Do Fish Sleep at Night?

DO FISH SLEEPAT NIGHT? By Stephan Reebs,Ph,D. Queen'sUniversity, Kingston,Ontario, Canada The title questionwas onceasked to me at the tank at night, with a lower respiratory that an internal circadian clock is dictating a scientific meeting where I had presented rate and no detectable eye movement, and when the fish go to sleep. data on the nocturnal behavior of parental that it does not respond as readily as during Aquarists can easily make similar three-spined sticklebacks, Gasterosteus the day to stimuli such as electrical current observationson their own favorite species, aculeatus. The enquirer was a specialist in or the delivery of food (Shapiro and provided they are willing to stay up late. mammalian behavior. Although he had no Hepburn 1976).Juveniles, however, do not However, a note of caution is necessary problem with the concept of sleep in show such a behavior; it takesthem about 22 regarding the means of illumination. Some mammals - indeed, he had experienced it weeks tô develop these sleep patterns, speciesare known to react in a special way firsthand --- he'was curious about its unlessif adults are presentwith them in the to suddenbright light at night; they sink to existence in the mammals' distant tank (Shapiroet al., 1981). the bottom where they remain motionless ancestors. At the Academy of Sciencesof the USSR for severalminutes (see Davis 1962). This Of course, one need not be a mammalian in Leningrad, I.G. Karmanova and his team "light shock" may give the impressionof behavioristto wonder about sleèp in fish. have observed that the brown bullhead, sleep in species that could be normally Most aquarists have been similarly Ictalurus nebulosas,displays a typical sleep active at night. It is preferable to use very intrigued, maybe after coming late one night posture. The tail fin lies flat on the ground, dim illumination constantly throughout the and glancing at the aquarium in their half-lit other fins are stretchedout, and the body is night, or to switch on a veiled flashlight in living room. Armed with a flashlight and a inclined at an angle of l0-30o to the vertical. the middle of the night and rapidly make the lot ofcuriosity, we all have, at one point or The cardiac and respiratory rhythms are observation. another, peeked inside our tanks at night to much slower, and the fish is less sensitiveto I was able to obtain infra-red goggles determine what our fish were up to in the sound and tactile stimulation (Titkov 1976; from an Army Surplus store, and combined dark. Karmanovaet al. l98l). with a powerful rescueflashlight on which I What we saw (Plecostomus fanciers Other researcheshave shown that rest and attachedan infra-red filter (Kodak #878), I excluded)was certainly suggestiveof sleep. activity periods continue to alternate with used them to unobtrusively observe fish in Most fish in the aquarium trade are diurnal neu24h periodicity even when the lighting complete darkness. The use of infra-red (to enjoy them, we want them to be up and conditions are kept constant (see illumination ensured that the behavior I about at the same time we are). They are Schwassman1971). For example,Dr. witnessed was not influenced by the therefore ill-adapted to life in the dark, and Martin Kavaliers, now at the University of presenceof visible light, however dim. I accordingly they are fairly inactive at night, Western Ontario, has left killifish, concentrated on members of the family tending to rest motionless near the bottom, Fundulus heteroclitus, in complete Cichlidae. as these are common in the oet behindplants, or in cavitiesbetween rocks. darknessfor up to 20 days andrecorded their trade and known to be insensitiveto But is suchinactivity equivalentto sleepas activity with an ultrasonic system. He has infra-red light (Schwanzara 1967). What I we usually imagine it? found that the activity patterns remain found was that these fish are indeed Well, even the expertsdo not all agree. lt rhythmic, sleep and activity alternating at a quiescent at night . most of the time. depends on the criteria used to define periodicityof about26 h (Kavaliers1980). Adult oscars, Astronotus ocellatus, are "sleep." Most sleep researchers,being Thus the fish would start being active later sluggish and unresponsiveat night. focused on mammals, have relied upon andlaterevey dayby 2h. Such"freerun" in Typically they rest on the bottom with their typical patterns in the electrical activity of the absenceof time cues is similar to what eyesturned downwards. Severum cichiids, the brain to recognize sleep. Another has been observed in other animais, Cichlasoma severum, are also motionless criterion, used mostly in the field, or in including birds and mammals, and it shows although they can be near the bottom or up situations where electro-encephalograms are not available, is prolonged eye closure. Infra-red goggles and light