Chinese Journal of Oceanology and Limnology Vol. 31 No. 2, P. 267-281, 2013 http://dx.doi.org/10.1007/s00343-013-2150-y Morphotaxonomy and seasonal distribution of planktonic and benthic Prorocentrales in Karachi waters, Pakistan Northern Arabian Sea* Sonia MUNIR 1, ** , Zaib-un-nisa BURHAN 1 , Tahira NAZ 1 , P. J. A. SIDDIQUI 1 , Steve L. MORTON 2 1 Centre of Excellence in Marine Biology, University of Karachi-75270, Pakistan 2 National Oceanic and Atmospheric Administrations, 219 Fort Johnson Road, Charleston SC29412, USA Received Jun. 7, 2012; accepted in principle Aug. 14, 2012; accepted for publication Nov. 19, 2012 © Chinese Society for Oceanology and Limnology, Science Press, and Springer-Verlag Berlin Heidelberg 2013 Abstract Morphotaxonomy and seasonal abundance of dinoflagellates of the genera Prorocentrum and Mesoporos (Prorocentrales) were studied from nutrient-rich waters, Karachi Harbor and the mouth of the Manora Channel, Pakistan during May 2002–July 2003. Using both light and scanning electron microscopy, 13 species of Prorocentrales were identified according to cell shape, size, ornamentation of thecal plates, and architecture of apical platelets, apical pore area, marginal pores, and intercalary bands. P. sigmoides, P. arcuatum, P. scutellum, P. donghaiense, P. balticum, P. minimum, P. emarginatum, P. lima, P. faustiae, and Mesoporos perforatus constitute new records for sindh coast of Pakistan. The most abundant species were P. minimum/P. balticum (4.5×10 3 cells/L), P. micans (1.1×10 3 cells/L), P. gracile/ P. sigmoides (2.5×10 2 cells/L) and P. donghaiense (6.6×10 3 cells/L) at temperatures of 29–31°C and salinities of 35–40. Maximum abundance was observed in winter and lower abundance in summer. There was no significant change in the distribution of species between stations except for the benthic species which occurred close to Karachi Harbor waters. Significant positive correlations were observed between Prorocentrum spp. and temperature ( R 2 =0.27) and negative correlations with salinity ( R 2 =-0.32) except for P. minimum and P. emarginatum which has negative correlation with temperature (R 2 =-0.24) and positive with salinity (R 2 =0.08, 0.19). The finding of potential okadaic-acid producing species of benthic Prorocentrum call for monitoring for possible human health problems in this region. Keyword: ecology; morphology; planktonic/benthic Prorocentrales; harmful algae; scanning electron microscopy; northern Arabian Sea 1 INTRODUCTION Harmful algae and phycotoxin production by dinoflagellates is a global and economic problem throughout coastal areas of the world, generating adverse effects on marine fish resources and human health (Nakajima et al., 1981; Anderson et al., 1989; Hallegraeff, 1995). Planktonic dinoflagellate species such as Prorocentrum micans Ehrenberg, P. gracile Schutt, P. sigmoides and P. triestinum Schiller, P. arcuatum Issle, P. compressum (Bailey) Abé ex Dodge, P. balticum (Lohmann) Loeblich III, P. dentatum Stein have been reported to cause blooms and occasionally fish kills (Steidinger, 1983), and are distributed worldwide (Dodge, 1975). To date, none of these planktonic species have been found to produce toxic compounds. During the past two decades, many new epiphytic/benthic species of Prorocentrum have been described using both morphological and molecular characters. Many of these species produce the toxin, okadaic acid (Murakami et al., 1982; Faust, 1990, 1993a, 1993b, 1994; Morton, 1998; Morton et al., 1998; Holmes et al., 2001; Hoppenrath and Leander, 2008; Aligizaki et al., 2009, Nagahama et al., 2011, Chomerat et al., * Supported by Scholarship (IRSIP) of Higher Education Commission, Islamabad, Pakistan for Doctoral student Miss. Sonia Munir ** Corresponding author: soniaku2003@yahoo.com