Friday, May 26, 2017

How Do Fish Find Their Shoal Mates? by Lindsey Wentzel

A variety of studies concerning Japanese lates (Lates japonicas), damselfish (Chromis viridis), fighting fish (Betta splendens), zebrafish (Danio rerio), Brook stickleback (Culaea inconstans) and the Fathead minnow (Pimephales promelas), and guppies (Poecilia reticulata) have sought to discover why. It has been hypothesized that teleost fish form shoals, a social aggregation “held together by social attraction”, by innately grouping with other fish of analogous phenotypic morphologies to themselves with respect to group size and species characteristics (Peichel 2004). Additionally, it is thought that Shoaling is determined by a learned preference for homogenous coloration and other characteristics and may be useful in defense against predation, increased access to mates and reproductive success, hydrodynamic efficiency, group vigilance, predation dilution and kin dynamics, predation avoidance, and foraging success (Peichel 2004). Through shoaling behavior, many gregarious species may increase their fitness.

Reproductive Success.
Japanese Barramundi Lates japonicus. Source

            Among all taxa, not only fish, congregating with conspecifics generally increases reproductive success. Especially with rare or greatly dispersed populations, such as the endangered Japanese lates, shoaling behavior not only increases access to potential mates, but also directly increases number of potential mates (Gonzalvo et al. 2015). By shoaling with conspecifics, a fish may decrease the energy it expends to find mates and thus will have greater reproductive success. This behavior also greatly benefits migratory spawners as the journey to the spawning grounds will be safer travelling as a shoal than as an individual.

Hydrodynamic Efficiency.
School of Yellow Snapper  Source
            It has been hypothesized that shoals provide hydrodynamic efficiency and lift to participating fish. Within shoals, fish tend to be equally spaced and uniformly sized, yet, laboratory experiments have struggled to demonstrate hydrodynamic lift within shoals (Pitcher and Parish 1993; Hoare et al. 2000). However, many still believe that this hydrodynamic benefit may still be observed in the wild. Regardless, within shoals, fish continuously change leadership at the head of the shoal. In large, synchronous groupings, such as the shoal of yellow snapper depicted above, this leader position is beneficial in allowing the fish to direct movement and feed first. 

Increased Vigilance.
Green Chromis  Chromis viridis    Source
            Within many taxa, living within groups allows for individuals to spend greater time foraging while others employ vigilance to evade predation. Within shoals, this technique has been greatly demonstrated. One study assessed the metabolic health of a species of shoaling damselfish through the use of novel respirometry methodology (Nadler et al. 2016). It was determined that visual and olfactory cues signifying the presence of a shoal mate would produce a reduced minimum metabolic rate in individuals. Additionally, fish held within shoals displayed greater and healthier body condition than those held in isolation at the conclusion of the study (Nadler et al. 2016). It his hypothesized that this reduced need for individual vigilance aided in reducing stress and metabolic requirements of the fish, allowing for more energy to be spent on increasing body condition (Nadler et al. 2016). Not only does increased vigilance from shoaling aid in avoiding predation, but it also benefits the individual fish on a biological and metabolic level.

Predation Dilution and Kin Dynamics.
Siamese Fighting Fish Betta spendens   Source
The Siamese fighting fish is a small, brightly colored fish, typically associated with the aquarium trade. They are not typically thought of as a shoaling fish as males instinctively fight one another to defend their own territory. However, females of the species have been observed to associate with other females based on coloration. In one study conducted by Saint Joseph’s University and Amherst College, it was determined that female preference lead them to spend more time demonstrating typical shoaling behavior with larger groups of other females than solitary behavior (Blakeslee et al. 2009). When given the option of two uniquely colored groups of fish to swim by, the majority of females would choose the group that most closely represented their own coloration (Blakeslee et al. 2009). Like many other fish, this study demonstrates that shoaling behavior of fighting fish is strongly impacted by external characteristics and coloration. This tendency to swim next to large groups of similarly colored fish provides benefits to individual fish through predation dilution.

A learned behavior.
Zebra Fish  Danio rerio   Source
            The zebrafish is a small tropical fish known for its distinct lateral stripes. In a study conducted by Engeszer et al., zebrafish were tested for their innate ability to shoal with not only with similarly-colored fish, but also fish that had similar genetic characteristics (Engeszer et al. 2004). During the study, one group of wild type and nacre-morph larvae were raised in separate environments, some wild and nacre-morph fish were raised in environments of the opposite phenotype, and some fish were raised in isolation. Due to the controlled rearing of the fish, the study was able to isolate phenotypic selection as the primary factor in shoaling preference. While fish raised in isolation showed no strong preference in shoaling behavior, fish reared in groups of similar and dissimilar phenotypic expression were found to strongly select for the traits of the group in which they were reared (Engeszer et al. 2004). This study suggests that this behavior may be one that is learned through early developmental experience as fish tend to prefer grouping with those which they are familiar. This may be helpful in promoting kin dynamics amongst shoaling groups.

Predation Avoidance and Foraging Success.
Fathead MInnow Pimephales promelas  Source
The oddity effect refers to disposition of odd, rare, or solitary fish to experience disproportionately higher rates of predation than conspecifics who choose to swim near those similar in appearance. One study conducted by the Southwest Missouri State University tested the oddity effect among Brook stickleback and Fathead minnow (Mathis et al. 2003). Studying the shoaling behavior of solitary minnows and sticklebacks when faced with a threat, they determined that when faced with both low and high likelihood of alarm or predation, typically solitary minnows would choose to shoal with conspecifics rather than heterospecifics (Mathis et al. 2003). In respect to the sticklebacks, when faced with high predation risk, the sticklebacks would actually prefer to shoal with minnows rather than conspecifics. It was hypothesized that this behavior was due to the vulnerability of the minnows when paired with a stickleback (Mathis et al. 2003). Outside of a research setting, the predator would most likely attack or focus on the minnows before preying on the stickleback as the stickleback requires more energy to successfully catch and would be capable of performing greater evasive tactics (Mathis et al. 2003). This relationship demonstrates the predation aspect of shoaling preference. Interestingly enough, in times of low predation risk, sticklebacks would shoal with conspecifics rather than minnows. This may be because of competition over food resources as minnows consume a greater amount of prey than sticklebacks. In this sense, during low predation risk, to maximize foraging ability, the stickleback will choose to shoal with other conspecifics to avoid competition.
Brook Stickleback Culaea inconstans Photo by Uland Thomas.   Source 
A similar phenomenon of shoaling behavior increasing foraging success may be observed in guppies. Rather than discriminating based on similar coloration, guppies preferred shoals of fish that were large, similar in body size to their own, had previous foraging experience within the experimental environment, and a greater familiarity amongst one another (Lachlan et al. 1998). Through these shoaling preferences, the transmission of social behavior was observed as fish were more likely to adopt foraging behaviors utilized and taught by many shoaling individuals rather than one. This suggests that shoals benefit fish by transmitting novel feeding tactics to one another through the shoal, thus increasing foraging success (Lachlan et al. 1998).

            Fish choose their “shoal mates” based off of a variety of factors from phenotypic traits to experience. In every case, the decision of a fish to shoal with conspecifics (or even heterospecifics) is meant to directly increase its fitness. Through various methods, shoaling aids in defense against predation, increased reproductive success, hydrodynamic efficiency, group vigilance, predation avoidance, and foraging success.


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Nadler, L.E., S. S. Killen, E. C. McClure, P. L. Munday, and M. I. McCormick. 2016. Shoaling reduces metabolic rate in a gregarious coral reef fish species. The Journal of Experimental Biology 219(18):2802-2805.
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