Zebra-fishNatural Habitats, Animal Behaviour and Ecological importance. Debtanu Chakraborty
Introduction:Zebrafish (Danio rerio) is increasingly used as a model organism in researchworldwide. One can enlist ‘n’ number of biological disciplines in which this smallcreature finds acceptance. The initiators of using Zebrafish were Creaser(1934),Streisinger(1981) and Kimmel(1996). However, we find that its ecologicalparameters too play a great part in making it a preferred choice over other modelorganisms. We quickly look at its advantage as a model organism.In general:1) Cost effective:Its small, robust and cheap and so a large number of fish can be kept in thelaboratory. Food involves aquatic planktons that can be easily reared in Lab. It hasa small spawning time (2-3days) and releases hundreds of eggs at the spawningsite. Also interesting to note is that as there is very less Mate Choice, SexualDimorphism is very limited and thus the effect of sex-specific hormones likesTestosterone or Estrogen on normal body function is eliminated to a large extent.In Bio-medicine:2) Place in taxa:It is a vertebrate, so closer to humans than insects like Drosophila. Yet, epigeneticregulation is simpler and easier to determine than in higher organism like miceand rats. With its acceptance worldwide, its genetic sequence has been traced tooat the Sanger institute of Cambridge, U.K. Thus, it is increasingly being used tomodel human diseases and screen therapeutic drugs.In Developmental Biology and Genetics:3) External Fertilization and Rapid Development:Short Generation time (3-4 months), make it suitable for selection experiments.Zebrafish eggs are large relative to other fish, (0.7 mm in diameter at fertilization)and optically transparent, the yolk being sequestered into a separate cell as inmeroblastic cleavage. Furthermore, fertilization is external so live embryos areaccessible to manipulation and can be monitored through all developmentalstages under a dissecting microscope.
Development is rapid, with precursors to all major organs developing within 36hours with food seeking and active avoidance behaviour observed within five dayspost fertilization, i.e. 2-3 days after hatching (both the findings reported byKimmel et al., 1995).In Neuroscience and Cognition:4)Kimmel et al published details of Cell differentiation and Neural Development inZebrafish. Also, they roam in groups and like all social animals, exhibit matingrituals, aggression, dominance in hierarchy and related behviours which hint attheir developed learning skills and cognitive abilities. They have been studiedimmensely in the last two decades. Thus, they are a great model in NeuralSciences as well.Scope of this document:We discuss the following characteristics of the Zebrafish:1) Their schooling behavior especially: *No of fish per school *Natural Habitats and normal group size *Determination of Dominance and Hierarchy2) Their individual behavior especially: *Mating behaviour *Male Male interaction and Aggression *Female Female interaction *Male Female sex markers
Schooling (or Shoaling)Schooling is a tendency generally observed in predator and preys. Prey speciesencourage school formation to save themselves from predation. They travel inhordes and can give solitary predators a run for their life. However, on watchingclosely, we will see that their herd shape is rhomboidal. Note that in these figures,the ovals are individuals, the dominant ones at top and the arrows correspond todirection of movement within group.Some predator species too prefer to live in groups and attack and bring down abigger or stronger prey. However, the shape of the predator group is highlydifferent. This is because the prey wants to minimize its chances of encounterwith the predator while the predator wants the opposite.
Thus, area to perimeter ratio will be higher in prey and lower in predators.However, the shape of a Zebrafish school is entirely different.This is because although it is both a predator and prey. It sticks together to wardoff bigger fishes but is also competitive in getting access to nutrition sources.(Discussion needed).Shoaling behaviour is proved to be innate in zebrafish.(Engeszer et al, 2007b)(Kerr, 1963) showed that this behavior commences soon after hatching and littleones form shoals as soon as removed from isolation .McCann and Matthews(1974) showed that fishes grown in isolation could notdiscriminate between conspecifics and other species suggesting that recognitionis learned in the organism.Engeszer,Ryan and Parichy (2004) also showed that preferences for differentintraspecific phenotypes are also learned.(Rosenthal and Ryan ,2005) demonstrated that stripes are a key shoaling cue.Zebrafish have also been known to use olfactory cues in both species and kinrecognition (Gerlach & Lysiak, 2006).(Grant & Kramer, 1992; Gerlach, 2006; Spence & Smith, 2006).showed thatIndividual recognition may play a role in zebrafish since the species is known toestablish dominance hierarchies.
