Sanchez et al: Morphometry of juvenile and subadult Loligo pealei and L plei 



549 



Juvenile and subadult L. pealei and L. plei over- 

 lapped in all measured characters. For those char- 

 acters in which there was a significant difference in 

 the slopes of the regression lines and for which the 

 coefficients of determination were high ( FL, F W, G W, 

 NCL, and W), the overlap generally was higher in 

 the smaller size classes. Differences in W, NCL, and 

 GW were more pronounced in larger specimens. In- 

 creases in measured characters per unit increase of 

 mantle length were higher in L. pealei. 



Both Cohen ( 1976) and LaRoe ( 1967 ) reported dif- 

 ferences in fin lengths between L. pealei and L. plei 

 and suggested that this character could be used to 

 differentiate between the two species. Cohen ( 1976) 

 found that fin length in relation to mantle length 

 was similar in both species in small specimens. How- 

 ever, she noted that fin length was equal to or greater 

 than half the mantle length of L. pealei when mantle 

 length exceeded 55.0 mm, whereas fin length was 

 equal to or greater than half the mantle length in 

 specimens of L. plei only after mantle length ex- 

 ceeded 95.0 mm. Fin length/mantle length propor- 

 tions in the present study corroborate these data for 

 juvenile and subadult specimens from the northern 

 Gulf of Mexico. Fin lengths were less than 50% of 

 the mantle length in both species in all specimens 

 under 70.0 mm ML. In L. pealei, the mean fin length 

 of specimens from 90.0 to 99.9 mm ML was equal to 

 50% of the mantle length, and this percentage in- 

 creased with increasing size. Fin length in relation 

 to mantle length was 50% in L. plei only at mantle 

 lengths of 120.0 mm and larger. LaRoe (1967) also 

 listed width of the gladius and length of the funnel 

 cartilage as useful characters to separate the spe- 

 cies; however, Cohen (1976) noted that the overlap 

 in values for these characters negated their useful- 

 ness. The present data show overlap in GW in all but 

 the larger size classes. Overlap in FCL was more preva- 

 lent in smaller size classes, but in specimens greater 

 than 90.0 mm ML, it may be a useful character. 



The shape of the gladius and the area of the junc- 

 tion of the vane and free rachis were distinct in both 

 species over the size range of individuals examined. 

 The gladius in L. pealei was broader and more 

 rounded than was the gladius in L. plei. It was 

 feather-shaped in all specimens and the vane was 

 without ribs. The junction of the vane and the free 

 rachis was gradual and not distinct. The gladius in 

 L. plei was more slender, elongated, and resembled 

 a sword or dagger in shape. Ribs in the vane were 

 more obvious in larger specimens. The lateral mar- 

 gins of the vane were usually straighter and the ribs 

 in the vane were more pronounced in males than in 

 females. The junction of the vane and free rachis was 

 abrupt and distinct. 



The best discrimination between the two Loligo 

 species was based on combinations of measurements 

 of the characters and calculated indices associated 

 with cartilaginous structures. Funnel or nuchal car- 

 tilage lengths combined with rachis width measure- 

 ments or indices enabled greatest percent separa- 

 tion of the species. Characters and indices based on 

 gladius shape also had high percentage levels of sepa- 

 ration. Highest correct classification ofL. pealei (95%) 

 by discriminant analysis was achieved by combin- 

 ing RW with either FCL or GW or by combining the 

 GW/ML and RW/ML indices. Loligo plei were classi- 

 fied with 100% accuracy by combining FCL and RW. 



Numerous studies cite the GW/RW ratio as a reli- 

 able index for species differentiation. Roper et al. 

 ( 1984) summarized taxonomic data for the two spe- 

 cies and reported GW/RW ratios from 2.4 to 3.7 for 

 L. pealei and from 1.5 to 2.4 for L. plei. Cohen (1976), 

 Whitaker ( 1978), and Hixon ( 1980) also used the GW/ 

 RW index to distinguish the two species of Loligo and 

 used the ratio of 2.4 to differentiate between the spe- 

 cies: below 2.4 for L. plei and above 2.4 for L. pealei. 

 The present study is the first to use IEF to identify 

 juvenile and subadult L. pealei and L. plei and thus 

 provides a more accurate base from which to assess 

 morphological variation between the species. The 

 GW/RW ratios in the present study ranged from 1.9 

 to 2.7 for L. plei and from 2.1 to 3.8 for L. pealei. 

 Separation of the species was best accomplished with 

 a GW/RW ratio of 2.7. Applying the ratio of 2.4 to the 

 present data resulted in overlap in both species; 10% 

 of the L. plei had an index greater than 2.4, and 4% 

 of the L. pealei had an index below 2.4. By using 2.7 

 as the critical value, all the L. plei were separated 

 and 91% of the L. pealei could be identified. Hixon 

 ( 1980) also found specimens of L. plei with GW/RW 

 ratios above 2.4. He identified 21 mature females and 

 one male L. plei from Texas waters with GW/RW in- 

 dices ranging from 2.4 to 2.9. He attributed the 

 greater index in the females to the wider vane asso- 

 ciated with sexually mature individuals. This does 

 not explain the higher GW/RW indices recorded for 

 some specimens of L. plei in this study, because all 

 individuals were immature. Also puzzling are the 

 specimens of L. pealei with GW/RW ratios below 2.7. 

 Because all specimens in the present study were iden- 

 tified biochemically, errors in identification should 

 be negligible. According to Cohen ( 1976 ), L. pealei is 

 composed of morphologically variable populations 

 and greater variation occurs in areas where L. pealei 

 co-occurs with L. plei. In these areas of sympatry, 

 there is a closer resemblance between species. Cohen 

 (1976) found a single specimen of L. pealei with a 

 GW/RW ratio below 2.4, and this specimen was from 

 an area where both species occur. The possibility of 



