RESPONSES TO SALINITY CHANGE AS A TIDAL TRANSPORT MECHANISM OF PINK SHRIMP, PENAEUS DUORARUM

¹ Institute of Marine Sciences, University of Miami, Miami, Florida

1. The inshore movements of postlarval pink shrimp and the subsequent offshore movements of the juveniles are facilitated by flood and ebb tides, respectively. This investigation concerns the behavioral mechanisms involved in the selective use of one tide and the evasion of the other.

2. Salinity changes, similar to those occurring with change in tide in the inshore environment usually occupied by pink shrimp, were imposed on both postlarvae and juveniles in a constant-current apparatus.

3. Juvenile shrimp were almost invariably positively rheotactic. However, with a decrease in salinity the sign of the response was reversed, resulting in active downstream swimming. This often gave way to passive drifting.

Under conditions of low light intensity postlarvae were active in the water column, and being unable to withstand even slow currents, were easily displaced. With a decrease in salinity they sank to the substrate or remained low in the water column where they were better able to maintain position.

4. Responses of postlarvae at a discontinuity barrier between bodies of water differing in salinity indicated their ability to perceive differences as small as 1. There was an "aversion" to penetrating such a barrier into water of lower salinity.

5. Smaller postlarvae were more "averse" to the barrier than others approximately a week older.

6. If similar responses are elicited in nature during the flood tides, juveniles would orientate and swim against the current in an offshore direction, while post-larvae, by being active in the water column, would be displaced shoreward. Following the decrease in salinity which accompanies the ebb tide the juveniles would swim, or be passively displaced, with the current, again in an offshore direction, and the postlarvae would sink low in the water column or settle on the substrate from where they are better able to resist displacement.