Journal of Fish Biology (2013) 82, 658–670 doi:10.1111/jfb.12020, available online at wileyonlinelibrary.com Ontogeny of body density and the swimbladder in yellowtail kingfish Seriola lalandi larvae L. D. Woolley and J. G. Qin* School of Biological Sciences, Flinders University, G. P. O. Box 2100, Adelaide, SA, 5001, Australia (Received 2 March 2012, Accepted 8 November 2012) The ontogeny of larval body density and the morphological and histological events during swim- bladder development were investigated in two cohorts of yellowtail kingfish Seriola lalandi larvae to understand the relationship between larval morphology and body density. Larvae <3 days post hatch (dph) were positively buoyant with a mean ± s.d. body density of 1·023 ± 0·001 g cm −3 . His- tological evidence demonstrated that S. lalandi larvae are initially transient physostomes with the primordial swimbladder derived from the evagination of the gut ventral to the notochord and seen at 2 dph. A pneumatic duct connected the swimbladder to the oesophagus, but degenerated after 5 dph. Initial swimbladder (SB) inflation occurred on 3 dph, and the inflation window was 3–5 dph when the pneumatic duct was still connected to the gut. The swimbladder volume increased with larval age and the epithelial lining on the swimbladder became flattened squamous cells after initial inflation. Seriola lalandi developed into a physoclist with the formation of the rete mirabile and the gas-secreting gland comprised low-columnar epithelial cells. Larvae with successfully inflated swimbladders remained positively buoyant, whereas larvae without SB inflation became negatively buoyant and their body density gradually reached 1·030 ± 0·001 g cm −3 by 10 dph. Diel density changes were observed after 5 dph, owing to day time deflation and night-time inflation of the swimbladder. These results show that SB inflation has a direct effect on body density in larval S. lalandi and environmental factors should be further investigated to enhance the rate of SB inflation to prevent the sinking death syndrome in the early life stage of the fish larvae. © 2013 The Authors Journal of Fish Biology © 2013 The Fisheries Society of the British Isles Key words: buoyancy; fish larvae; morphology; rete mirabile. INTRODUCTION Yellowtail kingfish Seriola lalandi (Valenciennes 1833), belonging to the family Carangidae, occurs primarily in the cool temperate waters of the Pacific and Indian Oceans (Kolkovski & Sakakura, 2007). At present, the average survival rate of S. lalandi larvae is <20% under controlled laboratory conditions (Fielder & Heasman, 2011), which is unsatisfactory for larval fish production at a commercial scale. High mortality occurs throughout the larval stage before metamorphosing into juveniles (Tanaka et al ., 2009). Factors contributing to high mortalities include inadequate nutrition and environmental factors, deformities and swimbladder malformations. *Author to whom correspondence should be addressed. Tel.: +61 8 82013045; email: jian.qin@ flinders.edu.au 658 © 2013 The Authors Journal of Fish Biology © 2013 The Fisheries Society of the British Isles