Ocean spiders, more accurately known as sea spiders or pycnogonids, are marine arthropods that are not true spiders. Despite their common name and superficial resemblance, they belong to the class Pycnogonida, distinct from the class Arachnida, which includes terrestrial spiders. This ancient lineage has existed in Earth’s oceans for hundreds of millions of years, with fossil records tracing back to the early Paleozoic era. Over 1,300 known species are found across diverse marine environments globally.
Distinctive Features and Physiology
Sea spiders possess a body plan distinct from terrestrial spiders and other arthropods. Their bodies are slender, with a small trunk and an elongated, tubular proboscis often exceeding their body length. This proboscis functions as their mouth, used for sucking fluids from soft-bodied prey. While many species have eight legs, similar to true spiders, some can have as many as ten or twelve legs extending from their small central body.
Unlike terrestrial spiders, sea spiders lack specialized respiratory organs. They absorb oxygen directly through their thin exoskeleton, or cuticle. Their circulatory system is unique; their heart, which is relatively weak, primarily circulates hemolymph (the arthropod equivalent of blood) to the central body, including the head and proboscis. The long legs, which contain extensions of their digestive system and reproductive organs, receive oxygen and nutrients through peristaltic contractions of their gut.
Global Habitats and Diversity
Sea spiders exhibit a global distribution, inhabiting all oceans from the frigid polar regions to temperate and tropical waters. They are found across a wide range of depths, from shallow intertidal zones to the abyssal plains of the deep sea, at depths exceeding 7,000 meters. This widespread presence highlights their adaptability to various marine ecosystems.
The class Pycnogonida exhibits considerable diversity in size and morphology. While many shallow-water species measure only a few millimeters in leg span, deep-sea and polar species can display “polar gigantism.” Some Antarctic species, such as those in the genus Colossendeis, can reach leg spans of up to 70 centimeters (2.3 feet). They camouflage well among rocks and algae in coastal areas and can be found associated with various marine organisms like coral reefs and kelp forests.
Feeding and Reproduction
Sea spiders are carnivorous, feeding on soft-bodied invertebrates. They use their proboscis to puncture prey and suck out fluids. Their diet includes organisms such as sea anemones, hydroids, sponges, bryozoans, and other marine worms. The digestive process is distinctive, with their gut extending into their slender legs, allowing for nutrient absorption.
Reproduction in sea spiders involves separate sexes, with external fertilization occurring after the male and female position themselves together. Females release unfertilized eggs through openings called gonopores, located at the base of their legs. The male then fertilizes and collects these eggs using specialized appendages called ovigers. The male carries the developing eggs and sometimes young larvae on his ovigers until they hatch. While most species involve the male carrying eggs, some deep-sea Antarctic species attach thousands of eggs to the rocky seafloor instead.