535 



AbStfelCt. - Two species of Loligo are 

 abundant in northern Gulf waters: the 

 long-finned squid, Loligo pealei, and the 

 arrow squid, Loligo plei. Variability 

 within species and similarities between 

 the species often hamper accurate iden- 

 tification. The two species more closely 

 resemble each other in areas of sympa- 

 try, and there is overlap in almost all 

 of the diagnostic characters. Small 

 specimens of Loligo are not easily iden- 

 tified, and there are few studies detail- 

 ing their morphometry. Because of the 

 taxonomic uncertainties associated 

 with the identification of juveniles and 

 subadults of L. pealei and L. plei, the 

 species were differentiated by isoelec- 

 tric focusing, and morphometric char- 

 acters and indices of potential use in 

 species separation were evaluated. Em- 

 phasis was placed on those taxonomic 

 characters suitable for use in the field. 

 Best discrimination between the two 

 species was achieved with combina- 

 tions of measurements of characters 

 and calculated indices associated with 

 cartilaginous structures (funnel carti- 

 lage length, gladius width [GW], nuchal 

 cartilage length, and rachis width 

 IRW]l and the shape of the gladius. An 

 arbitrary cutoff in GW/RW ratio of 2.7 

 (ratio of the greatest width of the 

 gladius to the width of the free rachis 

 at the junction of the vane) correctly 

 classified 100% of the L. plei and 919c 

 of the L. pealei. The overall shape of 

 the gladius ( broader and more rounded 

 in L. pealei), presence or absence of 

 marginal ribs in the vane of the gladius, 

 and the nature of the junction of the 

 vane and free rachis (junction gradual 

 and not distinct in L. pealei) were also 

 useful in distinguishing the two species. 



Morphometry of juvenile and subadult 

 Loligo pealei and L plei from the 

 northern Gulf of Mexico 



Guillermo Sanchez 

 Harriet M. Perry* 

 Christine B. Trigg 



Gulf Coast Research Laboratory 



PO Box 7000 



Ocean Springs, Mississippi 39566-7000 



Michael Vecchione 



NMFS Systematics Laboratory, NOAA 

 National Museum of Natural History 

 Smithsonian Institution, Washington, D C. 20560 



Clyde Roper 



Department of Invertebrate Zoology, Molluscs 

 National Museum of Natural History 

 Smithsonian Institution, Washington, D C. 20560 



Patricia M. Biesiot 



Department of Biological Sciences, University of Southern Mississippi 

 Hattiesburg, Mississippi 39406-5018 



Manuscript accepted 1 March 1996. 

 Fishery Bulletin 94:535-550 ( 19961. 



Although squid are conspicuous 

 components of the marine ecosys- 

 tem and form a substantial portion 

 of the invertebrate biomass in the 

 northern Gulf of Mexico, many as- 

 pects of their life history and struc- 

 tural characteristics are poorly 

 known. Three species of loliginid 

 squid are abundant in northern Gulf 

 waters (Rathjen et al., 1979): the brief 

 squid, Lolliguncula brevis (Blainville 

 1823), the long-finned squid, Loligo 

 pealei Lesueur 1821 (also known as 

 the longfin inshore squid [Roper et 

 al., 1984]), and the arrow squid, 

 Loligo plei Blainville 1823. The lack 

 of knowledge about the species of 

 Loligo in the Gulf can, in part, be at- 

 tributed to the difficulties in identi- 

 fying juveniles. Recent interest in lo- 

 cating new or underutilized squid 

 species, or both, and the recognition 

 of the importance of documenting 

 changes in marine biodiversity have 



increased emphasis on the need to 

 improve taxonomic resolution in this 

 important invertebrate group. 



Although the adults of L. pealei 

 and L. plei have been described by 

 numerous authors (e.g. Haefner, 

 1964; LaRoe, 1967; Cohen, 1976; 

 Roper et al., 1984), variability 

 within species and similarities be- 

 tween the species often hamper pre- 

 cise identification. Species variabil- 

 ity has been documented in the 

 long-finned squid, L. pealei. Cohen 

 (1976) noted that populations of 

 long-finned squid varied widely 

 both within and between geographic 

 areas. In areas of sympatry with L. 

 plei, the two species more closely 

 resemble each other, and with the 

 exception of gladius shape and the 

 hectocotylus in adult males, there 



Author to whom correspondence should be 

 sent. 



