Jackson and Yeatman Phenotypic plasticity in Photolohgo sp. I 



teria; the differences between stages 4 and 5 were 

 found to be mostly morphological for both sexes 

 (Sauer and Lipinski, 1990). Dorsal mantle length 

 (ML), measured on fresh material, was used as the 

 standard squid length. 



Statolith preparation and ageing 



Statoliths were stored dry in a plastic cavity block. 

 For increment analysis, statoliths were mounted in 

 the thermoplastic cement Crystal Bond. Increments 

 in statoliths, from small individuals, could be counted 

 directly without any further preparation. The sta- 

 toliths from larger individuals needed grinding and 

 polishing (see Jackson, 1990a) to reveal all the in- 

 crements from the nucleus to the edge. The incre- 

 ments within the statolith microstructure were very 

 clear and easy to count. All counts were carried out 

 in the dorsal or lateral region of the dorsal dome by 

 following the increments with a camera lucida at- 

 tached to an Olympus BH microscope (x400). Total 

 increment number was taken as the mean of three 

 replicate counts that differed less than 10% from the 

 mean. 



The periodicity of the increments was not deter- 

 mined in this study. However, the increments were 

 very similar in appearance to validated daily statolith 

 increments of other tropical loliginids — Loligo 

 chinensis and Loliolus noctiluca (Jackson, 1990b). 

 Daily periodicity in statolith increments was also 

 validated for the tropical loliginid Sepioteuthis 

 lessoniana (Jackson, 1990a; Jackson etal., 1993). The 

 increments within the statolith microstructure of 

 Photolohgo sp. 1 in this study were therefore assumed 

 to be daily. However, there is a need for validation of 

 increment periodicity in this species. 



Results 



Photololigo edulis is distinguished from other 

 Photolohgo species primarily by its sucker dentition, 

 which usually requires detailed morphological in- 

 spection. However, previous genetic analysis has re- 

 vealed two cryptic species exhibiting "edulis-\ike" 

 teeth in northern Australia (Yeatman and Benzie 

 1993). 



All of the individuals of Photolohgo sp. 1 from the 

 northwest shelf in this study were also used in a 

 population genetic study, each for eleven (five poly- 

 morphic) loci (Yeatman and Benzie, 1994). Genotypic 

 frequencies for all loci showed good conformance to 

 Hardy-Weinberg expectations for random mating, 

 and the F IS value was not significant, thus demon- 

 strating a lack of within-population structure. 



Size at age 



The length-at-age data for the Photololigo sp. 1 popu- 

 lation were linear over the size range sampled. This 

 species appears to have a short life-span; the oldest 

 individual is estimated to be less than 160 d (Fig. 2). 

 The regression equations were y = 1.12x - 57.46 

 [r 2 =0.77], andy = 1.02.* - 45.57 [r^O.90], (where x is 

 age in days and y is ML in mm) for males and fe- 

 males, respectively. Both regressions were highly sig- 

 nificant (P<0.0001 ), and there was no significant dif- 

 ference between males and females in the slopes or 

 elevations in the Age:ML relationships (ANCOVA, 

 P>0.05). Males that were aged ranged in size from 

 38 mm to 87 mm ML, and females that were aged 

 ranged from 32 mm to 115 mm ML. 



Maturation 



Males All males sampled (n=37) were between 

 maturity stages 2 and 5. Stage-5 males showed a 

 large range in mantle length (48 to 126 mm ML). 

 Some mature stage-5 males were smaller than im- 

 mature stage-3 and stage-4 males (Fig. 3A). 



On the basis of the subsample of aged males (n = 13), 

 the largest male aged was over twice as long as the 

 smallest male aged (Fig. 4A), whereas the oldest male 

 ( 119 d) was only 31 d older than the youngest male 

 (88 d). The age data (Fig. 4B) confirmed that some 

 males were maturing at not only a smaller size but 

 also at a younger age. 



120 



100 



80 



60 

 40 

 20 







O MALES 

 • FEMALES 



20 40 60 80 100 120 140 160 

 Age (d) 



Figure 2 



The relationship between age and mantle length for male 

 (n = 13) and female (n = 19) individuals of Photololigo sp. 1. 

 Solid regression line represents males, dashed regression 

 line represents females. 



