SCHOOL STRUCTURE OF THE SQUID LOLIGO OPALESCENS 



Ann C. Hurley' 



ABSTRACT 



The squid Loligo opalescens forms schools which are similar in many respects to those of obligate 

 schooling fishes. These schools are marked by parallel orientation of individuals and strong cohesive- 

 ness. Laboratory experiments indicate that the main sensory modality regulating schooling is vision. 

 Squid on opposite sides of a clear rigid Plexiglas barrier readily schooled. The structure of schools of six 

 squid depended on size of individuals and was modified by environmental disturbance. Parallel 

 orientation was weaker in schools of smaller squid (ca. 7 cm dorsal mantle length) than it was in larger 

 ones (ca. 12 cm). In the field, L. opalescens schools are composed of uniformly sized individuals. 

 Laboratory experiments designed to determine whether this was due to actual size selection were 

 inconclusive, but they did suggest mechanisms which might be important in determining squid 

 position in the school. 



Considerable effort has been spent in understand- 

 ing the schooling behavior of fish in terms of 

 physiological mechanisms and possible survival 

 value and ecological consequences. (See reviews 

 by Radakov 1973 and Shaw 1970, 1978.) Virtually 

 no work has been done on schooling behavior of 

 invertebrates which occur in the same environ- 

 ments as schooling fish. The most evident school- 

 ing invertebrates in the pelagic environment are 

 the squid. Squid and fish play very similar ecologi- 

 cal roles and the two groups of organisms possess a 

 large number of similarities. (See Packard 1972, 

 for a discussion of convergent evolution.) 



Loligo opalescens is common off the west coast of 

 North America with a reported range from Baja 

 California to lat. 55°N (Fields 1965; Bernard 

 1970). Relatively little is known of the behavior or 

 general ecology of L. opalescens in spite of the fact 

 that there is a fishery for this species in California. 

 The fishery is based primarily upon the tendency 

 of squid to spawn in large aggregations in shallow 

 water (McGowan 1954; Fields 1965). Very little is 

 known about the distribution or location of newly 

 hatched squid as well as squid in later stages of 

 life. Attempts to catch the juveniles have often 

 been unsuccessful (Okutani and McGowan 1969) 

 and only recently have attempts been made to 

 catch nonspawning adults. Even though field 

 data are difficult to obtain, it is possible to keep 

 both juvenile (Hurley 1976) and adult L. opales- 

 cens alive in the laboratory. Schooling in the 



laboratory was examined to provide insights 

 about the function of schooling in squid. 



METHODS 



The squid used in the behavioral studies were 

 obtained either by dipnetting them after they had 

 been attracted to an underwater light or by pur- 

 chasing them from a local bait dealer. In the 

 laboratory, the squid were maintained in a 3-m 

 diameter circular tank with rapidly circulating 

 seawater. They were fed irregularly on small fish 

 (either mosquitofish, Gambusia affinus, or 

 goldfish, Carassius auratus). Mosquitofish were 

 taken much more readily than were the goldfish. 

 Occasionally, the squid could be trained to take 

 dead food. This was accomplished by first getting 

 them to accept live fish and by then throwing dead 

 fish in along with the live ones. In this manner, the 

 squid could also be coaxed to accept pieces of frozen 

 northern anchovy, Engraulis mordax. If the squid 

 were undamaged when they arrived at the 

 laboratory and there was an abundant supply of 

 small fish available, it was relatively easy to keep 

 them for over a month. 



Experiments designed to examine various as- 

 pects of schooling behavior were run in a 2 x 3 m 

 rectangular Plexiglas^ tank which was filled to a 

 depth of 0.4 m. The tank was painted flat white 

 and the primary source of lighting (in addition to 

 general room illumination) was provided by 



'Moss Landing Marine Laboratories, P.O. Box 223, Moss 

 Landing. CA 95039. 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Manuscript accepted November 1977. 

 FISHERY BULLETIN: VOL. 76. NO. 2. 1978. 



433 



