Indian Journal of Geo Marine Sciences Vol. 46 (07), July 2017, pp. 1371-1380 Diversity and abundance of epipelagic larvaceans and calanoid copepods in the eastern equatorial Indian Ocean during the spring inter-monsoon Kaizhi Li, Jianqiang Yin * , Yehui Tan, Liangmin Huang & Gang Li Key Laboratory of Tropical Marine Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China *[E-mail: jqyin@scsio.ac.cn] Received 20 August 2015; revised 29 September 2015 This study investigated the species composition, distribution and abundance of larvaceans and calanoid copepods in the eastern equatorial Indian Ocean. In total, 25 species of larvaceans and 69 species of calanoid copepods were identified in the study area. Although the average diversity and evenness indexes of larvaceans were lower than those of calanoid copepods, the abundance of larvaceans was higher than that of calanoids with means of 40.1±14.9 ind m -3 and 28.4±9.1 ind m -3 , respectively. Larvacean community was numerically dominated by Oikopleura fusiformis, Oikopleura longicauda, Oikopleura cophocerca, Fritillaria formica and Fritillaria pellucida, accounting for 83% of total larvacean abundance. The calanoid community was dominated by the following five species, represented 61% of calanoid copepods: Clausocalanus furcatus, Clausocalanus farrani, Acartia negligens, Acrocalanus longicornis as well as the copepodite stage of Euchaeta spp. This study highlights that the importance of larvaceans in the eastern equatorial Indian Ocean. [Keywords: Appendicularians, Calanoids, Water mass, Monsoon, Indian Ocean] Introduction It has been clear that small organisms of marine zooplankton have historically been under-sampled by coarse-mesh nets 1,2 . Larvaceans and copepods are important constituents of the mesozooplankton, and play pivotal roles from the microbial ecosystem to higher trophic levels in the tropical oligotrophic waters. Larvaceans (or Appendicularians), an important group of zooplankton, appear to show little taxonomic diversity with only 65 species currently described due to their general frailty 2 . In recent years, it has become obvious that the remarkable filtration systems and rapid population growth of larvaceans may be contributing to their population sizes equaling or exceeding those of copepods 3-8 . Larvaceans play a pivotal role, from the microbial ecosystem to upper trophic levels, and their elaborate ‘houses’ enable them to be efficient filter feeders of picoplankton and nanoplankton 9,10 . Copepods, with a length < 1 mm, are the most abundant metazoans on Earth, including adults and copepodites of the calanoid genera (such as Paracalanus, Pseudocalanus, Acartia, and Clausocalanus) and the cyclopoid genera (such as Oithona, Oncaea and Corycaeus 1 ). Most calanoid copepods are considered as ‘herbivorous’ zooplankton with a preference for larger particles. Small calanoid copepods often form a dominant zooplankton group in tropical pelagic environments 11 . They have a great influence on the efficiency of trophic coupling between the primary producers and other larger pelagic carnivores 1,12 . Although the important roles of larvaceans and copepods in marine food webs are well studied, little is known about their diversity, abundance or distribution, especially in tropical oligotrophic waters. Monsoons play a vital role in determining the various physical and chemical features of the northern Indian Ocean. The circulation of the upper layers undergoes drastic seasonal variations, with the southwest monsoon current occurring during May- September, and the northeastern monsoon current during December-February 13 . Another important current is the Wyrtki Jet, located between 2°S and 2°N in the equatorial Indian Ocean. The Wyrtki Jet is