The Robinson Library >> Science >> Zoology >> Class Pycnogonida
About Sea Spiders

Despite their name and appearance, sea spiders are not directly related to true spiders (arachnids); they are, however, members of the same phylum, Arthropoda.

sea spider


The typical sea spider has a body that is barely large enough to "support" its legs. Most sea spiders have four pairs of walking legs, but a few have five and one has six. The walking legs of all sea spiders are nine-segmented. Most of the commonly encountered intertidal species have short, thick legs and are quite sedentary, while deep-water bottom-dwelling species tend to have longer, thinner legs and to be more active. Many sea spiders are also known to swim periodically.

typical deep-dwelling sea spider

Internally there are no gaseous exchange organs, or organs for the removal of metabolic wastes. Their generally small size, and the long thin legs give the animal sufficient body surface area-to-volume ratio to supply all its respiratory needs. Many metabolic wastes are lost through the digestive tract. Others are stored in vacuoles in the cuticle and lost during the moult.

Sea spiders range in size from less than a half inch to almost two feet across the legs. The relative size of the legs compared to the body ranges from about 1-to-1 to about 7-to-1.

Distribution and Habitat

Sea spiders are found in every ocean, as well as in the Caribbean and Mediterranean seas. They are most common in shallow, intertidal waters, but some species live as deep as 23,000 feet.

Feeding Behavior

Most sea spiders are carnivorous, preying on bryozoans, hydroids and sedentary polychaetes. Some species, however, feed on algae that grow on polyzoan colonies, and a few other species feed on the detritus that collects around the bases of these colonies. Scientists have learned that adult sea spiders often have taste preferences for a particular type of prey that develop depending on what they were fed on while they are juveniles.

Food (often just body fluids and tissue fragments from their prey) is obtained by sucking it through a tube-like structure called a proboscis. The food is partially externally digested in most cases, and is generally in the form of fine particulate matter by the time it reaches the guts. Mucous cells lining the midgut absorb food particles and then, when they are full, they become detach and wander around in the circulatory system. They may later reattach to the gut wall for a short period of time, but are eventually expelled as waste.


Little is known about the courtship and mating rituals of sea spiders, except that eggs are fertilized externally and then brooded by the male. Both sexes may mate with multiple partners, and males carrying as many as 14 separate egg masses (indicating 14 separate matings) have been observed. The male carries the eggs around with him either until they hatch, or, in a few cases, until they have moulted several times. In this latter case, the females produce eggs with a lot of yolk and the young live off this even after hatching. In the former, the young are free-swimming larvae which feed on the same food items as the adults.

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The Robinson Library >> Science >> Zoology >> Class Pycnogonida

This page was last updated on 07/05/2017.