Author: Jan Scholten
English: Sea spiders; Pantopoda; Pycnogonids.
Genera: > 1300 species
Region: cosmopolitan, Mediterranean Sea Caribbean Sea, Arctic and Antarctic Oceans.
Pycnogonida are marine arthropods. Their size varies from 1 millimetres to 90 cm, quite large in deep Antarctic waters. They have eight, long legs in contrast to a small body size.
Pycnogonids do not require a traditional respiratory system. Instead, gasses are absorbed by the legs and transferred through the body by diffusion.
A proboscis allows them to suck nutrients from soft-bodied invertebrates, and their digestive tract has diverticula extending into the legs. The anterior proboscis has fairly limited dorsoventral and lateral movement, and three to four appendages including the ovigers, which are used in caring for young and cleaning as well as courtship. In some species, the chelifores, palps and ovigers can be reduced or missing in adults. In those species that lack chelifores and palps, the proboscis is well developed and more mobile and flexible, often equipped with numerous sensory bristles and strong rasping ridges around the mouth. The last segment includes the anus and tubercle, which projects dorsally.
In total, pycnogonids have four to six pairs of legs for walking as well as other appendages which often resemble legs. A cephalothorax and much smaller abdomen make up the extremely reduced body of the pycnogonid, which has up to two pairs of dorsally located simple eyes on its non-calcareous exoskeleton, though sometimes the eyes can be missing, especially among species living in the deep oceans. The abdomen does not have any appendages, and in most species it is reduced and almost vestigial. The organs of this chelicerate extend throughout many appendages because its body is too small to accommodate all of them alone.
The morphology of the sea spider creates an extremely well suited surface-area to volume ratio for any respiration to occur through direct diffusion. Oxygen is absorbed by the legs and is transported via the hemolymph to the rest of the body. The most recent research seems to indicate that waste leaves the body through the digestive tract or is lost during a moult. The small, long, thin pycnogonid heart beats vigorously at 90 to 180 beats per minute, creating substantial blood pressure. The beating of the sea spider heart drives circulation in the trunk and in the part of the legs closest to the trunk, but is not important for the circulation in the rest of the legs. Hemolymph circulation in the legs is mostly driven by the peristaltic movement in the part of the gut that extends into every leg. These creatures possess an open circulatory system as well as a nervous system consisting of a brain which is connected to two ventral nerve cords, which in turn connect to specific nerves.
Sea spiders are not true spiders, or arachnids; traditional classification as chelicerates; they may even be an ancient sister group to all other living arthropods.