Commensalism refers to the ecological relationship between two species in the biological world. One species will obtain survival benefits due to this relationship, but the other species does not obtain any benefits or harm in this relationship, but only drives the other to obtain benefits.
According to the above definition, Crucian carp has poor swimming ability. Its back turns into a suction cup and adsorbs on the shark. With the shark traveling all over the world, when the shark catches prey, crucian carp steals the remaining food belongs to commensalism.