Most sand-burrowing and coastal mysids perform a diel (24-hour cyclical) vertical migration, rising and dispersing into the water column at night and returning to deeper water towards dawn. Many are benthic by day and pelagic at night. A few species rest on algae during the day or on stones and cliff ledges; only a few mysids bury themselves in sand. Gastrosaccus throws up sand grains by moving its thoracic limbs while lying on the sand. Paramysis digs ditches in muddy sand with its first three pereopods. This species can dig for long periods of time, producing open ditches as long as 1.9-3.9 in (5-10 cm) within an hour.
Almost all bathypelagic and bottom-dwelling mysids, even those that burrow in sand, rise in the water column at night. The stimulus for this migration may be light intensity; the time it takes the mysids to rise depends on the speed and depth of water currents. This nocturnal pattern is most noticeable during spawning periods, and may be related to the dispersal of young mysids leaving the marsupium. Littoral species sometimes move to deeper water in fall and return to the shoreline in spring or summer.
Some species of mysids form swarms. These swarms may be several miles long and three or more feet in diameter.
Mysids are primarily swimmers; all members of the class can swim up, down, forward, and backward. The females have reduced pleopods and swim with the exopod (external branch) of their pereopods. They hold the exopods out to the sides and rotate them so that the tip describes an oval. They move their limbs continuously in a slightly different phase and draw water from the sides toward their upper surface. In this way two strong currents of water, one parallel to the abdomen and the other some distance from it, drive the body forward. Many species hold their bodies in a horizontal position with the dor-sum up while swimming. A few hold the anterior part of their bodies in an almost vertical position.
Some species swim in schools. School formation among mysids depends on optic signals during the day and probably sensitivity to water currents generated by the swimming movements of their neighbors at night.
Mysids that are suddenly disturbed jerk backward by flexing their abdomen and tail fan against their thorax. Bottom dwellers walk slowly on their endopods with their exopods also in constant motion. The eyes and statocysts keep the body horizontal even in the dark. If a mysid is illuminated from the side, it will turn its back toward the light. The angle of turning may be greater than 45° especially if the light is strong. The animal's optic control over body position is able to override the statocysts.
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