HURLEY; SCHOOL STRUCTURE OF LOLIGO OPALESCENS 



cysts. In the case of L. opalescens, the most likely 

 explanation for this change in spacing is that the 

 presence of the barrier physically limits the extent 

 to which each squid can compensate for the other 

 individual's movements. In the experimental 

 tank, squid seemed to differ in their motivation to 

 school. When the barrier was not present, a squid 

 with a strong tendency to school could always 

 maintain proximity to another squid. But if the 

 barrier were present, that squid could only follow 

 another squid as far as the barrier and had to 

 remain there until the other squid returned. 



I had hoped that the experiments with the three 

 squid separated by Plexiglas barriers would give 

 some clue as to whether the squid actively chose to 

 school with individuals of the same size, but the 

 results were inconclusive. The results did indi- 

 cate, however, a possible mechanism for mainte- 

 nance of spacing within a school. The center squid 

 tended to stay toward the middle of the compart- 

 ment, while the side squid maintained positions 

 very near the Plexiglas barrier. It is possible that 

 the center squid was attempting to equalize the 

 visual angle subtended by the squid on each side, 

 while the outer squid were attempting to get into 

 positions with squid on each side. The measure- 

 ments of visual angle which I can get from my 

 photographs are not accurate enough to determine 

 whether this is happening. If outer squid are con- 

 tinually trying to achieve a position where they 

 have squid on either side of them, individuals in a 

 school should be continually shifting positions. 

 Casual observations have indicated that this does 

 happen some of the time; but at other times, the 

 individuals maintain the same positions relative 

 to one another. 



An area where a comparison of squid and fish 

 schooling may be useful is in the speculation con- 

 cerning the evolution of the schooling behavior 

 and its possible advantages. Many recent papers 

 have concentrated on the hydrodynamic advan- 

 tages offish schooling (e.g., Breder 1976) and base 

 their explanations of many of the details of school 

 structure on the fish mode of tail-flip locomotion 

 and the vortices which are subsequently created. 

 Van 01st and Hunter ( 1970) suggest that the typi- 

 cal nearest neighbor distance in fish schools is 

 about one-half a body length and that this distance 

 may be explained by considering the amplitude of 

 the tail beat in swimming. It is interesting that in 

 squid, with their very different mode of locomotion 

 (jet propulsion as opposed to tail flips), the spacing 

 between nearest neighbors is still maintained be- 



tween one-half and one body length in undis- 

 turbed squid. 



Other investigators have speculated that a 

 primary function of schooling is as a defense 

 against predation. ( See reviews by Shaw 1970 and 

 Radakov 1973.) Squid have many of the same 

 reactions to disturbance that fish do. They both 

 clump more closely together as a result of distur- 

 bance and both have been seen to surround their 

 predators. Further evidence which suggests that 

 predator defense may be an important function of 

 squid schooling comes from the development of the 

 behavior in juvenile squid. In the course of rearing 

 L. opalescens (Hurley 1976), I made observations 

 on schooling behavior. The newly hatched squid 

 appeared to have no attraction to each other, but 

 after 6 or 7 wk schooling was occasionally ob- 

 served. This schooling was only evident in re- 

 sponse to disturbance (tapping on the tank or put- 

 ting a net into the water). When the squid were 

 feeding undisturbed, there was no obvious school- 

 ing behavior. 



ACKNOWLEDGMENTS 



I would like to thank P. Hartline, J. Hunter, and 

 G. D. Lange for their assistance during this study; 

 E. Shaw, G. Cailliet, and J. Nybakken for valuable 

 comments and suggestions; and Rosemary 

 Keegan for typing the manuscript. This work was 

 supported by NIH grant NS-09342 and NSF grant 

 GH-41809 to the laboratory of G. D. Lange, Uni- 

 versity of California, San Diego, and the South- 

 west Fisheries Center, La Jolla Laboratory, Na- 

 tional Marine Fisheries Service, NOAA, while I 

 held a NOAA associateship at that laboratory. 



LITERATURE CITED 



BERNARD, F. R. 



1970. A distributional checklist of the marine molluscs of 



British Columbia, based on faunistic surveys since 



1950. Syesis 3:75-94. 

 BREDER, C. M., JR. 



1959. Studies on social groupings in fishes. Bull. Am. 



Mus. Nat. Hist. 117:393-482. 

 1967. On the survival value of fish schools. Zoologica 



(N.Y.) 52:25-40. 

 1976. Fish schools as operational structures. Fish. Bull., 



U.S. 74:471-502. 

 CAHN, p. H. 



1972. Sensory factors in the side-to-side spacing and posi- 

 tional orientation of the tuna, Euthynnus affinis, during 



schooling. Fish. Bull., U.S. 70:197-204, 



441 



