They weigh nothing and take up no space.
Used to test pollution by heavy metals, using the Ecology level 1 skill.
It seems the more jugs you use to gather Water, the more Tadpoles you get.
Recently, several Egyptians have been caught "Power Poling", a technique where they gather water with a massive number of jugs repeatedly for no other reason than to collect tadpoles. The practice has lead to a firestorm of protest from the tadpole traditionalists, who claim that legitimate tadpoles are only those that happen to be gathered in the normal course of play. Advocates of power poling claim that any collection method is valid.
Initial Tadpole Analysis can be found here.