Life flourishes in the ocean’s shadows, and the fish that dwell in the caves, under ledges and behind rocks are no less wonderous than the bright and colourful species that roam the reefs by day. But to survive the daylight, and thrive at night, small nocturnal fish need some special adaptations...

Discover nocturnal wonders, and more, in our Skretting Diversity Gallery.

Eye spy

The first thing you might notice about our cave-dwelling fish is that they all have relatively large eyes – all the better to see you with my dears. These large eyes help them to see in the low-light conditions of their hiding places and, perhaps more importantly, they allow the fish to see their prey in the dark of the night.

For example, nocturnal squirrelfish have eyes more than three times the size of similarly sized diurnal fish.

The blotcheye soldier (Myripristis murdjan) uses its large eyes to spot tiny prey at night - which it can surprise with a quick burst of speed.

Red = invisibility

The colour red is predominant in many predatory and ambush fish species. This is because red is the first colour of light to be absorbed from the light spectrum by water, between depths of between 1 and 10 metres.

The image on the left is what these cave dwellers look like in "perfect" light, but the image on the right is what their world looks like in natural light.

The sun's light is white, which is a combination of all visible colours, but with the red wavelengths absorbed, this colour isn't reflected by red pigmented fish - so they appear dull and shadowy. 

Crowned squirrelfish (Sargocentron diadema) might look vividly coloured to us with good light, but at night their red colouration makes them nearly invisible in the water while they hunt zooplankton.

This is the perfect “invisibility cloak” – the reddish predatory fish cannot be seen by its prey in low light. Interestingly, water's ability to absorb long wavelengths of electromagnetism is also the reason you can't use Bluetooth underwater and why microwaves can warm up your food.

This scuba diver demonstrates how the red colouring of his clothing appears to "fade" the deeper he goes.

"Spidey sense"

Almost all fish have an organ called a "lateral line" - one running along each side of their bodies. These sense organs allow them to detect small changes in pressure, vibrations, electrical signals and movement in the water around them.

Nocturnal fish, like these smallscale cardinals (Apogon multitaeniatus) are especially reliant on their lateral lines - sensory organs that run along the side of a fish and help it detect tiny electrical signals in the water. 

Just like Spiderman, nocturnal fish rely on these senses, even more, to warn them about approaching predators and the presence of prey. Even with their large eyes, fish need every available advantage if they are going to be able to thrive in the dark and having the ability to feel the vibrations of a nearby heartbeat certainly help.

When it is almost impossible to see predators, being able to sense the slightest movement is even more important...

Nocturnal fish are not too bright

Being an awesome nightcrawler does have some disadvantages - notably that nocturnal fish have much smaller brains than their daytime counterparts. The reason for this is that although their vision is far more sensitive than the eyes of diurnal species, they don't have well-developed ability to see in colour and they have very poor depth perception, as in general things that are seen in the dark are all close by.

Experience a world of underwater wonders - these cave dwellers, and many other curious creatures, are yours to discover at the Two Oceans Aquarium.

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