Genetic cryptic species as biological invaders: the case of a Lessepsian fish migrant, the hardyhead silverside Atherinomorus lacunosus Giuseppe Bucciarelli a,1 , Daniel Golani b , Giacomo Bernardi a, * a Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95064, USA b Department of Evolution, Systematics and Ecology, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel Received 19 November 2001; received in revised form 8 March 2002; accepted 19 March 2002 Abstract Marine cryptic species, taxa that are morphologically identical but genetically distinct, may be important and underestimated components of the ecosystem. The understanding of several ecological interactions, such as marine bioinvasions, could be altered by the correct description of the bioinvaders. Here, we have focused our study on the hardyhead silverside, Atherinomorus lacunosus, a Lessepsian migrant. Lessepsian migrants are those species that are invading the Mediterranean from the Red Sea via the Suez Canal. We PCR amplified and sequenced the mitochondrial control region from individuals collected from the Mediterranean and the northern and southern Red Sea. We found that the two Red Sea populations are likely to correspond to two previously undescribed cryptic species. We also found that the Mediterranean individuals group with the northern Red Sea species. The Mediterranean population showed high levels of genetic diversity and did not share haplotypes with the northern Red Sea population. Lessepsian invasion by A. lacunosus probably occurred repeatedly and is likely to still be occurring. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Lessepsian migrants; Atherinomorus lacunosus; Cryptic species; Biological invasions 1. Introduction The number of cryptic species, taxa that cannot be distinguished morphologically but are genetically distinct, may be vastly underestimated in the marine environment 0022-0981/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0022-0981(02)00138-7 * Corresponding author. Tel.: +1-831-459-5124; fax: +1-831-459-4882. E-mail address: bernardi@biology.ucsc.edu (G. Bernardi). 1 Present address: Stazione Zoologica ‘‘Anton Dohrn’’, Villa Comunale I, 80121 Naples, Italy. www.elsevier.com/locate/jembe Journal of Experiment Marine Biology and Ecology 273 (2002) 143 – 149