used to But fish lack complex brain structuresand observe fish in complete darkness. eyelids and, therefore, the two above criteria cannot be used. Fortunately, other criteria exist that can be expanded throughout a vast array of animalclasses, including fish. Theseare ( I ) prolonged behavioral quiescence, (2) typical resting posture, (3) circadian organization, and (4) high arousal thresholds.Thus, most people would agree that an animal is, for all practical purposes, asleep when it remains almost completely immobile(criterion 1), in almostalways the same posture and same location (criterion 2), attbe sametime every day (criterion 3), and in a state of decreased sensitivity to disturbances(criterion 4). ,t..,g;: A few scientists have applied these f' t:à criteria to fish and taken the question to the l. laboratory. Dr. Colin M. Shapiro, now at the University of Edinburgh, has reported that the Mozambique mouthbrooderTilapia mossambicatends to rest on the bottom of 59 ls Thisscene Jusl A LittleToo Fqmiliqr? ,Ël lf you'retired of Theold routine.,, Thenit's obout time you try Tokethe WORK out of CLEAN qnd FILL til MondotoryAquorium flL" AQUARIUMMAINTENANCE SYSTEM Mointenonce Mqde with PRIDEin fhe U.S.A. A^a 4ZZ Northl5th Street Milwoukee,Wisconsin 53233 (414)273-1313 FAX:(414) 273-2213 P FI o D U C TS" in the water column. Rainbow cichlids, visually-orientedin its behavror. the parentalbehavior of other cichlids such Herotilapia multispinosa, and convict Better still, both night and day-spawners as C. severum, C. spirulum, and cichlids, C. nigrofasciatum, are often will keep on fanning their eggs at night H. multispinosa, and all showed intense immobile with their pelvic fins lowered all throughoutthe incubationperiod (about 3 fanningat night. The patternmay indeedbe the way down, but occasionallythey setoff days in my studies),and nocturnalfanning typical of the whole family. It makessense: on slow promenadeswith their pelvic fins levels tend to be much higher than during if fanning servesto bring new oxygenated brushingagainst the gravelly bottom. the day. Whereasfemales will spendabout water to the eggs, and the eggs keep on Things change when the fish are in 30Voof their time fanning during the day, consuming oxygen at night (a safe breedingconditions. My observationshere they will devoteabout 85Vo of their time to assumption),then the parents should havefocused on theconvict cichlid. If a pair this parental activity at night (Reebs and continue fanning at night. However, why is showing signs of being ready to spawn Colgan1991). The males usually do notfan, they shoulddo it at suchhigh levels is i still just prior to the lights going off at dusk, it either day or night. In day fanning, the unclear.Maybe it is a hard-wiredadaptation will carry on into the night. The fish will female is normally broadsideto the eggs, to lower oxygen availability causedby the keep on digging pits and shoving gravel 1-3 cm away from them, fanning mostly absence of oxygen-producing away;they will continueto nudgeand skim with the fins from one side; at night, she is photosynthesisby aquatic plants at night the spawningsubstrate in apparentcleaning muchcloser to her clutch, andher snoutand (this may happenin nature but not in the movements;they will even spawnl In my fins comein frequentcontact with the eggs. well-oxygenatedhome aquarium). Or experience, the broods of such This suggeststhat sheuses olfactory and/or maybethey simply do the oniy usefulthing night-spawnerstend to be more scatteredon tactilecues to locateher eggsin the dark. I in the absenceof other worries such as the spawning substratethan those of am currentlyconducting night experiments looking for food, threat, mate, etc. day-spawners.Yet they do not seem to with artificial odorlesseggs and with real Had I known all this a few years ago, I compromisemore unfertilized eggs. It is eggs in tea bags, and the results seem to would havebeen able to give a moreprecise worth emphasizingthat all of this takes indicatethat olfaction is the fish's primary answer to that inquisitive mammalian place in completedarkness (even the pilot meansof recognizingif and whereeggs are ethologist.As it were, I just madea vague lights of the heaterswere disabled),yet the presentin the nest. convict cichlid is normallv diurnal and I havemade preliminary observations on to page 186 PET SHOPCAREER KOt & LOTUS DISCUS Home-Study Puggrsm FANCY SOLID CAP HIGH BODY- GOLDFTSH 4p o-.Â--fr O, FOOD NO HORMONES SPECIAL FOR FAMA READERS ç,#*ks# {4) - 5 oz- containers for 919.95 t$3.00 S&H) $47.5O each All Aspæta ot P€t Shop Manag;nfft (MD res. add 5% tn). Reg. $6.50 each. PLEÂSE I .OO SEI/Dtl Fot PlH ioday! For Stock up nowfor sping. Great food with fish meaL 408-336-5655 DescùptiveColor Blochurc and Exciting Detailslo: shrimp meal, and spirulina. Color enhancers too! XÀTUFE!' 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