Sea Spiders belong to Class Pycnogonida (Phylum Arthropoda). One species is very common on hydrothermal vents at Axial Seamount.
Sea Spider (Ammothea verenae)
Sea Spiders are not Arachnids but belong to Class Pycnogonida, spider-like marine Arthropods. They have four pairs of walking legs, which make up most of their bodies. Sea Spiders are so thin that they don’t need a respiration system, and instead exchange gas through their bodies. Due to their size, their muscles are only a single cell, which is surrounded by connective tissue. They use a tubular proboscis to scavenge food or feed on microbes. Sea Spiders reproduce by forming mating clusters where males collect eggs from females and then fertilize them. The father than cares for the eggs and young.
Sea Spiders of the species Ammothea verenae are found on and around hydrothermal vents at Axial Seamount. These organisms are a mere 1 cm in length, from proboscis tip to abdomen tip.
References:
http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Arthropoda/Chelicerata/Pycnogonida/Ammothea_verenae.html
http://animal.memozee.com/view.php?tid=3&did=26581
http://www.pmel.noaa.gov/eoi/nemo/explorer/bio_gallery/biogallery-Info.00061.html