Morphological changes in the radula of abalone (Haliotis iris) during post-larval development

H. iris Gmelin post-larvae fed with diatoms (Cylindrotheca closterium) were sampled weekly from 10-60 days post-settlement. Radula morphology was examined by SEM, and compared with that of competent larvae (260-280 mu m shell length (SL)) and adults (125-130 mm SL). Larvae (130 degrees C.days old) had a well-formed radula with similar to 10 rows of teeth. Each row comprised 1 rachidian tooth, 2 pairs of lateral teeth, and 1 pair of marginal teeth. The number of rows of teeth increased to 26-29 by similar to 500 mu m SL (Day 10 post-settlement) and stayed at that level throughout the post-larval period (maximum size/age observed was 2.7 mm SL at Day 60). Marginal teeth were added steadily to give similar to 30 pairs per row at similar to 2.5 mm SL (compared to 60-80 in adults). Lateral teeth (L3-L5) were added between 1.0 and 1.7 mm SL to complete the adult complement. Progressive changes in the post-larval radula above 1-1.5 mm SL included: (1) reduction of serrations on the margins of rachidian and lateral teeth, particularly the more central (R, L1, L2) teeth; (2) increased spacing between adjacent rows of teeth; (3) lengthening of the outer lateral teeth (L3-L5) making them larger and taller than the central (R, L1, L2) teeth. The clearance angle of rachidian and lateral teeth was variable, but generally increased as post-larvae grew. Post-larvae <1 mm SL had highly curved teeth and apparent clearance angles around or below 0 degrees. Larger post-larvae had positive clearance angles. These developments suggest that the teeth of post-larvae <1 mm SL probably function as "scoops" which slide across the surface collecting small diatoms and other fine, loose particles. Radulae of post-larvae > 1mm SL become more suitable for collecting larger particles, and gouging feeding substrata.