What produces a waterproof super glue, acts like a vacuum cleaner, and even teaches scientists about gene repair？ The humble little shellfish known as the mussel (贻贝)。
Mussels are found worldwide. Some live in the sea. Others inhabit freshwater streams and lakes. When you try to move a mussel from a rock, you will discover what an incredibly firm grip it has a necessity if the mussel is to resist the sharp grab of a hungry seabird or the pounding waves of the sea. How does it manage to cling so tight？ When it chooses a place to set up home, it pokes its tongue-shaped foot out of its shell and presses it against a solid surface. Special glands give off a fluid mixture of proteins into a channel that runs the length of the foot. The liquid quickly hardens into a fine, elastic thread about an inch long. Then a tiny pad-like structure at the end of this thread gives off some natural glue-like substance, the mussel lifts its foot, and anchor line number one is complete. These strategically placed threads form. a bundle, which ties the mussel to its new home in much the same way that ropes hold down a tent. The whole procedure takes only three or four minutes.
Imagine having a very strong glue that is non-toxic and so flexible that it can penetrate the tiniest holes and corners, sticking to any surface, even under water. Shipbuilders would welcome it for repairing vessels without the expense of dry-docking them. Auto-body workers would like a really waterproof paint that keeps the rust out. Surgeons would value a safe glue to join broken bones and to close wounds... The list of possible uses appears endless.
However, scientists are not thinking of using the mussels themselves to produce this super glue. It would take some l0.000 shellfish to make just one gram of glue. So collection enough mussels to supply the world's demand for super glue would wipe out the mussel population, many species of which are already endangered. Instead, American researchers have isolated and cloned the genes for five mussel glue proteins, and they are about to mass-produce them in the laboratory. However, the mussel is still one jump ahead. Only the mussel instinctively knows the exact blend of proteins needed for each kind of surface. Molecular biologist Frank Roberto has asked admiringly: "How are you ever going to imitate that？"
A mussel grips a hard surface very firmly to

A. seal itself from being damaged by sea water
B. protect itself from being the food of other animals
C. protect itself from being blown away by strong wind
D. produce the waterproof super glue