536 



Fishery Bulletin 94(3). 1996 



is overlap in all other diagnostic characters (Cohen, 

 1976). This problem is particularly acute in the north- 

 ern Gulf of Mexico, where the two species are common. 

 Hixon (1980) noted that all external characters and 

 morphometric indices overlapped in immature speci- 

 mens taken in waters off Texas. Difficulties in sepa- 

 rating the species were best summarized by LaRoe 

 ( 1967) who stated, "there are differences between the 

 species, but there is a continuum between them in their 

 range of variation for most characters; there appear to 

 be no distinct boundaries to separate the two species." 

 The purpose of this study is to examine variability 

 in morphological characteristics between juveniles 

 and subadults of L. pealei and L. plei in the north- 

 ern Gulf of Mexico and to determine diagnostic cri- 

 teria for their identification. Emphasis is placed on 

 taxonomic characters suitable for use in the field. 

 Because of the taxonomic uncertainties associated 

 with identification of immature and small specimens 

 of the two Loligo species, an electrophoretic technique 

 (isoelectric focusing) was used to establish species- 

 specific protein patterns. Biochemical differentiation 

 of species provided more accurate assessment of 

 morphological variability between the two species. 

 This contrasts with previous studies in which mor- 



phological variability was assessed by using speci- 

 mens identified by morphological criteria. 



Materials and methods 



Field procedures 



Squid samples were collected by personnel of the 

 National Marine Fisheries Service, Mississippi Labo- 

 ratories, Pascagoula, Mississippi, aboard the Na- 

 tional Oceanic and Atmospheric Administration Ship 

 RV Oregon II. Samples used here were obtained dur- 

 ing the Southeast Area Monitoring and Assessment 

 Program (SEAMAP) resource survey Cruise 93-03 

 (205), conducted from 6 June to 21 July 1993. Otter- 

 trawl samples (n=230) were taken in an area from 88° 

 to 97°30"W (Mobile, Alabama to Brownsville, Texas) in 

 depths from 9 to 110 meters (Fig. 1). Station selec- 

 tion and field processing procedures followed estab- 

 lished SEAMAP protocol. 1 All specimens of Loligo 



1 Nichols, S., and G. Pellegrin. 1989. Trends in catch per unit 

 effort for 157 taxa caught in the Gulf of Mexico Fall Groundfish 

 Survey. 1972-88. Summary Rep., Mississippi Laboratories, 

 NMFS, NOAA, 158 p. 



Mississippi 



Alabama 



Texas 



96' 95" 



92" 9V 



Figure 1 



Location of trawl stations in the northern Gulf of Mexico where Loligo pealei and Loligo plei were 

 collected. 



