Short communication Qualitative and quantitative effects of constant light photoperiod on rainbow trout (Oncorhynchus mykiss ) peripheral blood erythrocytes Ariel E. Valenzuela a, * , Victor M. Silva b , Alfredo E. Klempau c a Graduate Programme in Biological Sciences, University of Concepcio ´n, PO Box 160-C Concepcio ´n, Chile b Faculty of Pharmacy, Dept. of Clinical Biochemistry and Immunology, University of Concepcio ´n, Chile c Faculty of Natural Sciences and Oceanography, Dept. of Oceanography, University of Concepcio ´n, Chile Received 21 August 2004; received in revised form 15 June 2005; accepted 15 June 2005 Abstract Immature erythrocytes or polychromatophils are commonly found in low percentages and at different maturational stages in fish peripheral blood. The concentration of these cells is indicative of the erythropoietic status, which can be modulated by environmental factors such as oxygen availability and temperature. To assess the consequences of constant artificial light on erythropoiesis, polychromatophils were quantified in two groups of rainbow trout. At the end of the experiment, fish exposed to two months of constant light showed an increase of immature erythrocytes from 0.07 Â 10 12 to 0.11 Â 10 12 cells/l along with a marked decrease in their polychromatophilic intensity, which almost disappeared 5 months after resuming natural photoperiod. In this group after an initial increase, a decrease in immature erythrocyte numbers was observed, suggesting that application of constant light causes a late depression of erythropoiesis in trout. The transient increase of polychromatophils in the same maturity state may be an indicator of a deviation towards other metabolic needs, such as initiation of reproduction. D 2005 Elsevier B.V. All rights reserved. Keywords: Haematology; Peripheral blood; Erythropoiesis; Polychromatophils; Oncorhynchus mykiss ; Artificial photoperiod 1. Introduction In fish peripheral blood, rounder erythrocytes (as compared to ellipsoidal mature erythrocytes) are gen- erally found, showing less dense chromatin and a violaceous cytoplasm (Fijan, 2002). These cells are called reticulocytes, if stained with bright cresil blue or polychromatophils, if stained with May Gru ¨ nwald- Giemsa. This latter by the cytoplasm staining ambi- guity, caused by a rosy colour due to haemoglobin affinity for eosin and a blue colour of stained ribo- somes in active protein synthesis (basophilia) (Beams and Sekhon, 1966). Thus, the less mature the erythro- cytes, the greater their basophilia. In peripheral blood, greater numbers of mature than immature erythrocytes are found and this is in direct proportion to their 0044-8486/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2005.06.012 * Corresponding author. Tel.: +56 41 204502; fax: +56 41 225400. E-mail address: avalenz@udec.cl (A.E. Valenzuela). Aquaculture 251 (2006) 596 – 602 www.elsevier.com/locate/aqua-online