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Figure 8. Experimental setup for measuring the swimming speed [27] .

discussing the problem of multiple robot fishes, we are most concerned with the problems of motion
coordination and communication of multiple robot fishes. As a result, we review the latest research on these
two issues in depth.

4.1. Motion coordination of multiple robot fishes
Fish frequently congregate in schools. Fish schools can not only effectively fight against natural enemies but
also save energy and help them survive in harsh environments. Researchers believe that schools of multiple
robot fishes can reap the same benefits. Therefore, we focus on coordinated swimming of multiple robot
fishes and related discussions. The current study is mainly concerned with tandem formation and parallel
formation. However, there have been studies on other planar formations.

Tandem formation refers to the connection of the heads and tails of two or more fish in a straight line, as
shown in Figure 9. The fish at the front of the line is known as the leading fish, and the fish behind it is
known as the following fish. The most basic formation of this is two fish swimming in tandem formation.
[47]
Tandem swimming of two 3D bionic fish was studied by Wu et al. . The results show that, in the absence
of any control by the two fish, the vortex generated by the leading fish deflected the path of the following
fish. Khalid et al. found that the undulating frequency of the following fish does not affect the vortex and
time-averaged drag of the leading fish at a certain Strouhal number . Furthermore, it appeared to be more
[48]
favorable for the leading fish when both fish kept swimming in tandem formation.

Parallel formation refers to two or more fish lining up in a row, as shown in Figure 10. Similarly, the fish at
the front of the line is called the leading fish, and the fish behind it is called the following fish. The most
basic form of this is two fish swimming in parallel formation. The efficiency of two fish when swimming in
parallel was analyzed by Doi et al. . The results show that the highest swimming efficiency was achieved
[49]
when the distance between the two fish (K1) was 0.4 BL under the premise of L = 0. A vortex phase
1
[50]
matching strategy for robot fishes was found by Li et al. . The following robot fish could conserve energy
when the front-back distance between two robot fishes was linearly connected to the tailbeat phase
difference. As shown in Figure 11, the following robot fish could save energy by vortex phase matching. By