Fish & Shellfish Immunology (2001) 11, 187–197 doi:10.1006/fsim.2000.0304 Available online at http://www.idealibrary.com on Effects of short-term crowding stress on the gilthead seabream (Sparus aurata L.) innate immune response J. ORTUN x O, M. A. ESTEBAN AND J. MESEGUER* Department of Cell Biology, Faculty of Biology, University of Murcia, 30071 Murcia, Spain (Received 7 July 1999, accepted after revision 28 August 2000) Gilthead seabream specimens were subjected to an intense short-term crowd- ing stress of 100 kg m 3 for 2 h. After 0, 1, 2, 3 and 4 days, blood glucose and serum cortisol levels, serum complement activity, phagocytic and respiratory burst activities of head-kidney leucocytes, and the percentage of monocyte/ macrophages and granulocytes in head-kidney and circulating blood were determined. An immediate e#ect of the stress was a depression in complement and phagocytic activities, both of which recovered after 3 or 2 days, respect- ively, while respiratory burst remained una#ected. The depression of phago- cytosis in head-kidney leucocytes seemed to correlate with stress-induced migration of active cells from the organ to circulating blood. The present results point to the importance of minimising intense short-term crowding stress in order to reduce possible states of immunodepression in farmed fish.  2001 Academic Press Key words: short-term crowding stress, innate immune response, comp- lement, phagocytosis, respiratory burst, cell tra$cking, gilthead seabream. I. Introduction The physiological e#ects of environmental stressors in fish depend on the duration and on the intensity of such stressors (Mosconi et al., 1998; Pickering, 1998). An acute stress response can be induced in fish as a result of short-term mild stressful conditions (Demers & Bayne, 1977; Rotllant & Tort, 1997; Ruis & Bayne, 1997), while a chronic stress response is induced in fish as a result of long-term severe stressful conditions (Pickering, 1998). The impact of these two categories of stress (acute and chronic) on the fish immune system has been the object of much study in recent years. The enhancement of fish immune response, rather than its immunosuppression, has been observed to follow acute stress (increased concentrations of specific plasma proteins, such as lysozyme or complement, as well as enhanced yeast killing by phagocytes), leading to a better protection against any possible damage (Demers & Bayne, 1997; Ruis & Bayne, 1997). Deleterious e#ects on several humoral and cellular immune parameters have also been observed as a consequence of chronic stress (Yin et al., 1995; Tort et al., 1996b; Bly et al., *Corresponding author. E-mail: meseguer@fcu.um.es 187 1050–4648/01/020187+11 $35.00/0  2001 Academic Press