Introduction: Chitons are marine molluscs, mostly grazers, resistant to tidal conditions and often living in crevices of rocks. The Oman Sea is located in the southeast of Iran and the northwest of the Indian Ocean and is heavily influenced by the monsoon winds of the Indian Ocean. Due to the occurrence of the monsoon process, the Makoran Sea has a rich variety of marine species, including molluscs. The aim of the present research was to investigate the density and distribution of Acanthopleura vaillantii in the beaches of Pozm, Konarak, Tis, Kolbe Ghawasi, Darya Bozorg and Ramin in winter 2018 and summer 2019.
Materials and Methods: In this study, the density and distribution of A. vaillantii was investigated in transects perpendicular to the sea and in the tidal zone, using a quadrat (1*1 m²). in the tidal zone of six stations on the coasts of Pozm, Konarak, Tis, Kolbe Ghavasi, Darya Bozorg and Ramin. A number of samples in each station were taken to the laboratory to measure biological parameters such as length, total wet weight, soft tissue moisture content, tissue moisture percentage and dry weight percentage. In each station, physical and chemical factors of water (temperature-salinity and acidity) were measured and recorded by WTW device.
Results: Investigations showed that the highest average density in the winter of 2018 was in Ramin station and was equal to 11.27 ± 1.46 m². In total, Ramin station had the highest average density in winter and summer with a value of 19.15 ± 2.67 m². Also, the lowest average density was related to the diving hut station in summer and the first transect of the lower part of the tide, which was not recorded due to the presence of a sandy bed. In total, the average density for two seasons was 0.85 ± 0.26 m² at the Kolbeh ghavasi station. Also, the statistical test showed a significant difference between the stations (P<0.05). The biological parameters showed that in Tis station, in winter and summer respectively, the maximum average length was 53.56±4.25 and 58.2±4.53 mm, weight was 23.19±7.43 and 22.8±51. 28.28 grams and at the Kolbeh ghavasi station for winter, the lowest average length was 32 ± 2.14 mm and weight was 2.99 ± 0.85 grams. In the Big Darya station, for summer, the lowest average length and weight were 43.44 ± 4.21 mm and 16.53 ± 4.89 grams, respectively.       
Discussion: According to the results of other researchers, Tis station has high biodiversity, but the results of the present study showed that in this station, A. vaillantii samples had more weight and length despite the lower density (probably due to competition). Also, there was more density and distribution in the crevices of rocks and tidal pools with algal cover and temperature of 22 to 26 degrees Celsius.