A body reduced almost to nothing between eight long legs, walking over hydroids it feeds on.
Not a spider and not a crustacean. Pycnogonids are their own class of chelicerate arthropods, and the striking thing about them is how little body there is: the trunk is so reduced that most of the gut and much of the reproductive system runs out into the legs themselves. What you see is eight legs, a proboscis, and almost no animal in between.
They feed by pushing that proboscis into soft-bodied prey — hydroids mostly, also soft corals, anemones and bryozoans — and sucking the contents out. So they are found where the hydroids are, walking slowly over the branches, and searching a hydroid stand carefully is a far better tactic than scanning open ground.
The males carry the eggs. A separate pair of legs, the ovigers, is used to gather the egg mass after spawning and hold it in cemented balls beneath the body until the larvae hatch, and a male carrying eggs is the photograph worth waiting for.
This entry is at class level on purpose. The Indo-Pacific pycnogonid fauna of muck sites is poorly documented, many of the animals encountered are small and undescribed, and identification to genus generally needs the proboscis and the chelifores under magnification rather than a photograph taken at 1:1. Logging one as Pycnogonida is honest; putting a species name on it usually is not.
They are small, thin, and the same colour as what they are standing on. Most are found by accident while photographing something else on the same hydroid.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.