Creature of the Month

Soldierfish
(Holocentridae)

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A member of the Holocentridae family, soldierfish are medium sized fish with big eyes. All species are primarily nocturnal. By day they hover in groups in or near caves and under coral formations. By night they swim above the bottom to feed on large zooplankton (invertebrates and young fish).

Like many nocturnal fish, soldierfish are mostly red. To us it is easy to spot during the day, but to other fish it blends into its dark crevice or cave. Long red light wavelengths don't penetrate water well, so fish colour vision tends to be tuned to the shorter, blue and ultra-violet, end of the spectrum. This means that red and pink fish are inconspicuous.

Their big eyes are another clue to their nocturnal habits. Large eyes, with a wider pupil and increased retinal surface can collect more ambient light for better seeing in the dark.

In fishes' eyes the rods, which detect brightness but not colour, are physically retractable. When light levels are high the rods are retracted into the back of the retina. When light levels fall and colour vision declines, the rods move upwards to provide low-light vision.

Nocturnal fish, who eat invertebrates, tend to eat larger food than their daytime counterparts. Possibly because they are easier to see in the dark.

You see soldierfish in the Indian, Pacific and Atlantic Oceans.

Pictures of soldierfish are at

http://www.scubatravel.co.uk/maldives_lobster.html

http://www.scubatravel.co.uk/maldives_soldierfish.html

Further Reading:

Coral Reef Fishes, Indo-Pacific and Caribbean <http://www.scubatravel.co.uk/fishbook.html>
 

Reprinted with permission:
Copyright SCUBA Travel - http://www.scubatravel.co.uk/