To start with I would like to copy the opening paragraph of a paper by Thomas E. Bowman from 1976 to underline how special this animal is:
The family Stygiomysidae was proposed by Caroli (1937) for an unusual new genus and species of mysid. Stygiomysis hydruntina from caves on the Salentine Peninsula, the “heel” of the Italian “boot”. Caroli gave a brief description of the male of his new mysid, stating that a more detailed study would be published later. The fuller description never appeared, however, and S. hydruntina remains inadequately characterized. The prefix Styg- means that these animals live in caves completely submerge and absent of any light.
As of today, the total number of Species in that Genus that are described at seven and all are stygobitic (see more information on the page of Dr. Thomas Iliffe). At the QRSS page you find the additional information that there are a total of four Stygiomysis sp. that reside in the underwater caves of Quintana Roo. These animals do not swim within the water column – as do the rest of anchialine critters. Species of stygiomysids are “crawlers”, preferring rocks, walls, and the ceiling of their stygian habitat. Cave divers often dislodge these animals from a cave ceiling with their exhaust bubbles, and misidentify the animal as Speleonectes tulumensis (Remipedia) as it happened to me very recently. Look for their square “head”.This was clearly a sign to distinguish for me, however, I also have them documented to do a bit of a swimming.