Who hasn’t sat in awe, watching large coordinated schools of fish, shimmering in unison through complicated maneuvers with each member precisely spaced apart? These schools seem to effortlessly turn, expand, contract, wishbone apart and then come back together…all without missing a beat.
There are actually two types of fish aggregations – schools and shoals. A shoal is a loose aggregation and sometimes comprises different species. These fish hang out together for social reasons but are not organized. Schools, are shoals on steroids. They are highly structured and coordinated. There is no clear rule for calling a shoal a shoal and a school a school, since all schools are shoals by definition.
The shape of a shoal can vary widely. Traveling shoals, may appear as long, thin lines, or they may be oval, square, or akin to an oozing splat. However, fast moving, traveling schools are generally wedge shaped. And, feeding shoals tend to be circular.
Why do fish Shoal/school? There’s a few theories on this. First, the regular spacing of fish in schools suggests that there’s probably a hydrodynamic advantage to being in a school – at least for fish that are behind other fish. Another advantage is increased ability for finding food. Fish that shoal throughout their lifetimes also have a reproductive advantage. They don’t have to expend energy to find a mate, its right there.
And finally, shoaling reduces predation risk. Most small shoaling fish are silvery, and it is difficult for a visually oriented predator to pick an individual out of a mass of twisting, turning fish, and then fix on that individual, and attack before losing it in the shoal. Shoaling fish that do get eaten often have been separated from the shoal, or they possess visual cues such as heavy parasite infestations.
How do fish Shoal/school? There’s still a lot scientists don’t fully understand when it comes to shoaling behavior, but they do know that both vision and the lateral line are involved. The fact that most schools (but not necessarily shoals) break up at night suggests vision is a main sensory link. Many laboratory experiments have temporarily blinded fish to demonstrate the importance of vision. One early study also found that fish would school their mirrored image.
Several studies have suggested that fish use optomotor reaction to maintain spacing in the school. Optomotor reaction is the response fish have when placed in a container with black and white vertical stripes rotating around it. The fish stops the rotation of the stripes by fixating on one stripe and then swimming at the same speed at which it’s rotating. This response is useful for schooling fish, because position in the school can be maintained by visually fixing on the side of neighboring fish.
Up until recently, scientists thought schooling fish only changed speed to make turns. However, a 2011 study from Uppsaia University found that shoaling mosquito fish actively changed their speed to avoid or move toward neighbors (much like we speed up or slow down in a car in response to the car in front of us). They also found that a single nearest neighbor dominates all social interaction. Moral of the story – follow the nearest fish and do what they do!
Where does the lateral line fall into all of this? Several studies from the 70s & 80s that looked at the lateral line found that blinded pollock, with an intact lateral line could maintain their positions in a school but with greater distance between neighbors. Pollock with normal vision but with their trunk lateral line cut were also able to school, but with closer distance between neighbors. Only when both the trunk lateral line was cut and the fish were blinded, did schooling behavior stop.
A 2010 study found that when the entire lateral line system (both along the trunk and head) was inactivated, that fish were unable to maintain a shoal; however, after the lateral line system was restored, shoaling behavior resumed. This study was the first to demonstrate the entire lateral line system was crucial to shoaling behavior. But how needed more research.
A couple more recent studies have looked at the role genetics play in schooling behavior. Researchers studying sticklebacks found that whether fish are motivated to school and how well they school are linked to two separate regions of the genome (it’s not a learned behavior). In a more recent study, the same researchers found the specific gene responsible for sticklebacks’ schooling skills. It’s called Ectodysplasin or Eda for short, and interestingly is also linked to the presence of bony armor and the sensory neuromasts (lines of sensory hairs) that make up the lateral line.
Marine sticklebacks have a pair of sensory hairs on each bony plate and they school. Freshwater sticklebacks have fewer bony plates, only a single line of sensory hairs, much lower amounts of Eda protein, and do not school. When scientists genetically manipulated the freshwater fish with high levels of Eda, the fish began schooling, although not as well as their marine counterparts.
These studies, published in 2013 and 2016, suggests a single gene could cause fish to detect their environment differently, and supports the long held notion that schooling behavior is controlled in part by the lateral line.