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Many of us who have out
of curiosity or urgent necessity probed the deep shadows of a reef
aquarium with a flashlight in the dead of night were astonished to find
a myriad of hitherto then unobserved creatures. Such experiences
underscore a general lack of awareness of the fundamental ecological
roles that nocturnal fauna play in reefs and reef aquaria. In
light of the fact that tropical coral reefs are for half of every day
cloaked in darkness, and that an entirely different (though equally
complex) assemblage of fauna is active at that time, it can reasonably
be said that most of us have but half of a reef aquarium (Moe
1992). Possible reasons for
this lack of attention are many. Ornamental reef fish harvests
are, for practical reasons, typically undertaken during daylight hours,
resulting in a relative scarcity of night-active species within the
trade. The typically curtailed treatment of nocturnal reef
ecology in aquarium science literature has thus far contributed little
to generate much interest in (much less commercial demand for) these
animals. Some aquarists might consider displaying nocturnal
livestock, but elect to pass up on the additional expense and effort
associated with its care; others simply have never really thought of
viewing their tanks at night, and so have never purposefully sought
night-active specimens; others are wary of introducing these
overwhelmingly predatory creatures to their decidedly
"community" aquarium systems. That notwithstanding,
those who endeavor to take a few necessary (though simple) steps to
properly care for and view nocturnal reef fishes will foster a
potentially more dynamic and interesting aquarium environment.
Ecological
influence of daily light cycles on the coral
reef Coral reefs occur along
the Earths equator (i.e., in the tropics), where seasonal fluctuation
of photoperiod is negligible; here, sunlight is more or less
recurrently divided into 12-hour on and 12-hour off phases.
Particularly within the first 10 meters of depth, however, daily
changes in sunlight intensity, spectrum, and polarization are
dramatic. The transition from daylight to darkness a period in
which these changes are most pronounced commences approximately 15
minutes prior to sunset, with a duration of approximately 45 minutes
(the darkness to daylight sequence is essentially identical, but
reversed) (Hobson 1972).
As visibility changes
throughout the course of this daily cycle, so does the balance of
advantage in many competitive and predatory interactions. Many
species have consequently evolved to concentrate their most perilous
activities (namely seeking food) during the hours in which
interspecific pressures are minimal; that is, the times at which these animals emerge
to forage is not (necessarily) when feeding rates are maximized, but
when the risk/food intake ratio is minimal (Hobson 1972, Sale
1991).
One rather important and
conspicuous example of this phenomenon can be found in the nighttime
vertical migration of zooplankton. Their dusk-to-dawn foray into
the photic zone is thought to be a means of feeding on phytoplankton
while the threat of becoming food themselves unto visually advantaged
predators is least. Shared by diverse zooplankton phyla, this
behavioral pattern exerts an enormous ecological impact on the reef
from the very base of its food chain (Levinton 2001).
Most reef fishes can be
described as diurnal
(primarily active during the day), nocturnal (primarily active at
night), or crepuscular (primarily active at
twilight). Light conditions exert an influence so great that one
group might almost completely supplant another within a mere 15 minutes
of sunset or sunrise. An evaluation of one tropical reef revealed
that approximately half of its fish species could be observed at
night approximately half of which could be observed only at night. Species evenness
was found to be greater among night-active fishes, suggesting a lower
incidence of species dominance (Wilson 2004). This intricate
temporal niche partitioning can allow for higher degrees of biological
diversity in virtually every natural (and indeed captive)
environment.
Typically, diurnal reef
fish assemblages include the families with which marine aquarists are
most familiar (e.g., angelfish, Butterflyfish, tangs, wrasses,
damselfish, Anthias). This group is predictably more visually
oriented. With its primary hours of operation scheduled in the
full light of the sun, it is easier not only to locate, but to become,
prey. Thus, many of its members have adopted a wide range of
adaptations to alter their appearance. These adaptations may
include cryptic form, texture, and pigmentation for camouflage, as well
as more ostentatious features (e.g., eye spots, disruptive patterning,
countershading) that confuse attackers, advertise toxicity, or promote
identification within protective shoals. If not built for optimal
speed and agility, members of this group are protected by (oftentimes
cumbersome) weapons and/or armor. They tend to exhibit more
highly developed social behavior, and include nearly all herbivorous
species. Diurnal shoaling species tend to congregate during the
day, but disperse to seek individual hiding places at dusk.
