548 



Fishery Bulletin 94(3). 1996 



Figure 1 7 



Illustrations of gladii traced from photocopies: Lolign pealei 

 (A) female (Bl male; Loligo plei (Cl female (D) male. 



Discussion 



Isoelectric focusing of mantle proteins seems to pro- 

 vide a reliable means of distinguishing between the 

 two species of Loligo in the Gulf of Mexico and to 

 enable accurate identification of small, immature 

 specimens. Garthwright et al. (1989) also biochemi- 

 cally separated L. plei and L. pealei but used the tech- 

 nique of horizontal starch gel electrophoresis. Morpho- 

 metric analyses provided information on sexual dimor- 

 phism within the species and on species differences and 

 similarities in specimens from the northern Gulf of 

 Mexico, already identified to species by means of IEF. 

 Comparisons of morphometric characters and in- 

 dices between males and females of each species were 

 made in an effort to determine their value as a rapid 

 means for fishery biologists to distinguish the sexes 

 in the field. Although statistically significant differ- 

 ences were evident in many of the selected morpho- 

 metric characters and indices, both between sexes 

 within a species and between species, the differences 



were small and overlap usually was high. The ma- 

 jority of squids examined in this study were between 

 40.0 and 80.0 mm ML, and greatest overlap in char- 

 acters and indices occurred in this size range. 



Tests of sexual dimorphism in measured charac- 

 ters regressed against mantle length for L. pealei 

 indicated no significant differences between males 

 and females in the slopes of the lines for W and GW. 

 Females were heavier than males and the GW was 

 broader in females than in males in all size classes. 

 Weight and GW also were highly correlated to ML in 

 both males and females of L. plei; however, no sig- 

 nificant difference existed in the slopes of the lines, 

 and the 95% confidence intervals of the regression 

 intercepts overlapped. Differences in weight between 

 males and females were more pronounced in L. pealei. 

 Haefner ( 1964) noted little difference in the weight- 

 length relationship between males and females of L. 

 pealei below 95 mm ML; however, at mantle lengths 

 greater than 95 mm ML, females began to outweigh 

 males. Hixon (1980) also found that the rate of in- 

 crease in weight per unit increase in mantle length 

 was faster in females than in males. Both Haefner 

 (1964) and Hixon (1980) attributed this faster rate 

 of increase in females to sexual maturation and the 

 increase in weight of the female reproductive struc- 

 tures. For northernmost populations of L. pealei off 

 New England, females less than 170.0 mm ML gen- 

 erally weighed less than males (Lange and Johnson, 

 1981 ). Maturation at smaller sizes in southern popu- 

 lations of long-finned squid could account for this 

 difference in the weight-length relationship between 

 females and males in comparisons of data for north- 

 ern and southern populations. Both Cohen ( 1976) and 

 Hixon (1980) noted that L. pealei seems to mature 

 at smaller sizes in the warmer portion of its range. 

 Width of the gladius also has been linked to sexual 

 maturation in female Loligo. Cohen ( 1976) noted that 

 in mature females the broadest portion of the gladius, 

 the vane, covers the full ovary and may provide sup- 

 port and protection for the eggs. In the present study, 

 the gladius was broader in females of both species. 



Although there were differences in morphometric 

 characters and indices between sexes within a spe- 

 cies, these differences were subtle and usually were 

 more apparent in larger specimens. The high over- 

 lap both in measured characters and in indices pre- 

 cluded their use as a reliable means of distinguish- 

 ing the sexes in juvenile and subadult specimens. 

 Gender-specific morphological characters (e.g. 

 hectocotylus or sexual organs) appear to be the only 

 precise means of differentiating males and females 

 of the two species. In those specimens in which these 

 characters are undeveloped, no way was discovered 

 to distinguish between the sexes. 



