Cleaner wrasses—small reef fish that pick parasites from larger "client" fish—provide a textbook case of mutualism. Yet these cleaners occasionally cheat by taking bites of the client's protective mucus, a behavior that causes the client to flinch and sometimes flee. For years, the leading explanation for why clients tolerate this cheating was that they retaliate: clients chase, bite, or otherwise punish dishonest cleaners, thereby keeping the cheating in check. That view has been unsettled by recent experiments showing that tactile stimulation from the cleaner's pectoral fins can override a client's impulse to flee. Researchers discovered that when a wrasse rhythmically strokes a client's body, the client's levels of the stress hormone cortisol drop markedly, and the client becomes less likely to react aggressively even after being bitten. This hormonal shift, induced by nothing more than physical contact, means that a cleaner can effectively reduce the chance of being punished. The discovery makes sense of a longstanding puzzle: why client fish continue to visit cleaning stations despite the risk of being cheated. It also reframes the cleaner–client relationship not as a simple case of mutualism policed by client retaliation, but as an interaction in which the cleaner actively shapes the client's physiological state to its own advantage.

The passage cites which of the following as evidence that resolves the puzzle of why client fish continue to tolerate cheating cleaner wrasses?

A. Cleaner wrasses sometimes take bites of the client's protective mucus.
B. Clients often retaliate against cleaners that cheat by chasing or biting them.
C. The tactile stimulation provided by the cleaner's fins can lower the client's stress hormone levels.
D. The cleaner–client relationship has traditionally been viewed as a textbook case of mutualism.
E. The discovery helps account for the otherwise puzzling persistence of cheating in a supposedly cooperative system.



The word "induced" in the passage is closest in meaning to
A. caused
B. hypothesized
C. simulated
D. amplified
E. permitted