HURLEY. SCHOOL STRUCTURE OF LOLIGO OI'ALESCENS 



outer squid were then switched from one outer 

 compartment to the other and the squid were 

 again allowed to adapt for 15 min. They were then 

 filmed for 5 min (once every 10 s). Squid in these 

 experiments ranged from 9.2 to 15.3 cm mantle 

 length. 



It was hoped that this experimental design 

 would indicate whether the center squid, if given a 

 choice, would choose to school with a larger or 

 smaller squid or one closer to its own size. One way 

 to determine whether such a choice is being made 

 would be to determine whether the center squid 

 spends more of its time closer to one outer squid 

 than to the other. Each 5-min run was considered 

 as a unit and each frame was scored according to 

 which outer squid the center squid was nearest. 

 For each run, the data were compared with a 

 binomial distribution which assumed that the 

 center squid had an equal probability of being 

 closest to either outer squid. Of the 17 runs, 16 

 showed a significant deviation from the expected 

 binomial distribution (Ps:0.05 for 1; P^O.Ol for 

 15). These 16 runs were now grouped according to 

 whether the center squid was closest to the larger 

 or smaller outer squid. In 8 of the 16 cases, the 

 center squid was nearest the larger outer squid, 

 while in the other 8 cases, it was nearest the small- 

 er squid. There is no evident preference for large 

 versus small squid. The data can also be arranged 

 to determine whether the center squid spent most 

 of its time near the squid closer to its own size. 

 There were 14 runs for which it was possible to say 

 that the center squid was closer in size to one of the 

 outer squid. Of these 14 runs, the center squid was 

 significantly nearer to the squid closer to its own 

 size 9 times and nearer to the squid farther from 

 its own size 5 times. 



These experiments may be viewed in another 

 way by looking at the absolute position of the 

 squid in the tank. The nearest distances of the 

 squid to the Plexiglas barriers were calculated for 

 each frame. These data are summarized in Figure 



3 for the 17 runs. The side squid usually are very 

 near the barrier which separates them from the 

 center compartment, while the center squid varies 

 his position within the center compartment, but 

 approaches the Plexiglas barriers much less often. 



DISCUSSION 



Pelagic fish and squid represent a striking case 

 of convergent evolution, not only morphologically 

 (Packard 1972), but behaviorally as well. One as- 

 pect of behavior where this is particularly appa- 

 rent is schooling. Since many of the same ecologi- 

 cal pressures exist for both pelagic groups, it is not 

 surprising that some sort of schooling behavior 

 would have developed in both fish and squid. What 

 is surprising, given the very different physiology 

 and mode of locomotion, is that so many aspects of 

 this behavior are the same. 



Loligo opalescens fits Breder's (1967) definition 

 of obligate schoolers. Single L. opalescens are 

 rarely caught in the field, and they immediately 

 come together when placed in a tank in the 

 laboratory. As has been reported for many species 

 offish (Radakov 1973), L. opalescens schools con- 

 sist of individuals of approximately the same size. 

 It has been suggested that the reason that fish 

 school in such groups has to do with swimming 

 speed. Small and large individuals would not 

 swim at the same speed and thus would not nor- 

 mally stay together. This is possibly also true for 

 squid, but data on the swimming speed of large 

 and small L. opalescens are not available to sub- 

 stantiate the argument. For several reasons, the 

 swimming speed hypothesis seems less plausible 

 for squid than for fish. In schools offish which show 

 parallel orientation, the fish continually maintain 

 forward motion and thus swimming speed is likely 

 to be an important factor. But field and laboratory 

 observations have indicated that individuals in 

 squid schools spend much of their time hovering in 

 the same position in the water column with only 



16 



14 



12 

 10 



8 



6 . 







2 . 







m- 



Figure 3. — Histograms of mean 

 nearest distance between Loligo opales- 

 cens and barrier in the 17 three-squid 

 experiments. Distances are broken up 

 into 10-cm intervals. From left to right: 

 left outer squid to left barrier, center 

 squid to left barrier, center squid to 

 right barrier, right outer squid to right 

 barrier. 



439 



