[4] The specific epithet, achylos, is constructed from the Greek achlys or akhlús (ἀχλύς), meaning "mist, darkness, and obscurity", in reference to both the species' coloration and the rarity of its sightings.

The bell color is a distinctive opaque dark purple to nearly black, with the margin having a lighter brown reticulated pattern.

[7] These sightings seem to coincide with incidents of red tides, which consist of the zooplankton that black sea nettles feed upon.

Upon contact, the cnidocil will immediately initiate a process that ejects the venom-coated filament from its capsule and into the target.

In addition, the black sea nettle's stomach is lined with a fibrous network of vessels that attach themselves to a swallowed victim and darkmaws for quick digestion or breaking apart large prey, though the maws will close when exposed to bright lights, hence their name.