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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 Âµmol/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 Âµmol/m2/sec) 3000K fluorescent illumination from 6:00 A.M. to 6:00 P.M., high-intensity (~150 Âµmol/m2/sec) 6500K fluorescent illumination from 7:00 A.M. to 5:00 P.M., and very high-intensity (~300 Âµmol/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 Âµmol/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).
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, StÃ©phan G. "Sleep in Fishes." UniversitÃ© 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>.
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Levinton, Jeffrey S. Marine Biology: Function, Biodiversity, Ecology. 2nd Ed. New York, NY: Oxford University Press, Inc., 2001