Sea spiders are captivating marine creatures that inhabit oceans worldwide. A common question is whether these animals, with “spider” in their name, spin webs like their terrestrial counterparts.
The Truth About Sea Spiders and Webs
Sea spiders do not spin webs. Unlike land spiders that construct intricate silk traps for prey, these marine animals have no silk-producing glands or the behavioral mechanisms to create webs. While they share a superficial resemblance to terrestrial spiders, sea spiders are not true spiders (arachnids).
Sea spiders belong to their own distinct class of marine arthropods called Pycnogonida. True spiders are classified under the class Arachnida. Both groups are part of the larger phylum Arthropoda and share a distant common ancestor, having evolved along a separate lineage for hundreds of millions of years. There are over 1,300 known species of sea spiders, found in diverse marine environments from shallow tide pools to the deepest abyssal plains.
Life Without a Web: How Sea Spiders Thrive
Without webs, sea spiders employ different strategies to survive and obtain food in their marine habitats. They typically move by walking along the seafloor on their long, multi-jointed legs, which can be several times the length of their small bodies. Some species are also capable of swimming using a pulsing motion or treading water. Their spindly legs allow them to navigate various bottom terrains without getting stuck in soft sediments.
For feeding, sea spiders use a specialized, straw-like mouthpart called a proboscis, which they extend into soft-bodied invertebrates. They then suck out the internal fluids of prey such as anemones, sponges, hydroids, and soft corals. In some cases, sea spiders may act more like parasites, weakening their prey without killing it outright.
Physiological Adaptations
Sea spiders have unique physiological adaptations for respiration and circulation, as they lack dedicated gills or lungs. Oxygen is absorbed directly through their thin, porous exoskeleton, particularly across the large surface area of their legs. Their internal organs, including parts of their digestive and reproductive systems, extend into these legs. While they possess a heart, it is often weak and primarily circulates fluid within the central body. Much of the circulation, especially in the extremities, is instead driven by the rhythmic contractions of their gut.