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Page 181 Yu et al. Intell Robot 2022;2:180-99 https://dx.doi.org/10.20517/ir.2022.10

efficiency is only 40%-50%. Furthermore, their shapes employ a non-bionic structure that cannot be
integrated into the underwater environment, making close observation of underwater organisms difficult.
[1]
Fish have undergone extensive natural selection and can swim with an efficiency of more than 90% . Fish
[2-6]
also have distinct advantages in terms of speed, maneuverability, and stealth . For example, swordfish can
reach a speed up to 30 m·s . Bionic robot fishes, which treat fish as bionic objects, can effectively absorb
-1[3]
these advantages to overcome the defects of traditional underwater robots and become more effective tools
for ocean exploration.

The propulsion modes of fish are usually classified into two categories according to the body parts used for
propulsion, namely body and/or caudal fin (BCF) propulsion and median and/or paired fin (MPF)
[7,8]
propulsion . It is worth noting that the median fin refers to the dorsal or anal fin, while the paired fin
refers to the pectoral or pelvic fin. Taking tilapia as an example, the structure and position of each fin are
shown in Figure 1. The BCF propulsion mode, in which the body and/or caudal fin acts as a propeller, is the
most common in fish and first discovered by researchers. This propulsion mode has the advantages of high
swimming speed and quick start performance, making it suitable for applications requiring high speed or
[9]
instantaneous acceleration . The median and/or paired fin acts as a propeller in the MPF propulsion mode.
This propulsion mode has the advantages of high maneuverability, high propulsion efficiency, and good
stability, making it suitable for applications requiring maneuvering to turn or long-term swimming, as well
as scenes with rapid water flow . After summarizing recent research results, we show that existing robot
[10]
fishes already have the BCF and MPF combined (BCF-MPF) propulsion mode. This propulsion mode is
based on the cooperation of the caudal and pectoral fins. With proper design, it is capable of balancing
swimming speed and propulsion efficiency, which has a wider application than either individually.
Furthermore, it is a promising research topic. The basic elements of the three propulsion modes are
summarized in Table 1.

Some review papers focus on the motion control of robot fishes [11-14] , while others focus on the design,
fabrication, and propulsion methods of robot fishes [15-17] . There is also a review paper that focuses on the
[18]
perception of robot fishes . However, the majority of them were published more than five years ago. In
these years, unprecedented attention has been paid to the study of bionic robot fishes. Related achievements
have proliferated and enriched the research in the field of robot fishes. As a result, this paper provides a new
survey on various major fields of robot fishes, addressing some gaps in related fields. Figure 2 depicts the
paper’s framework, which includes three objectives. Firstly, we provide a comprehensive survey of the most
recent designs of robot fishes, as well as the most recent progress in the study of motion mechanism. A new
field of study, namely motion coordination and communication of multiple robot fishes, is discussed.
Second, based on the survey, the challenges of current research and potential future research directions are
summarized. Three aspects are included: the gap between robot fishes and fish in terms of swimming
performance, methods to study the swimming mechanism of robot fishes, and the motion coordination and
communication of multiple robot fishes. Finally, a summary of the paper is provided.

The rest of the article is organized as follows. Section 2 elaborates on the latest designs of robot fishes.
Section 3 analyzes the motion mechanism of robot fishes. The motion coordination and communication of
multiple robot fishes are discussed in Section 4. Section 5 provides a comprehensive discussion on the
challenges and future works. Finally, in Section 6, some concluding remarks are made.

2. DESIGNS OF ROBOT FISHES
According to their body structure, robot fishes are classified into three types: rigid, soft, and rigid-soft
coupled. The strengths and weaknesses of different body structures are summarized in Table 2. The rigid