Complex sentence: About the length of a person's arm, the species is unique, due to the fact that Irkuandju jellyfish are usually the size of a finger nail and that none of the specimens found so far have tentacles.
Split into simpler sentences: About the length of a person's arm, the species is unique because Irkuandju jellyfish are usually the size of a finger nail. Moreover, none of the specimens found so far have tentacles.