Zooplankton and zooneuston observations were made at seven stations (four from the Gulf of Aqaba and three from the northern Red Sea), during September and October 2016. The main objective of this study was to assess the variability of nycthemeral fauna in relation to the sampling methods using two different types of nets namely, WP2 net and Neuston net along the two study sites, i.e., the Gulf of Aqaba and the northern Red Sea. Zooplankton was sampled vertically using a standard WP2 net from a depth of 200 m to the surface, whereas zooneuston was made using a standard Neuston net from a depth of 0-10 cm of the water surface. Total zooplankton density was maximum during night time ((617.83 ±201.84) ind./m³) at the Gulf of Aqaba and total zooneuston was maximum during night at the northern Red Sea ((60.94±29.48) ind./m³), respectively. The most abundant taxa were Copepoda, Gastropoda, Bivalva, Chaetognatha, Tunicata and Ostracoda. The abundance was almost 50% higher at night time at both the Gulf of Aqaba and the northern Red Sea. Overall, 30 taxa covering 10 phyla and 27 taxa covering 8 phyla were recorded in the Gulf of Aqaba and the northern Red Sea.

The northern Andaman Sea off Myanmar is one of the relatively high productive regions in the Indian Ocean. The abundance, biomass and species composition of mesozooplankton and their relationships with environmental variables in the epipelagic zone (~200 m) were studied for the first time during the Sino-Myanmar joint cruise (February 2020). The mean abundance and biomass of mesozooplankton were (1916.7±1192.9) ind./m³ and (17.8±7.9) mg/m³, respectively. A total of 213 species (taxa) were identified from all samples. The omnivorous Cyclopoida Oncaea venusta and Oithona spp. were the top two dominant taxa. Three mesozooplankton communities were determined via cluster analysis: the open ocean in the Andaman Sea and the Bay of Bengal (Group A), the transition zone across the Preparis Channel (Group B), and nearshore water off the Ayeyarwady Delta and along the Tanintharyi Coast (Group C). Variation partitioning analysis revealed that the interaction of physical and biological factors explained 98.8% of mesozooplankton community spatial variation, and redundancy analysis revealed that column mean chlorophyll a concentration (CMCHLA) was the most important explanatory variable (43.1%). The abundance and biomass were significantly higher in Group C, the same as CMCHLA and column mean temperature (CMT) and in contrast to salinity, and CMT was the dominant factor. Significant taxon spatial variations were controlled by CMCHLA, salinity and temperature. This study suggested that mesozooplankton spatial variation was mainly regulated by physical processes through their effects on CMCHLA. The physical processes were simultaneously affected by heat loss differences, freshwater influx, eddies and depth.