Radula Development in Abalone Haliotis Discus Hannai from Larva to Adult in Relation to Feeding Transitions

FISHERIES SCIENCE 2001; 67: 596–605 Original Article Radula development in abalone Haliotis discus hannai from larva to adult in relation to feeding transitions Tomohiko KAWAMURA,*1a Hideki TAKAMI,1 Rodney D ROBERTS2 AND Yoh YAMASHITA1 1Tohoku National Fisheries Research Institute, Shiogama, Miyagi 985-0001, Japan and 2Cawthron Institute, Private Bag 2, Nelson, New Zealand 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. KEY WORDS: abalone, development, diet, feeding, larvae, postlarvae, radula. INTRODUCTION diatom diets (sensu Kawamura et al.2 and grow more rapidly on efficiently digested diatom strains Feeding habits in the early life stages of abalone (the second transition). The final transition is at have been studied in detail in recent years1–8 and approximately 5–10 mm SL, from a diatom- three major transitions in feeding have been iden- dominated diet to a macroalgae-dominated diet. tified.9 The first is the transition from lecithotrophy The ingestion and digestion of diatoms appear to particle feeding around the time of metamor- to be affected by the action of the radula. The phosis. Benthic diatoms are a principal food for ability of post-larvae to ingest large diatom cells post-larval abalone after metamorphosis, and increases as post-larvae grow.7,8 Physical rupturing post-larvae smaller than approximately 0.8 mm of diatom cells, which directly affects digestibility, shell length (SL) grow at similar rates regardless of frequently results from the action of the radula.2 diatom strain, provided they receive an adequate Radula morphology is also important when ju- supply of biofilm material. Post-larvae bigger than venile and adult abalone graze macroalgae,10,11 but this size become responsive to the ‘digestibility’ of information on the action of the abalone radula during grazing macroalgae is limited. We previously observed the morphological *Corresponding author: Tel: 81-3-5351-6499. Fax: 81-3-5351- changes in the radula of a New Zealand abalone 6498. Email: [email protected] aPresent address: Ocean Research Institute, The University of Haliotis iris during the post-larval period, and Tokyo, 1-15-1, Minamidai, Nakano, Tokyo 164-8639, Japan. showed that post-larval radula developments are Received 14 April 2000. Accepted 15 December 2000. consistent with preparation for the transition from Radula development in abalone FISHERIES SCIENCE 597 microbial to macro-algal diets.12 It was suggested older post-larvae and young juveniles (~1–15 mm that the teeth of post-larvae < 1 mm SL probably SL) for radula observations. The main food source function as ‘scoops’ that slide across the surface for the abalone on the pregrazed plates was the collecting small diatoms and other fine, loose par- diatom film, which was dominated by Cocconeis ticles. Radulae of post-larvae > 1 mm SL became spp. more suitable for collecting larger particles and gouging feeding substrata. That paper is the first report to describe the progressive development of Radula observation the abalone radula throughout the post-larval period.12 Other reports present only limited infor- Samples of post-larvae and young juveniles were mation about the abalone radula, often at a single preserved in 5% seawater formalin at intervals point of larval or post-larval development.4,13–17 of 4–17 days from 6 to 109 days post-settlement. Larval and post-larval radula development has Competent larvae after 6–7 days at 20°C were also been studied only for polyplacophorans, apla- preserved in the same manner for observing the cophorans and pulmonates.18 larval radula. Radulae of these larvae, post-larvae In the present paper, changes in radula mor- and young juveniles were removed with a pipette phology of abalone Haliotis discus hannai from and an inverted microscope following dissolution larva to adult are described. We focus on radula of tissues by soaking in sodium hypochlorite (0.6% development in relation to transitions in feeding, Cl concentration; Wako Pure Chemical Industries and compare these developments with those of Ltd, Osaka, Japan) for several minutes. Radulae H. iris. were then serially pipetted through several distilled water baths to remove residual sodium hypochlorite. This procedure was selected after we con- MATERIALS AND METHODS firmed that the structure of radula teeth was not changed by the procedure. Post-larval and juvenile Abalone rearing SL (the longest shell dimension) were measured individually before dissolution. Radulae of older Larval abalone were hatched in May 1997 at the juveniles and adult abalone (~15–98 mm SL) were Iwate Sea Farming Association (Iwate, Japan) or dissected from fresh animals obtained from Iwate in October 1997 at the Akita Prefectural Hatchery Sea Farming Association and reared at TNFRI in a Center (Akita, Japan) using the procedures running seawater tank at 20∞C and fed brown alga described by Uki and Kikuchi.19 Four days after fer- Laminaria japonica. tilization at 20°C, the veliger larvae from Iwate were Radula length, width, number of transverse rows transported to Tohoku National Fisheries Research of teeth (Fig. 1a), and gap between rachidian teeth Institute (TNFRI, Miyagi, Japan) within 4 h. of adjacent rows (only for older juveniles and Five-day-old larvae from Iwate reared at 20°C, adult) were determined using an inverted micro- which were competent to metamorphose,20 were scope and video micrometer. Radulae were then transferred to plastic water tanks (diameter 28 cm, transferred to scanning electron microscope (SEM) height 14 cm), the inside of which were covered stubs, laid flat with the teeth upwards, and allowed with a mono-cultured benthic diatom Cocconeis to air dry before sputter coating with gold for SEM scutellum that had been pregrazed overnight observations. by juvenile abalone (2 cm SL). Cocconeis scutel- The length and width of the rachidian tooth, lum coated with trail mucus of juvenile abalone lengths of the L3 teeth and the gap between the has a high potential for the induction of settle- rachidian teeth of adjacent transverse rows ment/metamorphosis of larvae and provides a (Fig. 1b) were measured using SEM photographs. suitable food source for all stages of post-larval The clearance angle of rachidian and lateral teeth abalone.5 The metamorphosed animals were (Fig. 1c) of post-larval radula was also measured as maintained as a source of young post-larvae described elsewhere.12 The clearance angle of teeth (<~1.5 mm SL) for radula observations by being fed was defined by Padilla, who suggested that it pro- C. scutellum, as described elsewhere.21 vides information on the function of radula teeth.11 Four-day-old competent larvae from Akita were Rake angle (Fig. 1c) was also suggested to be settled on pregrazed plates and reared continu- important in the function of radula teeth,11 but ously in running seawater at 20°C at the hatchery. we could not measure it directly on most teeth Seven days after settlement, post-larvae were because of their shape and orientation in the SEM transported to TNFRI on the plates in a water tank preparations. We did not measure the clearance within 5 h. These post-larvae were reared on the and rake angles of juvenile and adult radula plates in running seawater at 20°C as a source of because accurate measurements were difficult 598 FISHERIES SCIENCE T Kawamura et al. Fig. 1 Scanning electron microscope photographs showing terms used in the text to describe radula morphology. (a) An entire radula showing length and width. (b) Transverse rows of radula teeth. R, rachidian tooth; L1–L5, lateral teeth 1–5; M, marginal teeth; G, gap between rachidian teeth of adjacent transverse rows; WR, width of rachidian tooth; LR, length of rachidian tooth; LL3, length of L3 tooth. (c) Side view of a row of radula teeth showing individual teeth. CA, clearance angle (positive); TTA, tooth tip angle; RA, rake angle. even on the outside laterals because of obstruction by the overhanging marginal teeth. Measurements of the angles of rachidian tooth and inner lateral teeth of juveniles and adults were impossible because of their positions behind the lengthened outer laterals. These measurements were done for several teeth rows located in the middle part of the whole radula.

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