Weird Wonder Wednesday: Sea Spiders

The Pycnogonida, commonly known as sea spiders, are not actually arachnids but a class of marine chelicerates, which crawl along the sea floor on long, spider-like legs.  These odd looking creatures, which bear strong resemblance to the 'face-hugger' reproductive life-cycle stage from the Aliens movies, consist of little more than legs and a digestive system.  Indeed, female pycnogonids even carry their eggs within their legs as shown by the following illustration:

Pycnogonids are mainly predatory carnivores, feeding on hydroids, soft corals, anemones, bryozoans, and sponges.  Many species apply their proboscis to the prey and use it to suck up tissue, while others cut off pieces of food with their chelicerae and pass them to the mouth at the tip of the proboscis.

During reproduction, the male hangs beneath the female and fertilizes the eggs as she emits them  from the gonopores in her legs.  The male gathers the fertilized eggs into his legs and cements as many as 1,000 eggs into an adhesive mass, which he broods until they hatch.

Sea spiders are only rarely found as fossils, but are known as far back as the Late Cambrian Period.  Sea spiders are near and dear to my heart; the specimen shown below is a fossil pycnogonid that was found during my field season in the summer of 2008 by palaeontologists from the Manitoba Museum, the Royal Ontario Museum, and the University of Saskatchewan from the Late Ordovician of Manitoba.  It is the oldest adult sea spider known from North America.