Morley Read / Slow motion clip of a Forest whiptail lizard (Kentropyx pelviceps) blinking its eye, showing nictitating membrane crossing the eye, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Richard Robinson / A Blue shark (Prionace glauca) just below the surface, with nictitating membrane partially shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

Bence Mate / Dalmatian pelicans (Pelecanus crispus) underwater view of group feeding, Lake Kirkini, Greece. Highly commended in the GDT competition 2013. Commended in the Wildlife Photographer of the Year Awards c...

Jack Dykinga / Raven (Corvus corax) with nictitating membrane over eye, in a blizzard on Hurricane Ridge, Olympic National Park, Washington, USA. Sequence 2/2

Jordi Chias / Whale shark (Rhincodon typus), closing nictitating membrane, South Ari Atoll, Maldives Islands, Indian Ocean.

Nick Upton / Rook (Corvus frugilegus) calling while perched in a Willow tree and blinking to show its blue nictitating membrane, Gloucestershire, UK, February.

Dave Bevan / European common frog (Rana temporaria) in a pond, closing its nictating membranes to lubricate its eyes, Carmarthenshire, Wales, UK, February.

Juan Carlos Munoz / Bateleur eagle (Terathopius ecaudatus) portrait with nictitating membrane closed, Captive, occurs in Africa.

Richard Robinson / Blue shark (Prionace glauca) with nictitating membrane partially shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

Richard Robinson / Blue shark (Prionace glauca) with mouth open and nictitating membrane almost completely shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

Richard Robinson / A Blue shark (Prionace glauca) just below the surface, with nictitating membrane partially shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

David Tipling / Rook (Corvus frugilegus) calling, Norfolk, February.