Group SizeThe number of fish per school is not at all fixed. In lab cultures, there is nopresence of a distinguished shoal if the fish density is high in aquarium. If it is low,There will be one primary shoal in the whole aquarium. In the nature, there canbe numerous shoals of numerous sizes depending on natural conditions. Theseconditions are determined by experiments.Shoal size and activity level are important parameters factoring shoaling decisionsin zebra-fish.(Pritchard et al,2001) showed an individual preferred to be with the larger shoaland during tough compromising environmental conditions like lowertemperature, the choice shifted to the most active shoal, regardless of the size.(Rhul and McRobert, 2005) also designed and carried out experiments tocorrelate sex with shoaling preferences. Males preferred to be in the group withthe most females but females preferred the group with most members regardlessof the sex ratio.(Krause et al.,1999) found that fishes could judge the nutrition levels within theirspecies and preferred to shoal with the well-fed ones. It was also shown that suchfishes subsequently enjoyed better foraging success.Gerlach et al. showed that the inter-individual distance between two fishes was afunction of their phenotypical relatedness; The more the related, the lesser thedistance. This also hints at the cognitive abilities of the zebra-fish..Dominance and HierarchyFemales and Males both deploy tactics to dominate over each other and form asocial hierarchy. Display of might involves chasing the competitor and often onserious cases, biting.Aggression is displayed by both the fishes aligning head to tail and opening of finswhile moving upwards in a helical motion. (R.Spence was the first to observe).(Larson, et al., 2006) were successful in showing that aggression is decreasedfollowing formation of a dominion order. They also showed that the Dominantfish in a pair is darker and utilizes the whole aquarium while the sub-ordinate ispale and restricted to a small area.
Once established, Dominance ranks remain fairly the same with time, notably inexperiments lasting 5 days. (Grant & Kramer, 1992; Spence and Smth,2005).G.Gerlach was instrumental in showing that the same rank was establishedbetween males after they were segregated for 4 days and then re-united.(Grant and Krammer ,1992) demonstrated that Sex is not a factor in determiningthe hierarchy in the group.It has been demonstrated that group formation is a function of food-area densityand food/individual density.Mating BehaviourThe mating behaviour of Zebrafish is very distinct and well-established.(Brederand Rosen,1966) showed that photo-periodism or the amount of light anddarkness influences egg laying in zebrafish. Males court females by chasing themoften nudging her flanks with his snout and attempting to lead her to a spawningsite (where egg-laying is preferred). The male fish displays courtship behaviour byswimming around or in front of the female in proper circles or double circles.Dis-interest on the part of the female is answered by the male shuttling betweenthe site of egg-laying and the female.On passing the spawning site, the males align genital pore with the female andoscillate with drastic frequencies to induce ovi-position. 5-10 eggs are laid atevery such site. The exercise can last upto an hour although the intensity isdecreased after the first 30 minutes. (reported by Darrow & Harris, 2004).(Spence et al, 2007b) reported similar behaviour In Wild varieties although thelatter are known to use the entire column (Surface to Water Bed; height).Also, 3to 7 fishes are involved in the mating ritual.Zebrafish follows the same mating behaviour pattern with the other fishes of theCyprinid order. They spawn in unison and scatter their egg preferably on surfaceswhere the eggs will have lower risk of predation.(Spence and Smith, 2005) werethe first to report this behaviour.In large groups, males chase females. However, in low density, males confine theirmovement to a territory central to a spawning site and a few body-length oftheirs in diameter. They chase away other males when they approach.Males and Females upon introduction to the same sex establish hierarchy asdescribed before.
