Diatom Diet Selectivity by Early Post-Larval Abalone Haliotis Diversicolor Supertexta Under Hatchery Conditions*
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Chinese Journal of Oceanology and Limnology Vol. 28 No. 6, P. 1187-1194, 2010 DOI: 10.1007/s00343-010-0019-x Diatom diet selectivity by early post-larval abalone Haliotis diversicolor supertexta under hatchery conditions* ZHANG Yuyu (张玉宇)1, GAO Yahui (高亚辉)1,**, LIANG Junrong (梁君荣)1, CHEN Changping (陈长平)1, ZHAO Donghai (赵东海)1, LI Xuesong (李雪松)1, LI Yang (李扬)1, WU Wenzhong (吴文忠)1, 2 1 Key Laboratory of Ministry of Education for Coast and Wetland Ecosystems and School of Life Sciences, Xiamen University, Xiamen 361005, China 2 Dongshan Entry-Exit Inspection and Quarantine Bureau, Fujian Province, Dongshan 363400, China Received Sep. 3, 2009; revision accepted May 28, 2010 © Chinese Society for Oceanology and Limnology, Science Press, and Springer-Verlag Berlin Heidelberg 2010 Abstract Benthic diatoms constitute the primary diet of abalone during their early stages of development. To evaluate the dietary preferences of early post-larval abalone, Haliotis diversicolor supertexta, we analyzed the gut contents of post-larvae that settled on diatom films. We compared the abundance and species diversity of diatom assemblages in the gut to those of the epiphytic diatom assemblages on the attachment films, and identified 40 benthic diatom species in the gut contents of post-larvae 12 to 24 d after settlement. The most abundant taxa in the gut contents were Navicula spp., Amphora copulate, and Amphora coffeaeformis. Navicula spp. accounted for 64.0% of the cell density. In the attachment films, we identified 110 diatom species belonging to 38 genera. Pennate diatoms were the dominant members including the species Amphiprora alata, Cocconeis placentula var. euglypta, Cylindrotheca closterium, Navicula sp. 2, and A. coffeaeformis. Nano-diatoms (<20 µm in length) accounted for a considerable proportion of the total species number and cell density of the diatom assemblages in the gut contents and on the films. This suggests that nano-diatoms are important to the efficient production of abalone seed. The difference of the composition and abundance of diatoms between in the guts and on the biofilms suggests that early post-larval grazing was selective. An early post-larval abalone preferred nano-diatoms and the genera Navicula and Amphora during the month after settlement. Keyword: benthic diatoms; feeding; Haliotis diversicolor supertexta; post-larval abalone 1 INTRODUCTION hatcheries, diatom films on plastic plates have been used as a settlement substrate and an initial food Abalones are commercially important in many source for post-larvae (Ebert et al., 1984; Kawamura, countries. Mass culture of abalone began in the mid 1996; Nie et al., 2004). Successful seeding is closely 1980s in China and developed rapidly in the 1990s. related to the quantity and quality of benthic diatoms. Haliotis diversicolor supertexta is the species most At present, the dietary value of the different commonly cultured in south China, constituting diatom species is still not well understood. A number around 65% of the total yield of abalone in 2000 (Nie of studies have examined the relationship between et al., 2004). In recent years, nursery rearing has post-larvae growth and feeding behavior using a become more difficult because mortality occurs range of diatom species. The dietary value of benthic frequently during the first month after settlement. diatoms is influenced by their morphology, Maintaining viable juveniles past settlement is the attachment strength, frustule strength (Kawamura biggest challenge for abalone seed production. It is et al., 1995a, 1998a; Kawamura et al., 1995b; generally believed that an inappropriate diet is the Roberts et al., 1999), the biochemical composition of major cause of high mortality in this (Nie et al., 2004; Zhang et al., 2004) and other species of abalone (Gordon et al., 2006). * Supported by Special Project on Basic Research of China, Ministry of Science and Technology (No. 2006FY120100) and Natural Science Benthic diatoms are the primary food choice for Foundation of Fujian Province (No. B0510005; 2005YZ1024) early post-larval abalone. In abalone seed production ∗∗ Corresponding author: [email protected] 1188 CHIN. J. OCEANOL. LIMNOL., 28(6), 2010 Vol.28 the diatom cell contents (Dunstan et al., 1994; 832±84 µm, 1 285±192 µm, and 1 570±290 µm on Gordon et al., 2006), and digestibility. In addition, days 12, 16, 20, and 24, respectively. post-larval size has a significant effect on their ability We used a sterilized blade to collect a sample of to consume different sized diatoms (Xing et al., diatoms from a 10×10 cm2 area surrounding the 2008). However, research into juvenile abalone diets post-larvae. Each blade was rinsed with 10 mL has focused on the use of monocultured species of filtered sea water to flush the diatoms into a 10-mL benthic diatoms. Little is known about the use of sample bottle, to which a few drops of formalin were hatchery diets or feeding behavior