THE EFFECT OF THE BOTTOM ON THE FAST START OF 

 FLATFISH CITHARICHTHYS STIGMAEUS 



p. W. Webb* 



ABSTRACT 



Fast starts of the speckled sanddab, Citharichthys stigmaeus, were initiated by a 1 volt per centimeter 

 direct current electric shock and recorded on movie film at 250 frames per second. Observations on 

 kinematics and performsince were made for fast starts offish accelerating in the water column and from 

 a grid located at a distance of 0, 1, 3, or 6 cm above the true bottom. During acceleration the body was 

 bent into a U-shape relative to the bottom. The body sustained a direct push against the grid when 

 accelerating from that grid. The amplitude of propulsive movements offish accelerating from the grid 

 was larger than acceleration movements offish in the water colimin away from the bottom, because the 

 potential for energy wastage due to recoil of the body was prevented by the grid. The distance between 

 the grid and the bottom had no effect on fast starts, ruling out any hydrodynamic ground effect. Motion 

 of fish accelerating in the water column was continuous and predominantly in the horizontal plane. 

 There was little motion offish accelerating from the grid until they started pushing against that grid. 

 Motion was predominantly in the vertical plane. The resultant distance traveled by fish accelerating 

 from the grid, measured at the end of the principal acceleration period exceeded that offish accelerating 

 in the water column. Velocities and acceleration rates were highest for fish accelerating from the grid. It 

 was concluded that contact with the sea bottom would enhance fast-start performance of fiatfish. 



Fish of the order Pleuronectiformes are unique 

 because the adults lie on one side. The habit of 

 inclining to one side is common among other 

 benthic fish that are normally vertically oriented, 

 but no other group shows the specialized mor- 

 phological adaptations of flatfish. Various obser- 

 vations on benthic fish suggest that inclining the 

 body or lying on one side is advantageous for 

 camouflage and crypsis (Norman 1966). Large 

 locomotor advantages could also accrue, espe- 

 cially to flatfish orienting their body axis parallel 

 to the ground. 



Improvements in locomotor performance of 

 flatfish could occur through two mechanisms: hy- 

 drodynamic ground effect and pushing against a 

 rigid substrate. The hydrodynamic ground effect 

 occurs through interaction of the down wash from 

 propulsive surfaces with the ground (i.e., the sea 

 bottom) and increases effective thrust and de- 

 creases effective drag (Bramwell 1976; Ligh thill 

 1979). It has been described for birds (Withers and 

 Timko 1977) and for pectoral fin propulsion of the 

 mandarin fish, Synchropus picturatus (Blake 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, La Jolla, Calif.; present ad- 

 dress: School of Natural Resources, University of Michigan, Ann 

 Arbor, MI 48109. 



1979). Pushing against the substrate is obv' msly 

 more energy efficient than pushing against water 

 because more muscle power is converted into body 

 motion rather than wasted in accelerating fluid. It 

 is utilized by macrurous decapods (Webb 1979). 

 The relative importance and effectiveness of these 

 mechanisms will depend on how flatfish move 

 their bodies near the ground. 



The following experiments were performed to 

 determine if the sea bottom could influence fast- 

 start (acceleration) movements and performance 

 of a typical flatfish. Emphasis was placed on fast 

 starts because of their importance in evasion of 

 predators and in catching elusive prey (Eaton and 

 Bombardieri 1978; Webb and Skadsen 1980). 



METHODS 



Fish 



Speckled sanddab, Citharichthys stigmaeus 

 Jordan and Gilbert (family Bothidae) were used. 

 Fish were caught in dip nets by divers along the 

 southern California coast, hence avoiding the 

 damage typical of trawl-caught specimens. Fish 

 were held in a 60,000 1 tank at 14° C. They were fed 

 twice a week on frozen brine shrimp. 



Manuscript accepted September 1980. 

 FISHERY BULLETIN: VOL. 79, NO. 2, 1981. 



271 



