Growth dynamics of the medullary cavity in the rostrum of juvenile paddlefish (Polyodon spathula)

Abstract

The prominent rostrum of the paddlefish has electroreceptive and mechanosensory functions. Not present in newly hatched paddlefish, the rostrum quickly grows to about one‐third the total body length of juvenile fish. Internally, the rostrum has a hyaline cartilage core that contains an adipose‐filled medullary cavity – thus resembling the long bones of vertebrates that have cavities filled with yellow marrow. To better understand formation, growth and function of the rostrum and medullary cavity, we compared cavity volume to rostrum length and total body length in six groups of size‐matched juvenile paddlefish. Medullary cavity volume was modeled as a simple cone. Each rostrum was mid‐sagittally sectioned and imaged, and cavity length and width measurements were obtained using ImageJ. Results show significant differences in ratios of cavity volume:rostrum length and cavity volume:body length between the developmental groups. In early development the medullary cavity increases in size rapidly, later the rate of cavity volume increase slows relative to body and rostrum growth. Elucidating the pattern of growth of the medullary cavity contributes to understanding of the functional anatomy of the rostrum and the biology of paddlefish.