Jellyfish are marine invertebrates belonging to the phylum Cnidaria. Mastigias papua is a jellyfish from the Scyphozoa class. This study investigates differences in environmental parameters, morphology, and shape of nematocyte cells using the histological method with (SEM) Scanning Electron Microscopy on M. papua jellyfish Kakaban Lake and Sea. Environmental parameters in Kakaban Lake and Kakaban Sea were measured for salinity, temperature, and pH. The t-test analysis results showed significant differences in environmental parameters in Kakaban Lake and Kakaban Sea (p < 0.05). Body diameter and length of jellyfish tentacles were measured and analyzed using SPSS 16 with a t-test. The t-test analysis showed that for M. papua jellyfish in Kakaban Lake and jellyfish in the Kakaban Sea showed a significant difference in the diameter and length of the body tentacles (p < 0.05). The SEM results of jellyfish in the Kakaban Sea showed that the tentacles of M. papua in the Kakaban Sea had nematocytes consisting of three forms, namely microbasic isorhiza, atrichouz isorhiza, and merotrichous isorhiza. M. papua in Kakaban Lake has nematocyst cells, which comprise of one type, namely Microbasic mastigophoran, where the number of nematocytes is minimal and has a smaller size than the Papuan Mastigias jellyfish that live in the Kakaban Sea. Nemeatocyte cells are stinging cells in jellyfish. SEM results show that the jellyfish in Kakaban Lake cannot sting because the size of the jellyfish tentacles is reduced to smaller, fewer nematocyte cells and different shape nematocytes. Based on the results of this study, it can be concluded that there are differences in environmental parameters, body dimensions, tentacle length, and the size and number of nematocytes between M. papua jellyfish in Dunau and in the Kakaban Sea.