Hydrobiologia 386: 27–36, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 27 Studies on the salinity and desiccation tolerances of Helice tridens and Helice japonica (Decapoda: Grapsidae) Koji Omori ∗ , Bambang Irawan & Yoshihisa Kikutani Department of Biology, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790, Japan ∗ Author for correspondence (E-mail: ohmori@sci.ehime-u.ac.jp) Received 17 March 1998; in revised form 4 September 1998; accepted 16 September 1998 Key words: desiccation, salinity, Helice tridens, Helice japonica, western Japan Abstract In the Shigenobu River Estuary (33 ◦ 48 ′ 18" N, 132 ◦ 41 ′ 10" E), Matsuyama (Japan), Helice tridens inhabits the mid-intertidal zone to the limit of the high tide, while H. japonica is found around the mid-intertidal zone. In addition, H. japonica is distributed further downstream than H. tridens. These differences suggest that H. japonica has a lower salinity and desiccation tolerance than H. tridens. Laboratory experiments on salinity tolerance confirmed the higher tolerance of H. tridens compared to H. japonica. Helice tridens survived at least a week under fresh water conditions, whereas most of the H. japonica died. Helice tridens was shown to be more able to tolerate desiccation than H. japonica, through behavioral, not physiological, adaptation to dry conditions. The present results demonstrate a stronger adaptation of H. tridens to semi-terrestrial and/or freshwater environments than H. japonica. Introduction Previous studies of grapsid crabs have demonstrated differences in ecological distributions between sym- patric species in an estuary, and some factors influen- cing these differences have been identified (Hartnoll, 1965; Warner, 1969; Griffin, 1971; Jones & Simons, 1982). The results of these various studies indicate that the characteristics of the substratum, as well as stress due to desiccation and salinity have important effects on the vertical and horizontal distributions of grapsid crabs in the intertidal zone. Amongst these factors, salinity is the most important factor controlling dis- tributions within a defined tolerance range; outside this range other factors may take over primary con- trol (Kinne, 1971; Jones & Simons, 1982). Jones & Simons (1982) reported that the vertical separation of Macrophthalmus hirtipes and Helice crassa appears to be the result not of interspecific differences in desic- cation tolerance but rather of feeding and burrowing behaviors related to the different substrata they oc- cupy. In some cases, biological interactions between species, for example, Helice tridens and Macrophthal- mus japonicus, may be a more important factor in the determination of small-scale distribution (Kurihara et al., 1989a, b). The closely related mud crabs Helice tridens de Haan and H. japonica Sakai and Yatsuzuka occur sympatrically in high densities in estuaries and salt marshes throughout western Japan (Sakai, 1976; Sakai & Yatsuzuka, 1979). In the Shigenobu River Estuary (33 ◦ 48 ′ 18" N, 132 ◦ 41 ′ 10" E), Matsuyama, they are horizontally separated, with H. tridens inhabiting the entire estuary, but Helice japonica being restricted to the lower estuary (Omori et al., 1997). In the present study, we examined the salinity and desiccation toler- ance of Helice tridens and H. japonica in an attempt to explain these distributions. Materials and methods Field measurements In 1990, six sampling stations were chosen in the Shigenobu River Estuary (Figure 1) to reconfirm the