Foraging Behavior of Bufo americanus Tadpoles in Response to High Densities of Micro-Organisms

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Abstract
Filtering small food particles from water apparently occurs in a variety of anuran larvae. The specialized social feeding on suspended micro-organisms is described for B. americanus from Emmet County, Michigan, USA. These include an abundance of protozoans, of which Chroomonas nordstedtii was most abundant and Dinobryon divergens next most common. Other common organisms were Oikomonas termo, Codonocladium sp., Bodo putrinus, Cercomonas crassicauda, Chilodonella uncinata, Chilodontopsis vorax, Cinetochilum margaritaceum, Strombilidium gyrans, Pleuromonas jaculans and Monas guttula. Data support the behavioral evidence which shows that this group of tadpoles was feeding on micro-organisms filtered from the water as they slowly swam through the containing water mass. Although protozoans are not necessary for growth and metamorphosis, it was demonstrated that toad tadpoles sometimes shift from their usual method of substrate feeding, on a variety of food, to filtering protozoans from the ambient water when such organisms occur in high density. Continuous feeding on micro-organisms in mid-water by B. americanus apparently is not common. Probably the conditions for development of sufficiently dense and extensive blooms of micro-organisms are not present often or for long. The feeding of several hundred or thousand tadpoles probably reduces the organisms rapidly. Such a group of feeding tadpoles constitutes a well-defined aggregation. The alteration of behavior in large numbers of tadpoles described here is rather striking evidence of the adaptability of B. americanus, an animal able to exist in a wide range of ecologic and geographic conditions, some of which are severe. This behavioral lability suggests that the larval stage may be more advanced ontogenetically in nervous system and behavior than is commonly assumed.

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