Sex markersSexually mature females are usually rounder-bellied, slightly less colourful and alittle larger than males. The males are torpedo shaped and shorter. They havegold stripes between the blue stripes; females have silver stripes instead of goldThe differences are especially clear when the fish are in spawning condition as themales intensify in colour and the females fill with eggs.(www.fishbase.org,www.wikipedia.org )More Information and References:1)www.sanger.ac.uk2) zfin.org3) CREASER, C. W. (1934). The technique of handling the zebrafish (Brachydanio rerio)for the production of eggs which are favourable for embryological research andare available at any specified time throughout the year. Copeia 1934, 159-161.4) STREISINGER, G., WALKER, C., DOWER, N., KNAUBER, D. & SINGER, F. (1981).Production of clones of homozygous diploid zebra fish (Brachydanio rerio).Nature 291, 293-296.5) KIMMEL, C. B., BALLARD, W. W., KIMMEL, S. R., ULLMANN, B & SCHILLING, T. F.(1995). Stages of embryonic development of the zebrafish. DevelopmentalDynamics 203, 253-310.6) ROWENA SPENCE1*, GABRIELE GERLACH2, CHRISTIAN LAWRENCE3 AND CARL SMITH Thebehaviour and ecology of the zebrafish, Danio rerio.7) ENGESZER, R. E., ALBERICI DA BARBIANO, L., RYAN, M. J. & PARICHY, D. M. (2007b).Timing and plasticity of shoaling behaviour in the zebrafish, Danio rerio. AnimalBehaviour (in press).
8) KERR, J. P. (1963). Grouping behaviour of the zebrafish as influenced by socialisolation. American Zoologist 2, 532-533.9) MCCANN, L. I. & MATTHEWS, J. J. (1974). The effects of lifelong isolation on speciesidentification in zebra fish (Brachydanio rerio). Developmental Psychobiology 7,159-163.10) MCCANN, L. I. & CARLSON, C. C. (1982). Effect of cross-rearing on speciesidentification in zebra fish and pearl danios. Developmental Psychobiology 15, 71-74.11) ENGESZER, R. E., ALBERICI DA BARBIANO, L., RYAN, M. J. & PARICHY, D. M. (2007b).Timing and plasticity of shoaling behaviour in the zebrafish, Danio rerio. AnimalBehaviour (in press).12) ROSENTHAL, G. G. & RYAN, M. J. (2005). Assortative preferences for stripes indanios. Animal Behaviour 70, 1063-1066.13) GERLACH G.& LYSIAK, N. (2006). Kin recognition and inbreeding avoidance inzebrafish, Danio rerio, is based on phenotype matching. Animal Behaviour 71,1371-1377.14) GRANT, J. W. A. & KRAMER, D. L. (1992). Temporal clumping of food arrival reducesits monopolization and defense by zebrafish, Brachydanio rerio. AnimalBehaviour 44, 101-110.15) GERLACH, G. (2006). Pheromonal regulation of reproductive success in femalezebrafish: female suppression and male enhancement. Animal Behaviour 72,1119-1124.16) PRITCHARD, V. L., LAWRENCE, J., BUTLIN, R. K. & KRAUSE, J. (2001). Shoal choice inzebrafish, Danio rerio: the influence of shoal size and activity. Animal Behaviour62, 1085-1088.17) RHUL, N. & MCROBERT, S. P. (2005). The effect of sex and shoal size on shoalingbehaviour in Danio rerio. Journal of Fish Biology 67, 1318-1326.18) KRAUSE, J., HARTMANN, N. & PRITCHARD, V. L. (1999). The influence of nutritionalstate on shoal choice in zebrafish, Danio rerio. Animal Behaviour 57, 771-775.19) LARSON, E. T., O’MALLEY, D. M. & MELLONI JR, R. H. (2006). Aggression andvasotocin are associated with dominant-subordinate relationships in zebrafish.Behavioural Brain Research 167, 94-102.20) GRANT, J. W. A. & KRAMER, D. L. (1992). Temporal clumping of food arrival reducesits monopolization and defense by zebrafish, Brachydanio rerio. Animal
Behaviour 44, 101-110.21) BREDER, C. M. & ROSEN, D. E. (1966). Modes of reproduction in fishes. New York,Natural History Press.22) DARROW, K. O. & HARRIS, W. A. (2004). Characterisation and development ofcourtship in zebrafish, Danio rerio. Zebrafish 1, 40-45.23) SPENCE, R. & SMITH, C. (2005). Male territoriality mediates density and sex ratioeffects on oviposition in the zebrafish (Danio rerio). Animal Behaviour 69, 1317-1323.