The relationship of the distribution of the planktonic worm, Poeobius meseres Heath, to the water masses of the North Pacific

The distribution of a planktonic worm, Poeobius meseresHeath, was determined from an examination of over 1800 quantitative plankton tows taken in the North and South Pacific. This distribution is compared with the distribution of water masses, as defined by temperature-salinity curves. Since the water mass concept involves a three dimensional unit of the ocean, its use in describing the environment of an animal whose distribution is also three dimensional, is preferable to the method of comparing plankton distributions with horizontal isotherms or isohalines.

The distribution of Poeobius coincides, for the most part, with that of the Subarctic water mass and the transition region of Subarctic water, the California Current. However, a few specimens were found in the eastern tropical Pacific. A satisfactory explanation for its restricted presence in this latter area is not possible at this time, but there is some evidence to indicate that this southern segment of the population is not endemic but has been carried in from the north. If this is true, then the occurrence of Poeobius here must be accounted for in considering the sources of the Intermediate water of the area.