The arthropod is able to crack open only relatively weak clam shells. if it could tell the difference, which variety might the arthropod seek out as it hunts for food? support your answer.

Well, if anthropods could tell the difference between relatively weak clams and tougher weak clams, they could better seek out the weaker clams. That way the anthropods would have a higher rate/chance of survival, because they would be able to locate food easier. There would be less anthropod deaths, which is why they would choose the weaker clams. In the end, anthropods would demonstrate excellent fitness in their environment and most likely thrive because of their ability to distinguish weaker clams from tougher ones.
I hope this helped you :)