It is interesting to note the great abundance of specimens in almost all samples from the hydrothermal vent field Rainbow (Mid-Atlantic Ridge). Indeed, a total of 700 specimens were caught in the different traps (KHRIPOUNOFF et al. 2001). They were regularly present from September to January and more markedly in October and November at different distances from the vents. They were always absent in traps from February to June, except for traps laid far off vent sites (2 km). Curiously, all the specimens were juveniles. Chemical analysis of the composition of particles from the stomach content has been done by EDS X-ray microanalysis. A comparison between T. parva specimens and specimens of the related species T. gregaria indicate that probably the T. parva population is more or less linked with the particle flux. Indeed, a peak of the Fe element is obvious and might be related with the high concentration of iron (7%) in the vent particles (KHRIPOUNOFF et al. 2001) that does not exist in T. gregaria (unpublished data). However, studies of T. parva specimens caught far from the vents have yet to verify this hypothesis. Curiously, there were no euphausiaceans in traps laid in the South Atlantic (Gulf of Guinea) and in the Pacific (East Pacific Rise: 13°N) (M. Segonzac, pers. comm.).