The effect of dietary administration of the fungus Mucor circinelloides on non-specific immune responses of gilthead seabream Alejandro Rodrı ´guez, Alberto Cuesta, M. A ´ ngeles Esteban, Jose ´ Meseguer* Department of Cell Biology, Faculty of Biology, University of Murcia, Campus Universitario de Espinardo, Espinardo, 30100 Murcia, Spain Received 10 March 2003; accepted 3 June 2003 Abstract The immunostimulant potential of the whole fungus Mucor circinelloides administered in the diet to gilthead seabream (Sparus aurata L.) was studied. Three different lyophilised strains were used: the wild-type (R7B) and two mutant strains in the carotene-production pathway which are enriched in lycopene (MU224) or -carotene (T31). Fish were fed diets containing 0 (control) or 10 g of one of the whole M. circinelloides strains per kg feed. After 2, 4 or 6 weeks of treatment the growth rate as well as humoral (lysozyme activity) and cellular (phagocytosis and cytotoxicity) immune responses were determined. The specific growth rate increased slightly with all the M. circinelloides- supplemented diets. Serum lysozyme activity increased slightly (P>0.05) in fish fed the mutant strain-supplemented diets. Of the cellular responses, phagocytosis significantly increased after 6 weeks, in fish fed the wild-type strain-supplemented diets while cytotoxicity increased after 4 weeks in fish fed the -carotene-enriched strain- supplemented diet. The immune responses were increased to some degree by diets containing whole M. circinelloides strains. These results are discussed in the light of the on-going search for new microorganisms, wild or mutant-type, for use as immunostimulants in fish farming.  2003 Elsevier Ltd. All rights reserved. Keywords: Mucor circinelloides; -carotene; Lycopene; Innate immunity; Leucocytes; Gilthead seabream (Sparus aurata L.) 1. Introduction Several whole microorganisms, live or not, such as bacteria, fungi or algae, increase disease resistance in mammals and fish [1,2]. In fish, as in other aquatic organisms, the whole microorganisms administered have mainly been bacterial species, which in the form of feed additives, have been shown to improve the intestinal microbial balance and increase the health status of fish, seemingly by colonising the gut and acting as * Corresponding author. Tel.: +34-968-364-965; fax: +34-968-363-963 E-mail address: meseguer@um.es (F. Meseguer). Fish & Shellfish Immunology 16 (2004) 241–249 Fish & Shellfish Immunology www.elsevier.com/locate/fsi 1050-4648/04/$ - see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1050-4648(03)00082-2