The thick-arm or 'fat-arm' jellyfish are a family of over 20 jellyfish species. These jellyfish have domed bells reaching upward of 40 cm in diameter. Members of this family have eight short arms surrounding the mouth. The arms are three-sided.
These jellyfish swim in jerks by contracting their circular and radial muscles,. They have no control over the direction of locomotion and drift with the currents.