Self-Inhibition of Growth in Rana pipiens Tadpoles

Abstract

Inhibition of the growth rate of Rana pipiens tadpoles occurs when they are reared under crowded conditions. Although all animals in a group originally may be the same weight and age, differences in growth rate soon become apparent. The slower growing members may be so severely retarded that they fail to survive unless they are removed from the group. This growth inhibition is known as the crowding phenomenon. The same adverse effect on growth occurs when individual tadpoles are reared in water previously occupied by growing tadpoles of the same species. Tadpole growth can be affected by a certain type of alga that has not been positively identified. These algal cells can be found in the gut and fecal material of R. pipiens tadpoles. Also, there seems to be a growth retarding substance which can pass through the water from one individual to another; it tends to be species specific. It not only affects larval growth, but also egg development. An hypothesis is proposed to explain the relationship between the algal cells and the specific inhibitor The specific substance responsible for the crowding phenomenon in R. pipiens tadpoles is produced by the tadpoles themselves. The algae are indirectly involved by making the inhibitor effective for longer periods of time and by making it possible to transfer inhibitor associated with the algal cells from one aquarium to another. The algal cells become associated with the inhibiting substance as they pass through the posterior half of the tadpole''s gut. The inhibitor is gradually released from the excreted cells into the water they occupy. If few or no algae are present, the inhibitor is active only for a short period of time, but still can affect tadpoles in the same aquarium. This hypothesis seems to be consistent with all experiments presented in this paper, and with the data regarding the crowding phenomenon in R. pipiens tadpoles presented by other investigators.