The differential leucocyte landscape of four teleosts: juvenile Oncorhynchus kisutch, Clupea pallasi, Culaea inconstans and Pimephales promelas K.B.T IERNEY ,A.P.F ARRELL * AND C.J.K ENNEDY Department of Biological Sciences, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada (Received 6 September 2003, Accepted 14 June 2004) The appearance and proportions of morphologically distinct leucocytes (monocytes, thrombo- cytes,lymphocytesanddiverseformsofgranulocytes)encounteredinthebloodoffourteleost species: coho salmon Oncorhynchus kisutch, Pacific herring Clupea pallasi, brook stickleback Culaea inconstans and fathead minnow Pimephales promelas, are presented, along with examples of how water quality influenced their relative proportions. The results clearly show thatrelativeleucocytenumberrespondssignificantlyanddifferentlytochangesinwaterquality. The value of using differential white cell counting as a quick and inexpensive component of assayingimmunesystemalterationsisdiscussed. # 2004TheFisheriesSocietyoftheBritishIsles Key words: basophil; eosinophil; leucocrit; leucocytes; monocyte; water quality. INTRODUCTION Invertebrates,whitebloodcells(leucocytes)areaprimarylineofimmunological defence.Accordingly,oneofthemostelementarywaystoassesstheimmunesystem is to explore changes in the number or appearance of the four main types of circulating leucocytes (lymphocytes, thrombocytes, granulocytes and monocytes). Oftheinexpensivemethodsavailabletoexaminethesecells,thedifferentialwhite blood cell (DWBC) count is one of the most time-honoured, having been in use with fishes for nearly a century (Ellis, 1977). The DWBC count succeeds with simplicity:adropofsamplebloodissmearedacrossamicroscopeslide,theslide isair-dried,stained,andleucocytesarecountedundermagnification.Testnumbers arecomparedtostandardsorsubjecthistoryprovidingquickresults.Thetesthas withstoodthetestoftimebecauseitdoesnotrequireextensivetraining,isinexpen- sive,rapid(minutes)andeasilyperformedinthefield,anditcanrespondtochanges intheorganism’sphysiologyandenvironment.Forexample,changesinleucocyte number and appearance have been correlated to alterations in immune system performance(Ransom et al.,1984;Hine&Wain,1988)andinfectiveagentssuch as Renibacterium salmoninarum (thecausativeagentofbacterialkidneydiseaseor *Author to whom correspondence should be addressed. Tel.: þ16042913647; fax: þ16042913496; email: farrell@sfu.ca JournalofFishBiology (2004) 65, 906–919 doi:10.1111/j.1095-8649.2004.00491.x,availableonlineathttp://www.blackwell-synergy.com 906 # 2004TheFisheriesSocietyoftheBritishIsles