THE INFLUENCE OF SOCIAL ENVIRONMENT ON SEX DETERMINATION IN HARLEQUIN SHRIMP (HYMENOCERA PICTA: DECAPODA, GNATHOPHYLLIDAE) G. Curt Fiedler Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, 3422 Sesoko, Motobu-cho, Okinawa 905-0227, Japan (e-mail: kaatosan@hotmail.com) ABSTRACT Harlequin shrimp, Hymenocera picta, are monogamous and pair-bonding, and are usually found in isolated singles and pairs in the field. The apparent rarity of this species in their habitat and high levels of aggression between consexuals suggests the possibility of some sort of plasticity in their primary sex determination. In this study, the influence of social environment upon primary sex determination in H. picta was examined experimentally in the laboratory. Naı ¨ve juveniles were placed in three novel social environments: paired juveniles, single juveniles, and adult/juvenile pairs. Spacing behavior and the onset of external differentiation were observed during long-term experiments for each social treatment. Gonadal development was also observed. Spacing behavior of paired shrimp reflected the sexual composition of social groups; high intrapair distance (IPD) for same-sex pairs and low IPD for opposite-sex pairs. Sex determination results were not so clear. Two of the three paired juvenile replicates displayed phenotype frequencies different from those expected from a random sample of gonochoristic organisms with a 1:1 sex ratio. However, the third replicate and the combined frequencies did not show any statistical difference from the hypothetical random sample. Furthermore, single juveniles and those paired with adults expressed sex in nearly 1:1 ratios, regardless of the sex of adult conspecifics. Histological examination of juvenile shrimp confirmed that sex was determined as early as four weeks after larval metamorphosis. Therefore, social environment had no clear effect upon sexual phenotype expressed. However, single females attained puberty at a significantly greater age and larger size than did females paired with males. Hence for H. picta, social environment had a measurable effect on the timing of female puberty. This is the first demonstration of this phenomenon in decapod Crustacea. The phenomenon of environmental sex de- termination (ESD) (Bull, 1983) is well docu- mented in many teleost fish families in the form of some sort of sequential hermaphrodit- ism (Atz, 1964; Smith, 1975; Yogo, 1987). The best such examples in fish involve the social environment and its effect on sex (e.g., Fricke and Fricke, 1977; Moyer and Nakazono, 1978; Warner, 1982). In crustaceans, however, there appears to be little direct evidence of so- cial control of functional sex. The best known examples occur in parasitic bopyrid isopods and protandrous hermaphrodite shrimp of the family Pandalidae. In bopyrid examples, the first individual to reach a host becomes female and may cause subsequent conspecifics to be- come male (Reverberi, 1944; Reinhard, 1949; Charniaux-Cotton et al., 1983). In pandalid shrimp, the timing of male-female sex change is thought to be influenced by the demo- graphics of their populations (Charnov, 1982). However, the mechanism of such a system is in doubt (Bergstro ¨m, 1997). Examples of hermaphroditism in the mala- costracan Crustacea are relatively uncommon and usually are in the form of protandrous sex change. Hermaphroditism is known from many species in a few subgroups of isopods and tanai- daceans (Charniaux-Cotton, 1975; Policansky, 1982). However, only approximately 42 species of decapod crustaceans are thought to be hermaphroditic, and 35 of these are caridean shrimp (see lists in Carpenter, 1978; Policansky, 1982; Bauer, 2000) (and for additional exam- ples see Kagwade, 1981; Sukumaran, 1981; Bauer, 1986; Nakashima, 1987; Gherardi and Calloni, 1993; Gavio et al., 1994; Rudolph, 1995; de Almeida and Buckup, 1997, 2000; Bauer and Holt, 1998; Fiedler, 1998). Further- more, at least two hippolytid carideans in the genus Lysmata display a form of simultaneous hermaphroditism (Bauer and Holt, 1998; Fiedler, 1998). Because marine decapods share many of the same ecological and habitat selec- tion pressures with teleost fishes, it is surpris- ing to find so few examples in this diverse 750 JOURNAL OF CRUSTACEAN BIOLOGY, 22(4): 750–761, 2002