By contrast, nocturnal
fishes (e.g., Cardinalfish, bigeyes, soldierfish, squirrelfish, true
eels, blowfish, Pineconefish, flashlightfish, coral catfish, snappers,
drums, some grunts) are almost exclusively predatory. Their
coloration and patterning is usually subdued, tending toward
monochromatic oranges and reds (which, as we will see, are the least
detectable spectra in low-light conditions) (Sale
1991).
Their eyes may be
enlarged and specialized for night vision, or (in cases where sight has
become, at best, a negligible part of their survival stratagem) greatly
reduced. Some distinctly nocturnal fishes (such as certain
Cardinalfish species) will venture into the light of day, but
nevertheless are fully active only when light intensity is below ~73
µmol/m2/sec (Debelius 1989). Relying less on
direction from visual cues, they characteristically possess fine-tuned
non-visual sensory organs; smell, taste, hearing, lateral line, and
electroreception senses are all commonly well developed in this
group. Often equipped with exceptionally sensitive taste buds and
nares (nostril-like
openings), these fishes might utilize a search mechanism called
klinotaxis, whereby a zigzag
swimming pattern is progressively adjusted as a stimulus (e.g., food
taste/odor) strengthens or fades. Highly sensitive ears and
lateral lines facilitate blind navigation as well as the
hunting/capturing of prey. Further, some species have pit organs
in their heads that detect the weak electric fields of their prey
(Reebs 2001). Nocturnal shoaling species tend to disperse to hunt
alone at night, but congregate near a shared shelter at
dawn.
Crepuscular fishes
(e.g., goatfish, lizardfish, jacks, groupers, lionfish, barracuda, some
grunts), while occasionally categorized as nocturnal (perhaps owing to
the habit of some to venture out on brightly moonlit nights), are best
placed in a group of their own. Members of this group are most
active during early-morning and/or late-evening hours; those that
prefer morning conditions are denoted as matutinal, while those that prefer
evening conditions are denoted as vespertine. These fishes are
able to make the best use of available light while neither diurnal nor
nocturnal predators/competitors are fully advantaged. These
fishes are often well-equipped to see the sharpened silhouettes of prey
in the refracted rays of the setting/rising sun. As one might
expect, while specially adapted to twilight conditions, they tend to be
highly versatile and possess key characteristics of both their diurnal
and nocturnal counterparts (Sale 1991, Hobson 1972). The order and manner in
which each respective group arises and retires throughout each day is
quite systematic. Early in the evening, diurnal fish begin to
seek cover; this occurs in an increasing order of size (that is,
from smallest to largest) both within and between species. Even
among shoaling species, individuals are rather protective of their
privacy at this time and might vigorously defend a hiding space from
conspecifics. Territorial disputes reach a climax in the lower
water column as diurnal fish fill prime shelter, presumably for the
reason that nocturnal fishes have yet to fully emerge from hiding
themselves and hungry crepuscular piscivores are increasingly on the
prowl. Following a period of relative quiet in the upper water
column, nocturnal fish begin their nighttime migrations in an order of
decreasing size. It
would seem that these patterns of changing size orders result from the
predictably rising and falling threat of crepuscular piscivores.
The morning transition
is similar, albeit reversed. Here, a notable difference between
diurnal and nocturnal fishes becomes evident; as the latter exhibit a
higher tendency to shoal during their resting period, much less
aggressive behavior is involved in their search for shelter (Sale 1991,
Hobson 1972). This all begs the
question, 'do fish actually sleep?' The best answer is
that some do, sometimes. Presumably to conserve energy, many
(though not all) fishes will enter a restful state of quiescence in the
course of daily-recurring periods of relative inactivity. Some
fishes do not sleep as juveniles, during migration, during spawning
season, or whilst caring for offspring. The effects of quiescence
range from slight sluggishness to near unresponsiveness. While at
rest, a deeply quiescent fish might be particularly vulnerable to
attack reason enough for the high priority placed on securing a safe
resting space (Reebs 2007).
