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A theoretical relationship has been developed between the environmental temperature and the survival index of a planktonic organism. The significant temperatures are the degree of departure from the optimum values of surface temperature (TS) and temperature at the depth of
10-12 illumination (T1-12). Allowance is included for cases where the optimum is closer to the upper than to the lower limit of tolerance, and also for a degree of physiological adaptation. It is considered that the survival index of a species in a given habitat should
be reflected approximately in the relative abundance in that habitat of the species in question among the total population. The geographical distribution of Euphausids in a considerable part of the North Atlantic and Mediterranean is charted. Charts are also given, for the same area, of
the distribution of the extinction coefficients of the water and of the calculated values of T1-12. The relation of the abundance of each of the commoner species to TS and T1-12 is given. A good agreement is found between theoretical and observed temperature
relationships if it is assumed that, for most species, there are two different temperature optima, one associated with the day level and the other with the night level. The likelihood of such a dual temperature control is discussed. For a few species, critical temperatures as indicated
by scattering layer records are compared with those indicated from geographic distribution. Finally, for Euphausia hemigibba, the geographic distribution, as predicted from physical conditions, is shown to agree reasonably well with actual observations.
The Bulletin of Marine Science is dedicated to the dissemination of high quality research from the world's oceans. All aspects of marine science are treated by the Bulletin of Marine Science, including papers in marine biology, biological oceanography, fisheries, marine affairs, applied marine physics, marine geology and geophysics, marine and atmospheric chemistry, and meteorology and physical oceanography.