Sea spider is a tiny torso on ample appendages
Southern Ocean giant sea spiders are all about their legs. They’ve got anywhere from eight to twelve of them, reaching up to ten inches across that they use to navigate the ocean floor (so no, they’re not actually spiders or even arachnids). Their legs actually dominate their body plan to such a degree that they appear to have no body at all where their legs come together. This isn’t just to look like a freaky experiment in exoskeleton design, but it’s also key to the animal’s physiology.
Such a tiny body means that, somewhat like a sea star, the sea spider’s vital organs are actually distributed throughout its legs, rather than at the nexus of its limbs like you’d expect. This creates a very high surface-area to volume ratio in the arthropod’s body, which means it can skip breathing through lungs or gills. Instead, oxygen from the water can diffuse through the legs, and therefore through critical organs, without any other physiological mechanism driving it.
This leaves the body to basically be a leg and proboscis mount. The proboscis is used to vacuum up jellyfish, worms and other slow or immobile soft-tissued animals. Sea anemones are sometimes snacked on, but more as a battery than actual prey as the anemone is usually left more or less intact after the encounter.
Living as far as three miles underwater helps the Southern Ocean giant sea spider retain some mystery, but we are chipping away at the story of its origins. Recent DNA studies have found that their family tree has diversified to as many as 20 different species millions of years ago, but have now rejoined to as few as five hybrid species. This is important to conservation efforts, where genetic diversity is a measure of the success of a species outside of raw population counts.
Source: Zoologger: The giant sea spider that sucks life out of its prey by Colin Barras, Zoologger