There is nothing that
necessarily prohibits nocturnal and crepuscular reef fish from being
maintained in a conventional reef aquarium (so long as tankmate
compatibility and stocking density issues are addressed, of
course). The primary consideration here is providing ample,
adequate shelter. This can be achieved mainly by creating as much
space between and under rocks, and within the substratum, as is
possible. To start, at least 3 inches of properly graded
substrate should be added to systems that include sifting foragers
(e.g., goatfish, coral catfish) or nighttime burrowers (e.g., wrasses,
Jawfish) (Spotte 1993). Try to avoid three great temptations when
aquascaping to place the largest pieces of rock on the bottom with
smaller pieces as filler on top, to cram rocks tightly together like
pieces of a puzzle, or to pile up a rock wall against the back of the
tank. Instead, situate smaller chunks (feet, if you will) widely
spaced on the substrate and place progressively larger pieces over them
as to create a course of caves, crevices, and overhangs. More
stable tunnels or chambers can be fabricated with PVC pipe and/or
perforated plastic boxes and hidden within the rockwork to great
practical effect. With some research (perhaps studying images of
particular species in their natural habitat), specialized types of
shelters can be constructed to suit the needs of specific fish
types. Once the rockwork is in place, and aquarium inhabitants
have laid claim to shelters, it should thereafter be left undisturbed;
reef fishes commonly use the same shelter throughout their life, and
their health can be adversely impacted by stress associated with their
hiding places being regularly dismantled (Spotte 1993). Where one is in the
position to plan and stock a reef system containing night-active
livestock from scratch, it is advisable to add all nocturnal specimens
(in an increasing order of size/territorial aggression) prior to adding
diurnal specimens (likewise in an increasing order of size/territorial
aggression); this can significantly facilitate the peaceable allocation
of shelter. Any tank housing
night-active fauna should be situated in an area that receives a
minimal amount of ambient light. One must take into account all
major factors in calculating light intensity (including type/strength
of bulbs, age of bulbs, angle/position of bulbs, effect of reflectors,
distance of bulbs from water surface, depth of water, clarity of water,
surface agitation, and even peculiarities of the aquascape).
Obtaining accurate intensity values can be appreciably simplified with
the use of a light meter.
Suddenly flooding the
aquarium with bright light can induce light shock, just as suddenly
immersing the aquarium in darkness can induce panic. A
conventional reef aquarium (or, almost any aquarium, for that matter)
is best equipped with a lighting system that gradually
increases/decreases levels of intensity (Reebs 2007, Spotte 1993); this
is most often accomplished with a number of bulbs of different output
and spectra. A wide variety of lighting techniques and
technologies has yielded demonstrably positive results; the subject of
reef aquarium lighting is yet a source of much debate and
experimentation among hobbyists.
The 12-hour day phase of such a lighting regime
might consist of low-intensity (~75 µmol/m2/sec) 3000K
fluorescent illumination from 6:00 A.M. to 6:00 P.M., high-intensity
(~150 µmol/m2/sec) 6500K fluorescent illumination from
7:00 A.M. to 5:00 P.M., and very high-intensity (~300
µmol/m2/sec) 10000K metal halide illumination from 9:00
A.M. to 3:00 P.M. Night lighting might not
require the use of any artificial illumination at all. Indeed,
many night-active fishes will tolerate (if not prefer) it that way;
even while some diurnal species can be spooked by total darkness and
will appreciate the nightlight, highly photophobic species (such as
flashlight fish) can experience significant stress when this lighting
is overzealously employed. All the same, successfully simulated
nighttime light conditions can elicit interesting, healthful behavior
(among many nocturnal, crepuscular, and diurnal creatures alike).
The 12-hour night phase of such a lighting regime
might consist of low-intensity (~15 µmol/m2/sec) actinic
blue LED lighting. Some aquarists might run them continuously
throughout the night, or from 7:00 P.M. to 5:00 A.M. to allow for
interim periods of darkness (which, as it is presumed by some, promotes
a stronger response to simulated nighttime light conditions). The
greatest concern here is replicating moonlight; while specific benefits
of their use have yet to be convincingly demonstrated (Hemdal 2006), a
wide variety of moon lights or lunar lights are available for this
purpose. Taking advantage of certain gadgetry, one might aim to
mimic the 29.5-day lunar cycle. This can be fairly easily
accomplished by way of electronic controllers with dimmers that
automatically alter bulb output according to the moons phases.
