Thomas and Moltschaniwskyi: Ontogenetic changes in size and shape of statoliths of Sepioteuthis lessoniona 



643 



total length and along the lateral dome and ventro- 

 lateral length axes suggests that relative growth of 

 these regions is slow and that statoliths retain the 

 elongate region of the rostrum as they grow. This 

 was reflected in the superior definition and clarity 

 of daily increment structures in the lateral dome re- 

 gion of adult statoliths when viewed under a light 

 microscope. This may also explain why optical acu- 

 ity of daily increment structures in the rostrum re- 

 gion of adult statoliths was poor. Slower growth of 

 statoliths in the rostrum indicates deposition of less 

 aragonite material compared with other faster grow- 

 ing regions of the statolith and in turn suggests that 

 daily increment widths in the rostrum will be nar- 

 rower and therefore harder to discern with light mi- 

 croscopy. In contrast, daily growth increments have 

 been reported as displaying optimal definition in the 

 rostrum of two other squid species (Photololigo edulis, 

 Natsukari et al., 1988; Berryteuthis magister, 

 Natsukari et al., 1993). Therefore, it is likely that 

 differences occur in statolith accretion between spe- 

 cies, possibly related to behavior. Optical definition 

 of daily increments in statoliths has important con- 

 sequences when ageing and growth studies are based 

 on counts and distances between increments. Age and 

 growth calculations based on narrower less distinct 

 increments may not reveal true patterns otherwise 

 evident in optically more distinct incrementation. 



Results from this study suggest that prediction of 

 age of S. lessoniana individuals from the size and 

 shape of their statoliths remains doubtful on account 

 of the disassociation between statolith and somatic 

 tissue growth. Changes in statolith shape during 

 growth may also be important if statoliths are to be 

 employed as a tool for constructing growth histories 

 of S. lessoniana. Variable levels of statolith accre- 

 tion ultimately leading to slight modifications in 

 shape may not necessarily be indicative of changes in 

 somatic growth. Ageing studies based on analyses of 

 statolith size from other squid species need to be aware 

 of the possible disassociation between statolith and 

 somatic tissue growth. The changes in size and shape 

 of Sepioteuthis lessoniona statoliths described in this 

 study are likely due to differing aragonite accretion 

 levels, possibly on a daily basis, in response to physi- 

 ological and behavioral adaptations during ontogeny. 



Acknowledgments 



We thank G. Jackson for providing constructive re- 

 views of early manuscripts. We also acknowledge J. 

 Semniens, G. Peel, and P. Martinez for their assis- 

 tance in the field and laboratory. This research was 

 completed in partial fulfillment of an Honours de- 



gree by RT and was supported by a Merit Research 

 Grant from James Cook University awarded to NAM. 



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