Do granular substrates enhance microbiota availability and growth in cultured Rana catesbeiana tadpoles?

A comparison was made of growth performance in bullfrog Rana catesbeiana tadpoles and biogenic capacity in culture tanks with gravel, sand and hard bottoms. Stage 25 (Gosner) tadpoles from a single cohort were stocked in 1-m^2 fibreglass tanks at 50 m^-2. Two treatments and a control were employed, with three replicates each: T1=tadpoles reared in tanks with 0.02 m deep layer of sterilized silica gravel (2360 (mu)m particle size); T2=tadpoles reared in tanks with 0.02 m deep layer of sterilized beach sand (250 (mu)m particle size); and C=a control treatment with tadpoles reared in tanks with no soft substrate (conventional method). Tadpoles were fed on a 40% protein, powdered specific bullfrog feed at 13% body weight day^-1. Results for survival, timing to metamorphosis, specific growth rate and weight gain exhibited no statistical differences between treatments (P>0.05). However, tadpoles in T1 had an apparent overall better performance based on numerical indicators. Biotic colonization was also statistically similar (P>0.05) between treatments, showing an overall poor biogenic capacity for the tested substrates. Only three taxa, at low abundances, were recorded for phytoplankton and phytobenthos: cyanophiceae, chlorophyceae, and bacillariophyceae. Zooplankton was represented by two microinvertebrate groups, rotifera and protozoa. The statistical similarities between treatments may reflect that bullfrog tadpoles do not exploit naturally occurring aquatic biota when sufficient suspended artificial feed particles are present.