Many of the latest LED units are quite attractive in that they offer
upgrades to allow for the simulation of daytime, dusk/dawn, and
nighttime conditions with a single fixture. Attempting to observe
the activities of nocturnal aquarium fauna presents inherent challenges
due to the necessarily subdued nighttime lighting. Even
temporarily or partially illuminating the aquarium with inappropriate
lighting during the night phase can easily disturb both nocturnal and
diurnal inhabitants. Some inventive aquarists have addressed this
problem with the use of low-frequency (red and infrared)
illumination. Within the spectrum of
visible light, red light has the longest wavelength and lowest energy,
and so is the first to be attenuated (i.e., diminished by scattering
and absorption) in seawater. While blue light (420-490 nm) might
penetrate over 150 meters, red light (630-780 nm) is all but absent
beyond only 10 meters (Bernal 2010, Levinton 2001). Hence, red
objects appear as gray even in relatively shallow depths, and are
particularly difficult to perceive in dim light conditions (just as
reef fishes evolved little capacity to detect light of this wavelength,
many nocturnal and deepwater species have evolved to assume red
coloration for the purpose of concealment). All variations of this
technique are based on the premise that the aquarist but not the fish can
perceive red light. This purposefully unnatural lighting
condition is used solely to enhance the aquarist's view of the
aquarium, and so may be applied at will (that is, it needs not operate
on any particular photoperiod). A variety of
"nocturnal" bulbs are commercially available, though some
types of lighting can be modified for this purpose. Standard
fluorescent tubes fitted with red plastic sleeves, or even red
incandescent "party" bulbs, have been used with some success
(Moe 1992). Recently, some manufacturers have developed red LED
lighting; some of these fixtures are very compact and even submersible,
enabling the aquarist to strategically place them in permanently shaded
areas (such as caves) to observe nocturnal fish sheltering during the
day as well as diurnal fish sheltering at night. Certain 1000 K
bulbs (some of which are marketed for nocturnal terrarium use) appear
to be red, but emit much more infrared light which is absolutely
imperceptible to fish and
fishkeepers. While infrared and near-infrared lighting is ideal
in that it ensures minimal disturbance of the aquarium inhabitants'
nighttime activities, the aquarist must use special equipment to detect
it. Reebs (2007) suggests the use of infrared goggles (available
online and at some army surplus stores) and a high-powered flashlight
fitted with an infrared filter (e.g., Kodak # 87B).
Conclusion Night-active fishes can
be maintained in most reef aquaria provided that certain, simple
husbandry issues are properly addressed. With a similarly
heightened attention to the maintenance of nocturnal invertebrates, one
can radically transform (if not complete) a captive reef
environment. Aquarists thusly may extend the hours during which
an active display can be presented each day, construct more diverse,
interesting, and successfully functioning aquarium systems, and quite
possibly develop a greater appreciation of the remarkably complex
habitats they strive to replicate.
Joshi, Sanjay Ph.D. "LED Lighting Tests:
Aquaillumination, Blue Moon, Eco-Lamp KR-91, Ecoxotic Panarama."
Advanced Aquarist's Online Magazine. 15 May 2010. 15 May 2010
<http://www.advancedaquarist.com/2010/5/aafeature2 >. Reebs, Stéphan G. "Sleep in Fishes."
Université de
Moncton, Canada. 2007. 4
May 2010 <http://www.howfishbehave.ca/html/sleep.html
>. Bernal, Christina E. "Light Transmission in the
Ocean." Water Encyclopedia:
Science and Issues. 30 April 2010. 4 May 2010
<http://webcache.googleusercontent.com/search?q=cache:GHElTtzTAkgJ:www.waterencyclopedia.com/La-Mi/Light-Transmission-in-the-Ocean.html>.
Wilson, Elizabeth V. et al. "Diel Patterns of
Coral Reef Fish Composition and Abundance in Four Habitats."
Dartmouth Studies in Tropical Ecology (2004):
212-216. Hobson, E.S. "Activity of Hawaiian Reef Fishes
During the Evening and Morning Transitions Between Daylight and
Darkness." Fishery
Bulletin 79.3 (1972): 715-740. Debelius, Helmut. Fishes for the Invertebrate Aquarium.
Stuttgart, Germany: Eugene Ulmer GmbH & Co., 1989. Spotte, Stephen. Marine Aquarium Keeping. 2nd Ed. New
York, NY: John Wiley & Sons, Inc., 1993. Reebs, Stéphan. Fish Behavior in the Aquarium and in the
Wild. Ithaca, NY: Cornell University Press, 2001. Moe, Martin A. Jr. The Marine Aquarium Reference: Systems
and Invertebrates. Plantation, FL: Green Turtle Publications,
1992. Hemdal, Jay F. Advanced Marine Aquarium Techniques.
Neptune City, NJ: T.F.H. Publications, Inc., 2006. Sale, Peter F., ed. The Ecology of Fishes on Coral Reefs.
San Diego, CA: Academic Press, 1991. Levinton, Jeffrey S. Marine Biology: Function, Biodiversity, Ecology. 2nd Ed. New York, NY: Oxford University Press, Inc., 2001 |
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