An amphibious fish, Periophthalmus argentilineatus is a mudskipper that spends most of its time on the mudflats of the tropical mangrove swamps of Africa, Asia, and Polynesia. It uses its stiff fins like walking sticks to perambulate with surprising speed and agility in the intertidal zone, seeking insects and small aquatic organisms. These remarkable fish actually spend nearly all of their time out of water on an exposed mudflat. A member of the goby family, this species can reach up to 12 cm at maturity and, although epipelagic, occurs exclusively on mudflats and mangrove swamps.

Its eyes can be raised and lowered, much like a genuine periscope. This remarkable task is accomplished by the use of a hammock-like structure composed of the inferior oblique and inferior rectus muscles. These muscles are stiffened in concert to elevate the globes. Conversely, when these retractile eyes are drawn closer to the body for protection, they are covered by puckered folds of skin.

When out of water, the eyes of mudskippers are able to focus in air because of powerful and rapid accommodation using a round piscine lens. The iris has a functional sphincter, but insufficient, if any, dilator musculature. Consequently, pupillary …