THE CYST HATCHING PATTERN OF THE THAI FAIRY SHRIMP, BRANCHINELLA THAILANDENSIS SANOAMUANG, SAENGPHAN & MURUGAN, 2002 (ANOSTRACA) BY NUKUL SAENGPHAN 1 ), RUSSELL J. SHIEL 2 ) and LA-ORSRI SANOAMUANG 1,3 ) 1 ) Applied Taxonomic Research Center, Department of Biology, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand 2 ) Department of Environmental Biology, University of Adelaide, South Australia 5005, Australia ABSTRACT We investigated factors influencing the hatching of the fairy shrimp, Branchinella thailandensis in relation to brood order, and also responses of cysts to wet and dry periods. Male-female pairs of B. thailandensis were cultured under static conditions at room temperature and fed ad libitum micro-algae (Chlorella sp.). Deposited cysts of the first, sixth and eleventh broods were monitored for cyst hatching under wet and dry conditions. Cyst hatching was scored daily for 5 days after incubation. Freshly laid cysts, immediately incubated, did not hatch in any of the experimental conditions (wet/dry). On the contrary, undried cysts immersed in their parental medium for 4 weeks showed the highest hatching in all brood treatments (76.67, 94.67, and 99.33% of the first, sixth, and eleventh broods, respectively). The cysts of the first brood hatched less successfully than did those of the sixth and eleventh broods. Cyst hatching mostly took place within 24 hours after incubation. The present study shows that a wet period has a strong influence on the cyst hatching success of B. thailandensis. The cysts require a period of retention in the parental medium for 2-4 weeks to complete their embryonic development before hatching. Hatching of the three representative broods showed a different pattern for each brood. The early broods required a longer wet period than did the late broods. In contrast to the hatching behaviour of many other species, drying is not absolutely essential for cyst hatching of this species. RÉSUMÉ Nous avons recherché les facteurs agissant sur l’éclosion de l’anostracé Branchinella thailanden- sis par rapport à l’ordre de ponte, et également les réponses des œufs de durée aux périodes humides et sèches. Des couples de B. thailandensis ont été élevés en conditions constantes à la température ambiante et nourris à volonté avec des micro-algues (Chlorella sp.). Les œufs déposés lors de la première, de la sixième et de la onzième pontes ont été suivis pour observer leur éclosion en condi- tions humides et sèches. L’éclosion des œufs a été notée chaque jour pendant 5 jours après le début de l’incubation. Les œufs fraîchement pondus, immédiatement incubés, n’ont pas éclos, quelles que 3 ) Corresponding author; e-mail: la_orsri@kku.ac.th © Koninklijke Brill NV, Leiden, 2005 Crustaceana 78 (5): 513-523 Also available online: www.brill.nl