Piglet Squid
Helicocranchia pfefferi
The Piglet Squid is a very small and delicate, transparent squid. It is found at depths between 400 to 1000 m. It has an unique siphon that is used for jet propulsion and it resembles a pig muzzle. The young piglet squids tend to live close to the surface, and steadily migrates downward as they grow. This behavior is called ontogenetic migration.
Photo credit: http://photo.cctv.com/2019/07/23/PHOAKMEBh8xJRaHXEUIGx8kE190723.xml
https://www.ourbreathingplanet.com/banded-piglet-squid/
Glass Octopus
Vitreledonella richardi
The Glass Octopus spend its entire life in the midwater section of the ocean and found at depths between 200m to 2000m. Even though it has no protection from predators, it achieves perfect transparency. The only part of the Glass Octopus that is visible is its digestive gland. However, the digestive gland is placed vertically to minimize detection. In addition, I find this octopus to be super cute!
Photo credit: https://www.mynumer.com/forums/topic/499/invisible-animals/view/post_id/859