Passage Summary: A stingray is personally healthier if it doesn't have parasites, yet if you find a population of stingrays with no parasites at all, it actually means the whole ecosystem is in trouble.

Reasoning: While parasites are detrimental to the health of an individual stingray, their total absence in a population indicates that the surrounding environment is suffering from significant stress or pollution.

Analysis: To resolve this paradox, we need a piece of information that explains why a benefit to the individual (no parasites) serves as a warning sign for the group's environment. The most logical bridge is that the parasites themselves are more sensitive to environmental toxins than the stingrays are. If the water becomes polluted, the parasites might die off long before the stingrays do. Therefore, while the stingray is temporarily 'healthier' without its parasites, the lack of those parasites functions as a 'canary in a coal mine' for the health of the entire ecosystem.