in newly settled added. larvae. Most hatcheries rely on a benthic diatom 2.2 Treatment community that forms on plastic plates placed in the tanks. Thus, the post-larvae ingest a variety of The analysis of gut contents was conducted in diatoms, rather than a single species. Given this, accordance with the method of Norman-Boudreau et knowledge of post-larval feeding preferences in the al. (1986). The post-larval abalones were treated with natural environment is likely to be invaluable for EDTA and sodium hypochlorite to decalcify the shell producing diets that allow successful culture of and dissolve the tissue, leaving only the siliceous post-larval abalone. frustules of the diatoms for counting and The dietary preferences of post-larval abalone can identification. Frustules were preserved in distilled be determined by identifying the algal species that water. are present in the gut. If certain species are preferred, 2.3 Relative abundance they are likely to be over-represented in the gut contents. Spawning and larval rearing of To determine the cell density of diatoms in the gut H. diversicolor supertexta occurs between May and contents and on the films, we placed 1 mL of the June in hatcheries in Dongshan, Fujian Province on preserved sample into a chamber slide. The number the southern coast of China. We analyzed the of cells was enumerated under a phase contrast composition and structure of the diatom community microscope. In addition, we calculated the size of in the gut of post-larvae during this rearing period in each cell. We calculated relative abundance using the 2003. We also documented the species composition following formula: and abundance on the surrounding diatom films. We Relative abundance (%) = (N/N0) ×100 then compared the composition and abundance of diatoms in the gut and on the diatom films. where N is the number of cells of one species per mL, and N0 is the total number of cells per mL. 2 MATERIAL AND METHOD We ignored cell frustules that were too damaged to be identified. 2.1 Sampling We tentatively identified the diatoms during the We obtained H. diversicolor supertexta post- counting procedure. Identifications were made in larvae in-situ from an abalone farm in Dongshan more detail by scanning electron microscopy (Philips between May and June, 2003. The larvae were reared XL30) and transmission electron microscopy 3 at the hatchery in flow-through tanks (30 m ) filled (JEM2100). Diatom taxonomic determination was with plastic plates at a temperature of 24.7±0.5°C. based on Jin et al. (1965, 1982, 1992). Approximately 6 d after spawning, the larvae had settled on the plastic plates that had been coated with 3 RESULT a thin film of naturally occurring diatoms. Seawater 3.1 Diatom assemblages in the gut contents of (salinity 29.6) was pumped from Dongshan beach post-larval abalone and filtered before it entered the tanks. We randomly sampled 10 post-larvae from the We identified 40 diatom species belonging to 13 diatom films 12, 16, 20, and 24 d after settlement. genera in the gut contents of the post-larvae (Table 1). The larvae were dislodged with a fine needle, washed Among these, pennate diatom species (38 species, with filtered sea water to remove external diatoms, representing 11 genera) accounted for 95.0% of the then placed into a bottle and preserved in 4% total number. The most common genera were: formalin. We measured the shell length (SL) of each Navicula, Nitzschia, and Amphora. individual under a microscope with an ocular The diatom taxa shown in Fig.1 were graticule. The mean SLs were 654±111 µm, representative of the most common diatoms in the No.6 ZHANG et al.: Diatom diet of early post-larval abalone under hatchery conditions 1189 Table 1 List of diatom species identified from settlement films and also (†) from post-larval abalone guts Achnanthes amoena Hustedt * † Gyrosigma sp. † A. bremeyeri Lange-Bertalot * Hantzschia amphioxys (Ehr.) Grunow * A. brevipes Agardh † Haslea sp. † A. brevipes var. angustata (Grev.) Cleve † Licmophora californica Grunow A. brevipes var. parvula (Kuetz.) Cleve * † L. flabellata (Carm.) Agardh A. campechiana Hustedt Melosira moniliformis (O. F. Muller) Agardh A. citronella (Mann) Hustedt M. juergensi Agardh * A. delicatula (Kuetz.) Grunow * Navicula britannica Hustedt et Aleem A. javanica var. subcontricta Meister N. complanata Grunow A. punctifera Hustedt N. duerrenbergiana Hustedt Achnanthes sp. 1 * † N. cryptocephala var. veneta (Kuetz.) Rabenhorst* † Achnanthes sp. 2 * N. gregaria Donkin * Achnanthes sp. 3 * N. johanrossii Giffen Actinocyclus ehrenbergi Ralfs N. mollis (W. Sm.) Cleve * Actinoptychus undulates (Bail.) Ralfs N. perminuta Grunow * † Amphipleura rutilans (Trent.) Cleve N. ramosissima (Ag.) Cleve † Amphiprora alata (Ehr.) Kuetzing * N. orthoneoides Hustedt * Amphora angusta Gregory † N. retrocurvata J. R. Carter * A. coffeaeformis (Ag.) Kuetzing * † N. subminuscula Manguin * † A. coffeaeformis var. borealis (Kuetz.) Cleve † N. voigtii Meister * A. copulata (Kuetz.) Schoeman & Archibald * † Navicula sp. 1 * † A. cymbelloides Grunow * Navicula sp.