Crabs are not sea spiders. While both are marine arthropods, they belong to entirely different taxonomic groups within the animal kingdom. They share a distant common ancestor as arthropods, but their evolutionary paths diverged significantly millions of years ago, leading to distinct body plans, habitats, and behaviors.
What Are Crabs?
Crabs are decapod crustaceans, which also includes shrimp and lobsters. Their defining characteristic is a thick, hard exoskeleton that covers their body. Crabs typically have five pairs of legs, with the first pair often modified into powerful pincers called chelae, which they use for defense, capturing prey, and manipulating food.
The body of a crab is characterized by a short, tail-like abdomen that is usually tucked entirely under the thorax, giving them their compact, rounded appearance. Crabs inhabit a wide range of environments, from all of the world’s oceans to freshwater and even land, often seeking shelter in crevices or burrowing into sediment. They are omnivores, feeding on various foods including algae, detritus, and other invertebrates. Crabs breathe using gills located under their carapace, which must remain moist to function.
What Are Sea Spiders?
Sea spiders, also known as pycnogonids, are marine arthropods. Despite their common name and resemblance to terrestrial spiders, they are not true spiders, nor are they arachnids. There are over 1,300 known species of sea spiders, found in oceans worldwide, from shallow waters to abyssal depths, with some of the largest species inhabiting polar regions.
The anatomy of a sea spider is distinct, characterized by a very small body, with their organs extending into their long, multi-jointed legs. They typically have eight legs, though some species can have ten or twelve. A prominent feature is their proboscis, a long, tube-like mouthpart. Sea spiders use this proboscis to suck fluids from soft-bodied prey like anemones, sponges, and corals. Unlike most marine animals, sea spiders do not possess gills or lungs; instead, they absorb oxygen directly through their body surface and the extensive surface area of their legs.
How Crabs and Sea Spiders Differ
Crabs are classified as crustaceans, placing them in the infraorder Brachyura, a diverse group of decapods. In contrast, sea spiders belong to their own distinct class, Pycnogonida, within the phylum Arthropoda, making them evolutionarily separate from crustaceans and true spiders. This taxonomic divergence reflects millions of years of independent evolution.
Crabs possess a robust, calcified exoskeleton that encases a body distinctly divided into a cephalothorax and a tucked-under abdomen. They typically have ten legs, including two large claws for defense and feeding. Sea spiders, however, have an extremely reduced central body, with many of their internal organs extending into their usually eight, slender, elongated legs. Their bodies are often so small that they appear to be little more than legs attached to a central hub.
Differences in feeding and respiration mechanisms further highlight their distinct adaptations. Crabs use their powerful chelae and specialized mouthparts to grasp and process a wide variety of food, ranging from detritus to other invertebrates. They breathe using gills, which extract oxygen from water and can be kept moist to allow for periods out of water. Sea spiders, conversely, feed by inserting a straw-like proboscis into their prey to suck out bodily fluids. They lack specialized respiratory organs, absorbing oxygen directly through their thin exoskeletons and the surface of their legs, a process facilitated by their small body size and large leg surface area.