Marine fish can be classified into groups, depending upon when they are most active. "Diurnal" fish are those that are primarily active during the day; those that are most active during the night are called "nocturnal." Some fish are especially active at dusk and dawn, and are referred to as "crepuscular." Various marine invertebrate species can also be classified into one of these three groups.
In aquarium settings, some nocturnal species, such as the Cardinalfish, can adapt to becoming more active during the day. Others, such as the Longspine Urchin remain primarily nocturnal.
| Nocturnal | Crepuscular | Diurnal |
|---|---|---|
| Cardinalfish, Big Eyes, Soldierfish, Squirrelfish, Pinecone fish, Scorpionfish, Snappers, Eels, Sea bass and groupers, Flashlight fish, Many invertebrates including lobsters, shrimp, crabs, and other crustaceans, mollusks, polychaete worms, and starfish, Polyps of many corals, such as Gorgonian, Torch coral, and Tubastrea | Barracuda, Jacks,Lizardfish, Goatfish | Angelfish, Anthias, Butterflyfish, Wrasse, Damselfish, Gobies, Hawkfish, Puffers, Triggerfish, Parrotfish, Surgeonfish |
Characteristics of nocturnal fish
Depending on when they are most active, fish may have developed certain physical and behavioral characteristics. For instance, nocturnal fish:
In general, are not designed to swim as fast as diurnal fish.
Tend to have larger eyes.
Tend to be more solitary, rather than living in large schools.
Are often more shy, and during the day are found hiding in caves or under overhangs.
Are often red or yellowish brown in coloration since these colors are absorbed by the water and are the first colors of the spectrum to become indistinguishable as light levels drop.
Tend to be carnivores versus diurnal fish who are often herbivores or omnivores.
May have a well-developed lateral line, like the Dwarf Scorpionfish, which allows them to sense water movement as an aid in finding prey in the dark or low light.
Setting up nocturnal aquariums
To establish a nocturnal aquarium you will need to create an environment with many caves and hiding places. Try to arrange the live rock and low-light corals to make hiding places that will still allow you to observe the nocturnal life under low light.
Even though it is a nocturnal aquarium, you will still need to provide light, especially if you will be including corals. In most cases, a reef aquarium with the intensity of lighting needed to maintain both live corals, and nocturnal fish will need to be very tall and wide. This will allow it to be aquascaped in a manner to make large enough caves and hiding places for the fish to get away from the light and feel at ease.
For nighttime viewing, a variety of lighting systems are available to allow you to see the inhabitants, yet provide the type of light that will bring out the nocturnal life. These include special lights often termed "lunar lights" or "moonlight lights." These lights produce light that is bluish in color and/or simulates moonlight. They are long lasting, draw little energy, and generate virtually no heat. Various styles can be mounted on the glass or in the canopy. Understanding the response of various organisms to changing light conditions is the subject of ongoing study. Many aquarium experts claim success simply with twelve hours of daylight and twelve hours of simulated moon light. You can easily automate this cycle using a timer. Because some corals and organisms appear to need a period of total darkness before moonlight appears, you might delay turning on the moonlight for an hour or more after turning off daytime lighting.
When choosing the inhabitants, consider their ease of care and temperament, since many may require extensive experience or be quite aggressive.