Direct Observations of the Behavior of Euphausia Superba and Euphausia Crystallorophias (Crustacea: Euphausiacea) Under Pack Ice During the Antarctic Spring of 1985

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Abstract
Direct field observations using SCUBA were made of Euphausia superba and Euphausia crystallorophias under the pack ice off Enderby Land, and aspects of their behavior were recorded using still, video, and cine photography. Swarms of E. superba were generally composed of 20-50 individuals (range 5-1,000), of which > 90% were seen feeding on algae growing on the undersurface of the ice. Euphausia crystallorophias formed larger swarms that occupied the region approximately 1-5 m below the ice. Preliminary fecal and ice algal composition analyses indicate that E. superba feeds selectively on cergain species of epontic algae and that the two euphausiid species do not compete directly for the same food source under the ice. Although a range of escape responses was displayed by both species, E. superba reacted generally with an individual back-flip response and E. crystallorophias with a coordinated pleopod swimming response. The adaptive significance of the range of responses in relation to swarm size and density and the proximity of the predator/disturbance is discussed by O''Brien (1987). Data from echo soundings combined with high resolution video images, obtained by lowering a camera into the scattering layer beneath the ship, revealed that adult E. crystallorophias undertake vertical migration at this time of year. However, evidence from a night dive suggested that swarms of juveniles and subadults did not migrate, but maintained their daytime distribution under the ice during darkness. Swarm formation and maintenance at this time of year are discussed in relation to feeding behavior in E. superba.