Shoaling Behavior of Zebrafish

What is “Shoaling” & Why is it Important?

Shoaling refers to groups of fish swimming together for social reasons, such as foraging, avoiding predators, and finding mates (Snekser et al, 2010). It’s analogous to the way humans form close group of friends or “cliques” who tend to always hang out together. For example, let’s look back to high school where there were popular kids and nerds. Popular kids “shoaled” among the popular kids, while nerds “shoaled” among themselves. In other words, popular students had their own “popular tables”, while nerds sat on their own separate “nerds tables” during lunch period.

About 25% of fishes shoal throughout their entire lives, while about 50% of the fishes shoal for only a part of their lives. There exists great ontogenetic variation of the timing among different fishes. Analyzing the shoaling behavior of zebrafish is significant and useful, as it may serve as an appropriate model in which its results can be applied to other vertebrates, such as humans. In other words, this study may serve as an aid in investigating the underlying mechanisms of social behavior among vertebrates (Engeszer et al, 2007).

Shoaling Video:

Here’s an example of shoaling behavior seen in fish inside a fish tank (thanks to YouTube uploader ncturk2006).

Example of Shoaling in a Fish Tank

Experiments:

Zebrafish can be studied to observe and analyze the behavioral mechanisms that affect shoaling, as they form shoals and are able to distinguish between conspecifics (belonging to the same species) and other co-occurring minnows. A couple of experiments have been conducted in order to explore the various factors that affect shoaling behaviors.

Binary Preference

In order to observe the shoaling preference of zebrafish, a binary preference test was used. This test involved placing a randomly selected wild-type fish in a “test tank” that contained both shoals of wild-type fish and nacre fish, which lack melanophore stripes that are present in wild-types (Fig. 1). The subject fish was isolated from each stimulus shoal by a transparent barrier (Fig. 2). Fishes of several different developmental stages were tested to observe the onset or the timing of the subject’s associations with the shoals. Different stages of zebrafish include preflexion larvae, early flexion larvae, postflexion larvae, metalarvae, juveniles, and adults (Engeszer et al, 2007).

Figure1. Adult and embryonic phenotypes: Adult on the left (a), exhibits the wild-type pigment pattern and on the right (b), the nacre mutant pattern. The embryo on the left (c), exhibits the wild-type embryonic pigment phenotype and that on the right (d), exhibits the nacre mutant phenotype (Engeszer et al, 2007).

The onset of shoaling was determined by recording the proportion of time spent in preference areas to the total observation time (Fig. 3). Preflexion larvae did not exhibit any shoaling due to immobility, while early flexion larvae showed little shoaling activity. The shoaling times for postflexion, metalarvae, juveniles, and adults did not differ significantly from one another. At the same time, only the juveniles and adults showed strong preference for wild-type shoals. This indicates that shoaling behavior of zebrafish arises during larval development and continues onto adulthood (Engeszer et al, 2007).

Figure 2. Test Tank. The schematic above represents the test tank. The dashed lines mark the interior boundaries of the preference areas. The double lines show both the position of double panes of Plexiglas and the outer boundaries of the preference areas. Stimulus shoals are shown in each of the outermost compartments and the subject is in the central “no preference” area (Engeszer et al, 2007).

Figure 3. The proportion of time spent in the preference areas to the total observation time ((time left + time right)/600sec) is shown for each stage. The dashed line represents the null expectation of 66% of the time spent in the preference areas. The circles represent the means for each stage. The letters A and B designate groups that are significantly different. The increase in the time spent in the preference areas between early flexion stage and the post flexion stage suggests the onset of shoaling behavior (Engeszer et al, 2007).


Stability of Shoaling Preference

An assay of visual preference of juvenile wild-type fish was used to study the stability of shoaling preference of zebrafish. First, each subject was raised with 3 wild-type siblings and then tested for binary preference in the test tank. Then, their social environment was manipulated for 30 days, housing half with the shoals of wild-type and the other half with the shoals of nacre fish. Lastly, these subjects were retested after 30 days to see if their preference of shoaling changed.

The results indicated that the preference of shoaling among zebrafish did not change even after the long manipulation of social environment. Neither the wild-type subjects nor the nacre fish subjects portrayed any significant change in the proportion of time they spent with the wild-type shoal group (Engeszer et al, 2007). This shows that the visual shoaling preference is stable and may possibly be resistant to change in the future.

Discussion and Conclusion

There were strengths and weaknesses present in the references. Both papers exhibited great background information of zebrafish that further explained the results. For example, the paper by Engeszer et al provided information about the preflexion stage of zebrafish from past experiments, which explained when and how the zebrafish utilizes functional visual system in response to food and shadows. By observing the timing of the development of visual system and the full complement of photoreceptors, it was possible to see it coincide with the timing of transition from preflexion larva to postflexion larva (Engeszer et al, 2007). In other words, the experimental results related to the onset of shoaling preference were strongly supported through the use of past experimental background information. On the other hand, weakness was shown in the paper by Snekser et al. The authors stated that the zebrafish that were used in the experiments were captive-bred animals obtained from commercial suppliers. This suggested that the fishes may have been in captivity for several generations, which means that they may have not been subjected to the same selective pressures as the wild types. In result, the data gathered from the experiment may be limited and unreliable (Snekser et al, 2010).

From these experiments, it is evident that shoaling in zebrafish occurs early during larval development, but zebrafish do not display preference for shoaling until the juvenile stage of their lives. In addition, acquired shoaling preference was shown to be stable regardless of any change in the environment. One thing is absolutely clear: zebrafish are undoubtedly shoaling. Through the understanding of social behaviors that affect shoaling in zebrafish, further studies in vertebrate social behaviors may be expanded. Perhaps shoaling-like behaviors among humans may become an important research matter in the near future.

References

  1. Timing and Plasticity of Shoaling Behaviour in the Zebrafish, Danio rerio.
    Raymond E. Engeszer, Laura Alberici Da Barbiano, Michael J. Ryan, and David M. Parichy
    NIH Public Access. Anim. Behav. 2007 November; 74(5): 1269 – 1275.
    http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2211725/pdf/nihms34872.pdf

  2. Influence of Sex and Phenotype on Shoaling Decisions in Zebrafish.
    Jennifer L. Snekser, Nathan Ruhl, Kristoffer Bauer, and Scott P. McRobert.
    International Journal of Comparative Psychology, 2010, 23, 70-81.
    http://www.comparativepsychology.org/2010ijcpissue1/05.Snekser_etal_Final.pdf

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