Trade is a particular type of exchange. If we exchange Christmas gifts, another species might interpret our behavior as trade. If monkeys groom each other that might be trade if each paid the other $5. Otherwise, it is more likely sociability rather than trade. Symbiotic species are not trading anymore than the plant and animal life that make up a coral reef are trading. Admittedly, trade can expand beyond market relations. Kids can trade baseball cards. But the examples proposed here do not appear to be trade -- an activity in which both parties are aware of a mutual advantage. Michael Perelman