Reproductive and developmental biology of rice field crab Esanthelphusa nani (Naiyanetr, 1984)

Abstract

The rice field crabs are freshwater crabs that can be considered as pest in paddy field as well as a stable food source by local people. Either of these roles requires a good knowledge of its reproductive biology to manage population in nature. In this study, reproductive and endocrine cycles of male and female, and development of embryo were examined. Adult Esanthelphusa nani were field caught from Nan province during November 2015 to November 2016. Crabs were euthanized and dissected for reproductive tracts. Accessory organ nearby the mandibular muscle was collected for endocrine activities in male, while hemolymph was collected for vitellogenin measurement in female. Gonadosomatic index (GSI) was calculated based on the relative weight of the reproductive tract and body weight. Male GSI showed a significant temporal difference with the peak in wet season (September). Testis can be divided into 3 stages with increasing in size from 1) resting, 2) developing, and 3) active stages. Microscopic examination revealed the different activity among three stages. Meanwhile, mature sperms can be found in male reproductive tract throughout the year, indicating prolonged spermatogenic activities. Moreover, mature sperms can be deposited and found in female spermatheca. Female GSI showed two significant peaks in dry season (February) and wet season (June and August). Female GSI profile is in agreement with levels of phospholipoprotein in hemolymph. Ovaries changed in size and color due to presence of yolk and mature oocytes. Microscopic examination of ovarian tissue showed that proportions of oocyte class in ovarian tissue could be used as an important tool for identifying ovarian cycle into five stages including 1) resting, 2) vitellogenic, 3) pre-mature, 4) mature, and 5) spawning and regenerating stages. Ovigerous crabs found during March and April, 2016 were collected for embryonic study. Eggs were randomly sampled and fixed in ethanol. Eggs were initially examined and recorded for embryonic stage under a stereomicroscope. Afterward, fluorescent technique was applied to observe morphology and identify stage of development under a confocal microscope. It was found that the embryonic development of E. nani encompass egg-cleavage, egg-blastula, egg-gastrula, egg-nauplius, egg-zoea and egg-megalopa, and can be completed within 12 days. Unlike other freshwater crab, E. nani larvae hatched at megalopa stage and took a few hours to metamorphose to juveniles. The normal developmental stage is finally established for E. nani