550 



Fishery Bulletin 94(3), 1996 



genetic introgression was considered by Cohen 

 ( 1976), but currently there is no evidence of hybrid- 

 ization between any of the loliginid species. 



The original intent of this study was to examine 

 selected morphometric characters of potential value 

 in differentiating the two common species of Loligo 

 in the field. Juvenile and subadult squids comprise 

 a substantial portion of the invertebrate biomass in 

 the northern Gulf of Mexico, and difficulty in sepa- 

 rating the species of Loligo has been a contributing 

 factor in our lack of knowledge of this group. No single 

 character or index enabled 100% separation of the 

 species in the size classes of squid under study. Over- 

 lap was high in many of the measured characters 

 and indices and precluded their use as reliable de- 

 terminants of species. LaRoe ( 1967 ) and Cohen ( 1976) 

 also found high overlap in measured characters and 

 indices for the two species and suggested that more 

 than one character or index had to be used to sepa- 

 rate the species. In the present study, combinations 

 of characters or indices associated with cartilaginous 

 structures (FCL, GW, NCL, and RW) enabled great- 

 est percent separation. 



The single GW/RW ratio of 2.7 enabled correct iden- 

 tification of 91% of L. pealei and 100% of L. plei. 

 This percent separation was similar to that achieved 

 in the discriminant analysis for the GW/RW ratio. 

 The most practical method for separation of the spe- 

 cies in the field is use of the 2.7 GW/RW ratio in con- 

 junction with the overall shape of the gladius, in- 

 cluding the presence or absence of marginal ribs. 



Acknowledgments 



This research involved the efforts of many people to 

 whom we express our sincere gratitude. Shiao Wang 

 (University of Southern Mississippi) and Joanne 

 Lyczkowski-Shultz (National Marine Fisheries Ser- 

 vice) reviewed the manuscript and provided helpful 

 suggestions. Kenneth Stuck and Hugh Hammer of 

 the Gulf Coast Research Laboratory ( GCRL) provided 

 training in electrophoretic techniques and allowed 

 use of their laboratory facilities. Biologists at the 

 National Marine Fisheries Service (NMFS), Missis- 

 sippi Laboratory, Pascagoula, MS, supplied the speci- 

 mens of Loligo used in this research. Gilmore 

 Pellegrin Jr. and Nathaniel Sanders Jr. supervised 

 collection of samples, supplied technical information 

 related to the capture of specimens, and provided as- 

 sistance with data analysis. Kevin Rademacher, Rob 

 Ford, Perry Thompson, Eva Kargard, and Bennie 

 Rohr assisted in field collections. Walter Brehm of 

 the Clinical Research Laboratory, Keesler Air Force 

 Base, and James Warren of GCRL helped with sta- 



tistical analyses of the data. Dennis Koi (NMFS) 

 supplied the computer program used to develop Fig- 

 ure 1, and David Donaldson (Gulf States Marine 

 Fisheries Commission) assisted with graphics. Marjorie 

 Williams (GCRL) aided in manuscript preparation. 

 Finally, we thank Thomas Mcllwain, former director 

 of the GCRL, and Scott Nichols, director of the NMFS 

 Pascagoula Laboratory, for providing financial and lo- 

 gistical support during the course of this research. 



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