Developmental polymorphism in the tadpoles of the spadefoot toad scaphiopus multiplicatus
Riverside, Calif., Univ. of California, Diss., 1981
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Tag hinzufügen | Zusammenfassung: | Riverside, Calif., Univ. of California, Diss., 1981 The tadpoles of Scaphiopus multiplicatus exhibit a polymorphism that is reflected in their morphologies and ecologies. The morphs--omnivores, intermediates, and carnivores--can be recognized in the ponds of southeastern Arizona as early as three days after egg-laying. The distinctive head of the carnivore is flat with a long snout and hypertrophied jaw musculature. In contrast, the omnivore has a rounded head and lacks the enlarged muscles. The carnivore gut is five times the snout-vent length; the omnivore, 19 times. Carnivores grow faster than omnivores up to initiation of metamorphosis. Size at metamorphosis in carnivores is 19-31 mm; in omnivores, 16-26 mm. The minimum time to metamorphosis in the carnivore is 13 days; the omnivore, 17 days. The carnivore is a predator with solitary and aggressive habits marked by rapid, continuous swimming and sharp turns. Its diet includes fairy shrimp, tadpole shrimp, smaller crustaceans, and omnivores. A fairy shrimp can be consumed in approximately six seconds. The omnivore is a browser-filter feeder displaying passive and gregarious habits. Its diet includes detritus, algae, mud, bacteria, feces, and fairy shrimp when prey density is high. This morph is an ineffective predator taking up to five minutes to consume a fairy shrimp. The diet of the intermediate is similar to the omnivore, but includes macroscopic detritus indicative of scavenging behavior. The morph frequency in ponds is variable. The predominant morph is usually the omnivore followed by the carnivore, but populations have been studied where intermediates and carnivores are the dominant morphs. Diet occupies a paramount role in the development of this polymorphism. One of 20 genetic lines produced one carnivore on a trout food diet while 12 of those lines produced carnivores on a regimen of shrimp and tadpoles. The size of the jaw-hyoidean musculature increases in field omnivores placed on this diet; field carnivores placed on trout food show reduced jaw muscles--to an omnivore size. Carnivores placed on shrimp and tadpoles exhibit less muscle reduction than when fed trout food. Field carnivores retain larger jaw muscles when fed lettuce, which requires shearing, than when fed trout food. Pond data show a positive correlation of carnivore jaw muscle size and fairy shrimp density. Although diet underlies the lability of this polymorphism, a genetic effect also exists. In tadpoles from 20 genetic lines raised on two diets, some lines showed greater jaw muscle growth than other lines. Embryonic development rate is positively correlated with later jaw muscle size when the larvae are fed shrimp and tadpoles. While egg size and color are quite variable, the jaw muscles of tadpoles hatched from large brown eggs and raised on trout food or shrimp and tadpoles are not different from tadpoles in the same clutch hatched from small gray eggs. Carnivores did not appear among hatchlings raised at high densities and fed trout food. In similar treatments maintained on a restricted diet of shrimp and tadpoles, the percent of carnivores was generally well below that observed in the field. Tadpoles treated with thyroxine show an increase in the size of the jaw-hyoidean muscles, an increase in development, a decrease in growth, and a decrease in gut length. Omnivores fed trout food in such treatments developed features which mimic the carnivore morphology. Carnivores given thyroxine and trout food retain their characters. |
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