Amazingly mobile germ cells: In the case of ray sperm, the head is not only a container for the genetic material, but also provides the drive together with the tail. Thanks to this dual drive system, the tiny ones can move particularly well in viscous liquids and even swim backwards. This concept also has technical potential, the scientists make clear by building a robot based on the model of ray sperm.
As is well known, human sperm look like small tadpoles: They have a rounded head, in which the genetic material is located, and a flexible tail that provides propulsion through flapping movements. This is how the tiny messengers of life set off on their journey to the egg cells. It is known that sperm appear differently in the animal kingdom. However, the diversity and functions of the characteristics have so far been considered as little researched. The study by the researchers working with Jiahai Shi from the University of Hong Kong now shows what amazing locomotion systems the germ cells can possess.
The scientists report that it was a chance discovery: the team originally wanted to develop artificial fertilization techniques for breeding rays in order to explore their potential as producers of antibodies against diseases. As part of this work, the scientists therefore also focused on the male germ cells of these fish. Under the microscope, the special structures and swimming movements of the ray sperm were revealed for the first time: The researchers discovered that their head is not round, but rather has a spiral structure – similar to a corkscrew. The elongated head seemed to screw itself through the liquid as it moved, while the tail flapped at the same time.
Additional thrust
Closer investigations then made it clear: Head and tail rotate in the same direction when swimming. The rigid, spiral head and soft tail are actually a dual propulsion system, say the researchers. The two parts are connected by a middle piece that supplies the energy for the rotational movements. In the ray sperm, the head is not just a container for the genetic material, but an element of the drive, the researchers sum up.
According to their calculations, the head movement of the ray sperm contributes around 31 percent to the total driving force. The studies also made it clear that the dual system significantly increases the efficiency of movement. An important effect is that they can move easily in both thin and viscous environments. “The unusual type of drive gives the ray sperm a high degree of adaptability to different environments and flexible maneuverability,” says co-author Yajing Shen. Since both the head and tail contribute to propulsion, a slight angle between them can create a lateral force on the entire structure that allows rotation, the researchers report. The team also found out that the ray sperm have a bidirectional swimming ability – by switching the direction of rotation, they can swim not only forwards but also backwards. Sperm with a spherical or rod-shaped head are unable to do this.
Technical potential
In order to investigate the function of the movement system and to show the potential for robotics, the scientists developed technical replicas of the ray sperm as part of their study. The devices also consisted of a rigid spiral head and a soft tail, with a motor system in the middle. The tests showed that the ray-sperm concept can also provide technical systems with advantages in terms of mobility and efficiency: The tests showed that the robots could move skillfully in media with different degrees of viscosity.
“In addition to helping us understand how biological units move, our results can also serve as inspiration for technical designs,” says Shen. According to the researchers, it is becoming apparent that the concept of ray sperm has potential in the development of micro-robots: cleverly driven tiny creatures could one day be used in medicine to perform healing missions in the human body with its complex liquid environments to meet.
Source: University of Honkong, professional article: PNAS: 10.1073 / pnas.2024329118