Sea Spider

Many sea spiders are recognised by their enormous walking legs in contrast to a reduced body region, resulting into the so-called "all legs" or "no body" appearance. The body segments (somites) are generally interpreted as three main sections (tagma): cephalon (head, aka cephalosoma), trunk (aka thorax) and abdomen. However, the definition of cephalon and trunk might differ between literature (see text), and some studies might follow a prosoma (i.e. cephalon and trunk)–opisthosoma (i.e. abdomen) definition, aligning to the tagmosis of other chelicerates. The exoskeleton of the body is tube-like, lacking the dorsoventral division (tergite and sternite) seen in most other arthropods.

Sea spiders live in many different oceanic regions of the world, from Australia, New Zealand, and the Pacific coast of the United States, to the Mediterranean Sea and the Caribbean Sea, to the north and south poles. They are most common in shallow waters, but can be found as deep as 7,000 metres (23,000 ft), and live in both marine and estuarine habitats. Pycnogonids are well camouflaged beneath the rocks and among the algae that are found along shorelines.

All sea spiders have separate sexes, except the only known hermaphroditic species Ascorhynchus corderoi and some extremely rare gynandromorph cases. Among all extant families, Austrodecidae and Rhynchothoracidae are the only two that still lack any observations on their reproductive behaviour and life cycle, as well as Colossendeidae until the mid 2020s. Reproduction involves external fertilisation when male and female stack together (usually male on top), exuding sperm and eggs from the gonopores of their respective leg coxae. After fertilisation, males glue the egg cluster with cement glands and using their ovigers (the oviger-lacking Nulloviger using only the ventral body wall) to carry the laid eggs and young. Colossendeidae is the only known exception that the egg mass was placed on substrate and well-camouflaged.