LILLELUND and LASKER: PREDATION BV MARINE COPEPODS 



POSITIVE PHOTOTAXIS AND SWIMMING 

 SPEED 



Labidocera jollae and L. trisplnosa are posi- 

 tively phototactic, and when confined to a beaker 

 of seawater illuminated from above, concentrate 

 at the water surface-beaker interface which is 

 the brightest area. Copepods were induced to 

 swim in the main body of water by wrapping 

 the beaker with black paper with 1 cm lapped 

 around the rim. This effectively eliminated the 

 bright area and resulted in random swimming 

 movements of the copepods near the surface. 



Swimming distances of copepods were traced 

 for 3 min in two dimensions on a clear acetate 

 sheet laid over a glass plate on top of a 3.5-liter 

 beaker. The distances were measured with a 

 map measurer. Vertical movements were very 

 slight, thus negligible, in these experiments be- 

 cause of the highly phototactic behavior of the 

 individuals. Labidocerans can swim continually 

 over relatively large areas in short periods of 

 time (Vlymen, 1970). Comparative speeds for 

 individuals are shown in Figure 4 ; on the aver- 



80r 



o 

 O 



Ql 

 UJ 



a. 

 o 

 o 



60- 



40 



E 

 u 



20 



-62 



= =-22 



-37 



z^33 



r 







Figure 4. — Swimming speeds for individuals of Labido- 

 cera. Each small horizontal bar represents the speed 

 of one animal; the large bar is the mean speed. 



age a L. jollae female swims 62 cm/min (3-4 

 body lengths/sec) and the male swims 22 cm/min 

 (1.5-2 body lengths/sec) . Both sexes of L. tris- 

 plnosa swim 33 to 37 cm/min (2-3 body lengths/ 

 sec) . Although L. jollae females swim in a seem- 

 ingly random pattern, the males usually swim 

 in straight lines for a few seconds then swim 

 in circles and cover a small area intensively. 



KILLING EFFICIENCY OF Labidocera 



We discovered that if the ratio of anchovy 

 larvae to L. jollae females was low (<10:1), 

 all or almost all the larvae in 3500 ml would be 

 killed within 20 to 24 hr in the dark. Two ex- 

 periments were done which illustrate this. In 

 the first, 30 anchovy larvae were confined with a 

 variable number of L. jollae females (Figure 5) 

 resulting in concentrations of larvae to copepods 



2 4 6 8 10 12 14 

 LARVAE PER COPEPOD 



Figure 5. — Mortality of Engraulis mordax larvae, to 

 1 day old, resulting from predation by different numbers 

 of Labidocera jollae females. In each experiment 30 

 larvae were presented to 2 or more copepods in 3500 ml 

 for 21 hr. Thus where 3 larvae per copepod is indicated 

 on the abscissa, 30 larvae and 10 copepods were used; 

 at the other extreme 15 larvae per copepod indicates 30 

 larvae and 2 copepods. The unbroken line is the theor- 

 etical 100% lax'val mortality curve. 



659 



