Pycnogonida, commonly known as sea spiders, is a class of marine arthropods that belong to the phylum Arthropoda. These unique creatures are characterized by their long, slender legs, a small body, and a peculiar appearance. The name "pycnogonida" is derived from the Greek words "pycno," meaning dense or closely packed, and "gonia," meaning joint or angle, reflecting the closely spaced joints on their legs.
Sea spiders are predominantly found in cold marine environments throughout the world's oceans, from intertidal zones to abyssal depths. With a wide range of sizes, some species can measure only a few millimeters, while others can reach lengths of over 70 centimeters. Despite their name, sea spiders are not true spiders, as they lack fangs and venom glands, and have a different body structure.
These marine arthropods have a unique method of respiration called cuticular respiration, which involves gas exchange through their thin exoskeleton. Their elongated proboscis and straw-like mouthparts enable them to feed on soft-bodied invertebrates, such as hydroids, sea anemones, and sponges. Pycnogonida have a simple circulatory system with a dorsal heart and a pair of lateral vessels that pump blood throughout their body.
The reproductive process of sea spiders is diverse, with different species exhibiting various strategies. Some species have internal fertilization, while others undergo external fertilization, where males transfer sperm directly onto the female's body. Eggs are then carried by the female or attached to her legs until they hatch into juvenile sea spiders.
Pycnogonida plays an important role in marine ecosystems as they are both predators and scavengers, contributing to the
