32 INVERTEBRATE PHYSIOLOGY 



vertical movements which compensate by a change in depth for the small 

 pressure change. Thus, partial vacua of 6 inches of sea water produce a 

 downward movement of approximately 6 inches. The converse operates 

 for positive pressures unless the animal is already at the surface, in which 

 case it still makes an effort to swim upward. The results of small positive 

 pressure changes are shown below : 



PRESSURE INCREASES REQUIRED TO PRODUCE UPWARD 

 SWIMMING IN VARIOUS ANIMALS 



Animals Pressure Increase 



(psi) 



Small pelagic annelids 2 



Hydrozoan medusae 5-10 



Temora 2 



Mncmiopsis 5 



Pteropods 0.1-5.0 



Pontella 0.1 



Sagitta 9 



Pleiirobrachia 9 



Paleomonetes larvae 9 



Acartia 9 



Cenfropages - 9 



CalUncctes megalops & zoea 9 



Lucifer larvae 9 



Pencils larvae 9 



The lowest threshold for positive or negative pressures in the Florida 

 marine forms was 3 inches of sea water. Such a low threshold to pressure 

 raises the question of mechanism. One possibility is that these organisms 

 possess small gas bubbles, although none is discernable with high dry 

 microscopy in the glass-clear medusa Pleurobrachia, or Sagitta. A test of 

 this hypothesis is available. If the pressure is so reduced that boiling occurs, 

 any gas bubble present would be expected to expand enormously and float 

 the organism to the surface. In none of the species tested did this happen. 

 The pressure decrease accompanying evacuation of a suction flask induces 

 a strong positive geotaxis, and the populations literally grovel on the 

 bottom while the water all around them is boiling at room temperature. 

 Gas bubbles, in the ordinary sense, at least, are not the mechanism. No 

 fresh-water organisms responded to this test either. 



Small continuous increments of pressure significantly increase the ve- 

 locity of upswimming of the Florida marine plankters until a pressure 

 of 15 to 30 psi is reached. When the pressure was maintained at these 

 levels for a few minutes, there resulted a destruction of the pressure-sensi- 

 tive mechanism. Recovery did not occur within a week in which these 

 animals were kept alive in the laboratory. Small continuous decrements of 

 pressure also increase the downswimming velocity until the sea water boils. 



