Sea Spiders suck the body fluids from their prey and digest them in their legs.
Pycnogonids (marine arthropods distantly related to the spiders) are almost all leg. Their very small body serves mainly to keep their legs together and to house the brain, heart and reproductive system. There is no respiratory system – ‘all legs’ gives a very large surface area (relative to body mass) and is sufficient for respiration by diffusion.
Sea Spider Digestion
The gut of a Sea Spider extends into all of the legs (usually eight) almost filling them (see photo) so that the liquid sucked from the living body of their prey almost fills the Pycnogonid. The digested food passes through the (open) blood system to supply the energy for the very weak muscles, allowing the Sea Spider to creep slowly towards the next meal.
Sea Spider Respiration
Pycnogonids are commonly found in cold water. They are usually small (less than one centimetre across) and all are very slow-moving. Because of their strange body-plan (which gives them a very large surface), and their efficient heart and circulatory system, Pycnogonids do not need a respiratory system – oxygen diffuses directly into the blood and is then carried to the muscles.
Sea Spider Reproduction
Almost all of the 1,000 or so species of Pycnogonid have separate sexes. The larvae of most types live in the plankton for a while before settling down to find a sea anemone, clam or worm (depending on the habits of that particular species), but some are almost parasitic in their early stages of development. Yet other types of larval development exist, but they are poorly understood. When there is parental care it is always the father’s job.
Size Matters - Size/Temperature Relationship
There is a relationship between the size of an animal and its surface/volume ratio. Small (less than a centimetre across) Pycnogonids can be found in warmer waters (Mediterranean and Caribbean), larger ones are very common in Arctic and Antarctic waters, and huge ones (up to almost a metre across!) live in the deep abyss where it is extremely cold. This size/temperature relationship is due to their unusual respiratory system, as is their extreme sluggishness. Because they feed on slow-moving prey they only need to creep around slowly when hunting – or maybe it is the other way round – because they can only move slowly they are restricted to equally slow-moving prey.
Classification - Spider or Ancient Cambrian Arthropod?
There is much debate about exactly what Pycnogonids are. Some people consider them to be quite closely related to spiders and their kin, while others believe that they are the last living representatives of an ancient (Cambrian) group of creatures who were the common ancestors of all the modern arthropods.
Ref: (and images of several Pycnogonids) Underwater Field Guide - Antarctica
Other articles by John Blatchford
