Zebrafish are commonly used as a model organism in research due to their low cost, small size, rapid development and reproduction. They naturally live in schools for protection against predators. School size can vary depending on environmental conditions, with fish preferring larger or more active schools. Both males and females establish dominance hierarchies through aggressive behaviors like chasing and biting. Males court females through behaviors like nudging and leading them to spawn sites, where the male aligns to induce egg-laying.

1. Zebra-fish
Natural Habitats, Animal Behaviour and
Ecological importance.
Debtanu Chakraborty

2. Introduction:
Zebrafish (Danio rerio) is increasingly used as a model organism in research
worldwide. One can enlist ‘n’ number of biological disciplines in which this small
creature finds acceptance. The initiators of using Zebrafish were Creaser(1934),
Streisinger(1981) and Kimmel(1996). However, we find that its ecological
parameters too play a great part in making it a preferred choice over other model
organisms. 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 the
laboratory. Food involves aquatic planktons that can be easily reared in Lab. It has
a small spawning time (2-3days) and releases hundreds of eggs at the spawning
site. Also interesting to note is that as there is very less Mate Choice, Sexual
Dimorphism is very limited and thus the effect of sex-specific hormones likes
Testosterone 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, epigenetic
regulation is simpler and easier to determine than in higher organism like mice
and rats. With its acceptance worldwide, its genetic sequence has been traced too
at the Sanger institute of Cambridge, U.K. Thus, it is increasingly being used to
model 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 in
meroblastic cleavage. Furthermore, fertilization is external so live embryos are
accessible to manipulation and can be monitored through all developmental
stages under a dissecting microscope.

3. Development is rapid, with precursors to all major organs developing within 36
hours with food seeking and active avoidance behaviour observed within five days
post fertilization, i.e. 2-3 days after hatching (both the findings reported by
Kimmel et al., 1995).
In Neuroscience and Cognition:
4)Kimmel et al published details of Cell differentiation and Neural Development in
Zebrafish. Also, they roam in groups and like all social animals, exhibit mating
rituals, aggression, dominance in hierarchy and related behviours which hint at
their developed learning skills and cognitive abilities. They have been studied
immensely in the last two decades. Thus, they are a great model in Neural
Sciences 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 Hierarchy
2) Their individual behavior especially:
*Mating behaviour
*Male Male interaction and Aggression
*Female Female interaction
*Male Female sex markers

4. Schooling (or Shoaling)
Schooling is a tendency generally observed in predator and preys. Prey species
encourage school formation to save themselves from predation. They travel in
hordes and can give solitary predators a run for their life. However, on watching
closely, 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 to
direction of movement within group.
Some predator species too prefer to live in groups and attack and bring down a
bigger or stronger prey. However, the shape of the predator group is highly
different. This is because the prey wants to minimize its chances of encounter
with the predator while the predator wants the opposite.

5. 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 ward
off 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 little
ones form shoals as soon as removed from isolation .
McCann and Matthews(1974) showed that fishes grown in isolation could not
discriminate between conspecifics and other species suggesting that recognition
is learned in the organism.
Engeszer,Ryan and Parichy (2004) also showed that preferences for different
intraspecific 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 kin
recognition (Gerlach & Lysiak, 2006).
(Grant & Kramer, 1992; Gerlach, 2006; Spence & Smith, 2006).showed that
Individual recognition may play a role in zebrafish since the species is known to
establish dominance hierarchies.

6. Group Size
The number of fish per school is not at all fixed. In lab cultures, there is no
presence 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 can
be numerous shoals of numerous sizes depending on natural conditions. These
conditions are determined by experiments.
Shoal size and activity level are important parameters factoring shoaling decisions
in zebra-fish.
(Pritchard et al,2001) showed an individual preferred to be with the larger shoal
and during tough compromising environmental conditions like lower
temperature, the choice shifted to the most active shoal, regardless of the size.
(Rhul and McRobert, 2005) also designed and carried out experiments to
correlate sex with shoaling preferences. Males preferred to be in the group with
the most females but females preferred the group with most members regardless
of the sex ratio.
(Krause et al.,1999) found that fishes could judge the nutrition levels within their
species and preferred to shoal with the well-fed ones. It was also shown that such
fishes subsequently enjoyed better foraging success.
Gerlach et al. showed that the inter-individual distance between two fishes was a
function of their phenotypical relatedness; The more the related, the lesser the
distance. This also hints at the cognitive abilities of the zebra-fish.
.
Dominance and Hierarchy
Females and Males both deploy tactics to dominate over each other and form a
social hierarchy. Display of might involves chasing the competitor and often on
serious cases, biting.
Aggression is displayed by both the fishes aligning head to tail and opening of fins
while moving upwards in a helical motion. (R.Spence was the first to observe).
(Larson, et al., 2006) were successful in showing that aggression is decreased
following formation of a dominion order. They also showed that the Dominant
fish in a pair is darker and utilizes the whole aquarium while the sub-ordinate is
pale and restricted to a small area.

7. Once established, Dominance ranks remain fairly the same with time, notably in
experiments lasting 5 days. (Grant & Kramer, 1992; Spence and Smth,2005).
G.Gerlach was instrumental in showing that the same rank was established
between males after they were segregated for 4 days and then re-united.
(Grant and Krammer ,1992) demonstrated that Sex is not a factor in determining
the hierarchy in the group.
It has been demonstrated that group formation is a function of food-area density
and food/individual density.
Mating Behaviour
The mating behaviour of Zebrafish is very distinct and well-established.(Breder
and Rosen,1966) showed that photo-periodism or the amount of light and
darkness influences egg laying in zebrafish. Males court females by chasing them
often nudging her flanks with his snout and attempting to lead her to a spawning
site (where egg-laying is preferred). The male fish displays courtship behaviour by
swimming 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 between
the site of egg-laying and the female.
On passing the spawning site, the males align genital pore with the female and
oscillate with drastic frequencies to induce ovi-position. 5-10 eggs are laid at
every such site. The exercise can last upto an hour although the intensity is
decreased after the first 30 minutes. (reported by Darrow & Harris, 2004).
(Spence et al, 2007b) reported similar behaviour In Wild varieties although the
latter are known to use the entire column (Surface to Water Bed; height).Also, 3
to 7 fishes are involved in the mating ritual.
Zebrafish follows the same mating behaviour pattern with the other fishes of the
Cyprinid order. They spawn in unison and scatter their egg preferably on surfaces
where the eggs will have lower risk of predation.(Spence and Smith, 2005) were
the first to report this behaviour.
In large groups, males chase females. However, in low density, males confine their
movement to a territory central to a spawning site and a few body-length of
theirs in diameter. They chase away other males when they approach.
Males and Females upon introduction to the same sex establish hierarchy as
described before.

8. Sex markers
Sexually mature females are usually rounder-bellied, slightly less colourful and a
little larger than males. The males are torpedo shaped and shorter. They have
gold stripes between the blue stripes; females have silver stripes instead of gold
The differences are especially clear when the fish are in spawning condition as the
males intensify in colour and the females fill with eggs.(www.fishbase.org,
www.wikipedia.org )
More Information and References:
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2) zfin.org
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