VERTICAL MIGRATION OF THE OPOSSUM SHRIMP, NEOMYSIS AMERICANA SMITH1

Abstract

Neomysis americana is rarely taken in a plankton net during the day, its normal habitat being on the bottom or immediately above it. N. americana undergoes a regular diurnal vertical migration which often includes a midnight sinking and a dawn rise. Fluctuations in the diurnal cycle are dependent upon the time of year and the number of hours the animals have spent in the dark. Light was found to be the most important single factor responsible for the migration. During their ascent, the mysids were located in areas of the water column where light intensities varied from 5 × 10‒1 to 1 × 10‒3 ft‐c; by the time morning surface light intensity values had reached 5 × 10‒1 ft‐c, the animals had descended to lower levels. Mature animals migrated only during the spawning season while immature and young forms migrated throughout most of the year. An increase in migrational tendencies was noted from August through January with the peak occurring in January.