Radula development in abalone Haliotis discus hannai from larva to adult in relation to feeding transitions
Abstract:
The radula morphology of Haliotis discus hannai
was examined by scanning electron microscope from the larval to the adult
stage. The radula of competent larvae contained 11–13 transverse rows of
teeth after 6–7 days at 20°C. The number of rows increased to 25–30
during the first several days after settlement, but then remained
approximately constant throughout the post-larval period, increasing again
in abalone larger than 4 mm in shell length (SL). In post-larvae < ~1 mm
SL, only two pairs of lateral teeth (L1, L2) were present in the larval
radula. An additional three pairs of lateral teeth (L3–L5) were added
progressively as post-larvae grew from 0.9 mm to 1.9 mm SL. Marginal teeth
were added steadily from one pair in larvae to 30–40 pairs at 3–4 mm SL,
70–80 pairs in 30–40 mm juveniles, and 70–90 pairs in 90–100 mm
adults. The serrations on the working edges of the rachidian (R) and lateral
teeth became less pronounced as the abalone grew. Nearly all serrations
disappeared from the rachidian (R) and inner lateral teeth (L1, L2) by ~2 mm
SL, and from the outer lateral teeth (L3–L5) by 20 mm SL. For abalone
larger than 1.5 mm SL, the L3–L5 teeth became longer and more pointed,
which increased the space between adjacent rows of teeth. Post-larvae < 1
mm SL had highly curved teeth with clearance angles of approximately or less
than zero, whereas larger abalone had positive clearance angles. These
radula developments appear to be related to transitions in feeding habits
from microbial to macroalgal diets.