Fish exist in two main forms, bony (teleosts) and cartilaginous (elasmobranchs). Sharks are cartilaginous and some can blink. They do this with a type of nictitating membrane (Figure), actually a fold in the lower eyelid which is drawn upwards and backwards across the eye by levator palpebrae nictitans, a muscle behind the eye which is innervated by the oculomotor nerve, the IIIrd cranial nerve (Gruber 1977) (Gruber 1975). The membrane is only protective, there being no requirement to moisten the cornea.
Nictitating membrane in a Lemon shark (Negaprion brevivirostris) from Gruber 1977.
Most bony fish do not blink; some have transparent ‘adipose eyelids’ which may cover most or all of the eye (Stewart 1962). These are immobile and there is uncertainty about their function.
One bony fish which can blink is the mudskipper, which spends much of its time with its eyes protruding out of water while living in shallow waters. It can also climb onto the adjacent mud flats and live for prolonged periods out of the water. In the videos below played back at 20% of the recording speed, blinks and winks (unilateral blinks) are seen in two Australian species.
Blink in a specimen of the Periophthalmadon genus in a tidal creek in the Botanical Gardens in Cairns. The globe sits partially embedded on the top of a black pigmented turret with no obvious separation into upper or lower (medial or lateral) lids. During the blink, the globes are completely retracted into the head of the fish leaving a slit. The lower lid moves more than the upper lid. Also seen is the imbibing of water which causes swelling in the region of the gills. The fish absorbs oxygen from this water before releasing it through its gills.
Wink in the same specimen seen from the front. The left globe retracts and the left lower lid rises, fully or partially.
Wink in a smaller species of mudskipper, also in Cairns. The pupil is elongated in the horizontal plane.
A wink followed by a blink in the same specimen.
A blink in a small mudskipper with green eyes in Darwin Harbour.
The mechanism by which the globe retracts in mudskippers does not appear to have been studied. In the giant guitarfish it is done by the inferior oblique extra-ocular muscle (Taketeru, 2016).
Gruber, S.H. and Schneiderman, N. 1975. "Classical conditioning of the nictitating membrane response of the lemon shark (Negaprion brevirostris)." Behaviour Research Methods and Instrumentation 7(5):430-434.
Gruber, S.H. 1977. "The Visual System of Sharks: Adaptations and Capability." Amer. Zool. 17:453-469.
Stewart, K.,W. June 1962. “Observations on the Morphology and Optical Properties of the Adipose Eyelid of Fishes”. Journal of the Fisheries Board of Canada, Volume 19, Number 6.
Takita, T., Larson H.K. and Ishimatsu A. 2011. "The natural history of mudskippers in northern Australia, with field identification characters." The Beagle. Records of the Museums and Art Galleries of the Norhern Territory 27:189-204.
Taketeru Tomita, Kiyomi Murakumo Kei Miyamoto Keiichi Sato Shin-ichiro Oka Haruka Kamisako Minoru Toda. 2016. “Eye retraction in the giant guitarfish, Rhynchobatus djiddensis (Elasmobranchii: Batoidea): a novel mechanism for eye protection in batoid fishes”. Zoology, Volume 119, Issue 1, February 2016, Pages 30-35.