The present dissertation is based on the systematics and distribution of holopelagic shrimp belonging to the family Sergestidae and the genera Sergestes and Sergia studied from the International Indian Ocean Expedition (IIOE) material (1960-65) collected by research vessels of nine nations in the Indian Ocean. Twelve species assigned to the Sergestes and Sergia are identified, described in detail and illustrated. They are: Sergestes cf brevispinatus Judkins, 1978; S. curvatus Crosnier and Forest, 1973; S. cf edwardsii Kreyer,1855; S. hamifer Alcock and Anderson, 1894; 8.henseni (Ortmann, 1893); S. orientalis Hansen, 1919; S. paraseminudus Crosnier and Forest, 1973; S. semissis Burkenroad, 1940; S.stimulator Burkenroad, 1940; S. verpus Burkenroad, 1940 and Sergia potens Burkenroad, 1940 and S. umitakae Hashizume and Omori, 1995. Out of the twelve species Sergestes er. brevispinatus Judkins, 1978; S. et edwardsii krøyer, 1855 and S. orientalis Hansen, 1919 are reported new to the Indian Ocean. Further the presence of Sergestes hamifer Alcock and Anderson, 1894 and Sergia umitakae in the material at hand are first subsequent records of these species after their original descriptions from the Indian Ocean.
The higher systematics deals with the diagnosis of the Decapoda, Dendrobranchiata and Sergestidae. Keys are given to the genera and the species in IIOE material. Description of each genus includes literature review, list of the world species, synonymy, diagnosis and type species, while species description includes synonymy, material examined, diagnosis, description of adult male and female, juvenile male and female, size range, geographical distribution, and appropiate remarks where necessary .
In the distribution part of this study frequencies of occurrence and bathymetric distribution of Sergestes and Sergia in different regions of the Indian Ocean is attempted based on the samples captured during the International Indian Ocean Expedition (1960-1965). The total number of Sergestids specimens was collected' 2409 from 1071 stations by the following vessels; Anton Brunn (611),Argo (256), Conch(II), Discovery (270), Diamantina (188), Gascoyne (44), Kistna (361), Koyo Maru (46), Meteor ( 122), Natal (29), Oshoro Maru (177), Patanela (51), Pioneer (72), Varuna (122), Vitiaz (44), Zulun (70). Nine Sergestes species with a total number of 1827 were collected from 746 stations as follows: Anton Brunn (436), Argo (199), Conch (11), Discovery (190), Diamantina (121), Gascoyne (5), Kistna (358), Koyo Maru (6), Meteor ( 103), Natal (29), Oshoro Maru (107), Patanela (51), Pioneer (31), Varuna (89), Vitiaz (10) and Zulun (70).
Two Sergia species with a total numbers 636 specimens from 325 stations as follows: Anton Brunn (175), Argo (57), Conch (11), Diamantina (67), Discovery (80), Gascoyne (39), Kistna (3), Koyo Maru (40), Meteor (19), Oshoro Maru (70), Pioneer (41), Varuna (33) and Vitiaz (34).
The material has been further investigated to determine the average numbers of adult males, adult females, juvenile males and juvenile females (Figs. 86,87,88,89).
The numerical count shows that Sergestes semissis is the most abundant species. It comprised of653 adult males and females of the total 1827 Sergestes specimens. Sergestes verpus was the second most abundant species, comprising of 210 both adult males and females and juvenile males and females of the total 2463 sergestids. The population of Sergestes cf. edwardsii was sparse, only 2 specimens were present in samples. While Sergia potens comprises specimens of 257 adult males and females and juvenile males and females and S. umitakae 379 both adult males and females and juvenile males and females of total 636 specimens.
The prevalence of sergestids under study is discussed, using of maps, tables and histograms.
Besides the adults and juveniles three larval stages have also been identified in these samples: Protozoeae, Zoeae and Postlarvae. Their occurrence and abundance is traced at different 'depths and time of the day.