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
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
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
You see soldierfish in the Indian, Pacific and Atlantic Oceans.
Pictures of soldierfish are at
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/