SOME BEHAVIORAL AND PHYSIOLOGICAL-RESPONSES OF THE ASCIDIAN STYELA-PLICATA (LESUEUR) DURING ACCLIMATION TO LOW-SALINITY

The sessile ascidian, Styela plicata (Lesueur, 1823), commonly inhabit s areas of fluctuating salinity; consequently, it must either tolerate or compensate for perturbations in body volume caused by environmenta l salinity changes. This study examines siphon closure as an avoidance response, and net effluxes of total ammonia and ninhydrin positive su bstances (NPS) during the first 48 h of acclimation of S. plicata to 2 0 parts per thousand. sea water. Experimental animals in 20 parts per thousand were compared with a control group in full strength sea water (35 parts per thousand). Typically, siphons of experimental tunicates remained closed until 6-8 h, isolating the animals from the environme nt. Water content in experimental tunicates was elevated 2-5% above co ntrol levels after 8 h and remained elevated in the soft tissue. Exper imental NPS net efflux increased by 12 h and remained elevated. After 24 h, experimental net efflux of ammonia increased. The increase in ex perimental NPS net efflux is probably due to the excretion of intracel lular organic osmolytes, which would reduce cell volume. The increase in ammonia net efflux suggests some deamination of NPS before excretio n.