Vol. 12: 191-197, 1983 MARINE ECOLOGY - PROGRESS SERIES Mar. Ecol. Prog. Ser. I Published May 30 Genetic differentiation between Penaeus kerathurus and P. japonicus (Crustacea, Decapoda) E. De Matthaeis, G. Allegrucci, A. Caccone, D. Cesaroni, M. Cobolli Sbordoni and V. Sbordoni Institute of Zoology, University of Rome, Viale dell'UniversitA. 32.1-00185 Rome. Italy ABSTRACT: Genetic differentiation and variability data have been obtained by electrophoretic analysis of 2 species of prawns (Penaeus kerathurus and P. japonicus). Both are fished commercially and used in either commercial or experimental aquaculture systems. Genetic distance calculated according to Nei's method (D = 0.887) shows the 2 species to be very different genetically. Similar values have frequently been found in other animal groups between congeneric species that are phylogenetically distant or between species in different genera. The contribution of different loci to the genetic distance, with respect to their metabolic role, is briefly discussed. Variability estimates for the 2 species differ cons~derably. In P, japonicus, mean heterozygosity expected at equilibrium (He)is 0.121; mean number of alleles per locus (A)is 1.484,and frequency of polymorphic loci (P)is 0.387. Respective values for P. kerathurus are Fie = 0.055, A = 1.265 and P = 0.265. Stochastic and deterministic factors that may possibly explain high heterozygosity in P. japonicus are hypothesized. Future research requires natural populations to be examined. INTRODUCTION Decapod crustaceans have recently been the object of electrophoretic studies (Lester, 1979; Mulley and Latter, 1980; Nelson and Hedgecock, 1980).However, only a few studies are concerned with commercially important decapod species (Hedgecock et al., 1976; Malecha et al., 1980), although they have been well studied both ecologically and physiologically. In the Mediterranean, Penaeus kerathurus (Forskal)has been considered for aquaculture since 1970 (Lumare, 1976). More recently, farming of P. japonicus Bate has been the object of initial research programmes (Lumare, 1980; Lumare and Palmegiano, 1980). This paper reports initial data on genetic divergence and variability obtained by electrophoretic analysis of Penaeus kerathurus and P. japonicus enzymes and proteins. P, kerathurus is found in Mediterranean and Eastern Atlantic regions (from Portugal to Angola). Adults live in muddy-bottom shelf regions in shallow- water environments (40 to 45 m deep) during winter; in summer they migrate to reproduce in coastal areas with low-salinity waters. P. japonicus has a wide geo- O Inter-Research/Printed in F. R. Germany graphical distribution in the Indo-West Pacific region and Mediterranean coastal waters of Turkey, Israel and Cyprus. Adults live on sand bottoms in shallow- water environments during winter. During spring- summer they migrate to reproduce in estuarine waters. Both species are eurythermic and euryhaline and show nocturnal activity. Our study sets out to establish the degree of genetic differentiation between Penaeus kerathums and P. japonicus in view of possible hybridization program- mes in which they might be involved. In addition, we have attempted to ascertain genetic variability levels, which could, in principle, be associated with the differ- ent physiological adaptive features shown by the 2 species. MATERIALS AND METHODS Samples of Penaeus kerathurus and P, japonicus were obtained from Italy's National Research Council's Istituto per 10 Sfruttamento Biologico delle Lagune (Institute for the Biological Exploitation of Lagoons) at