and thorax with thoracic appendages develop afterwards.
There are four types of larvae, the typical protonymphon larva, the atypical protonymphon larva, the encysted larva, and the attaching larva. The typical protonymphon larva is the most common. It is free-living and gradually develops into an adult.


Most sea spiders are carnivores and feed on sponges, bryozoans, polychaetes, and cnidarians. They also eat invertebrates such as corals, clams, and marine worms. Some species consume red algae.
They feed by sucking the juice from the soft-bodied invertebrates such as sponges, sea anemones, and sea squirts, through their long proboscis. Some species simply tear the prey to small pieces to eat.

Distribution and Habitat

Sea spiders are more commonly found in shallow waters. However, they can be found at a depth of about 7,000 meters (approx. 23,000 feet) all over the world. They are found in the Caribbean and Mediterranean Sea as well as the Antarctic and Arctic Ocean. They are also found in countries like Australia and New Zealand, and also the north and south poles.
They are found in the marine environment as well as estuaries, which is the place where freshwater areas and the saltwater from the rivers or seas meet each other and mix together. They are very widespread and diverse.


Sea spiders can vary in length. There are varieties that are a few millimeters in length, and then there are species like Colossendeis colossea, which is the largest of all. Its leg span is about 70 centimeters. The species Nymphon pixellae has a diameter of about an inch, whereas the Yellow Hairy Sea Spider Tanystylkum anthomasti is even smaller, about 0.5 inches in length.


Sea spiders are currently not facing any imminent danger of extinction. They are present on the earth in sufficient numbers so as not to raise a concern about their survival.