Sea spiders, with their long, spindly legs, are marine arthropods that often spark curiosity. Despite their common name and appearance, these creatures are not true spiders, nor are they related to crabs. They represent a unique and ancient lineage of marine arthropods, distinct from terrestrial spiders. This article details their classification, distinguishing features, and intriguing life habits.
Not Crabs But Pycnogonids
Sea spiders belong to their own distinct class of marine arthropods called Pycnogonida. This classification places them separately from true spiders (Class Arachnida) and crabs (Class Malacostraca). While all are arthropods, sea spiders represent an early diverging branch within this phylum. They are part of the subphylum Chelicerata, which also includes terrestrial spiders and horseshoe crabs.
The Pycnogonida class includes over 1,300 known species, found in oceans worldwide. Their unique taxonomic position means they have evolved independently for hundreds of millions of years, developing specialized features not seen in other arthropods. This distinct evolutionary path highlights why they are neither true spiders nor crabs, but rather a unique group with their own set of adaptations to marine environments.
Distinguishing Features of Sea Spiders
Sea spiders possess several unique anatomical features that set them apart from crabs and other arthropods. A striking characteristic is their unusually long legs relative to their body size, which can range from a few millimeters to over 70 centimeters in leg span. Their body plan is highly simplified, with many internal organs, including parts of the digestive tract and reproductive organs, extending into their slender legs. This adaptation is necessary because their central body is too small to accommodate all these systems.
Another defining feature is the proboscis, a long, tube-like mouthpart used for feeding. Unlike crabs, sea spiders lack a distinct abdomen and specialized gills, instead absorbing oxygen directly through their thin exoskeleton. Circulation within their legs is primarily driven by the peristaltic movement of the gut diverticula, which extend into each leg. Their head region, or cephalon, bears a dorsal ocular tubercle with up to two pairs of simple eyes, though some deep-sea species lack eyes.
Life and Habits of Sea Spiders
Sea spiders are found globally, inhabiting diverse marine environments from shallow intertidal zones to abyssal depths. They are benthic creatures, typically walking along the seafloor, but some species are also capable of swimming using a pulsing motion. Their diet primarily consists of soft-bodied invertebrates such as hydroids, anemones, sponges, and bryozoans. They feed by inserting their proboscis into prey and sucking out bodily fluids, a method sometimes described as suctorial predation. In some cases, they may act more as parasites, extracting fluids without immediately killing the host.
Reproduction in sea spiders often involves a unique form of parental care. In most species, the male takes on the role of carrying the fertilized eggs on specialized appendages called ovigers until they hatch. These ovigers are also used for grooming and cleaning by both sexes when not carrying eggs. While typical male parental care involves carrying eggs, some Antarctic species attach eggs to the rocky bottom for development.