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Fishery Bulletin 97(3), 1999 



individuals. This indicates a disassociation between 

 statolith and somatic (mantle length) growth in S. 

 lessoniana. The disassociation of otolith and somatic 

 growth has been demonstrated in several fish spe- 

 cies (e.g. Mosegaard et al., 1988; Secor and Dean, 

 1989; Francis et al., 1993). In squids, Loligo chiiiensis 

 (Jackson, 1995 ), and Todaropsis eblanae and Todarodes 

 angolensis (Lipinski et al., 1993) also display disasso- 

 ciation between statolith and somatic tissue growth. 

 Comparison of statolith size and shape between 

 Todarodes angolensis and Todaropsis eblanae also has 

 revealed distinct species differences (Lipinski et al., 

 1993). Jackson (1995) suggested that the use of the 

 relation of statolith length to mantle length as a pre- 

 dictor of squid ages should proceed with caution until 

 temperature-related effects on squid growth (e.g. sea- 

 sonal effects), as well as the relation between statolith 

 and somatic growth are better understood. 



The disassociation between statolith and somatic 

 growth rates is possibly a function of differences be- 

 tween the mechanisms responsible for these two pro- 

 cesses. A process related to metabolic rate, rather 

 than somatic growth, seems to govern the rate of 

 otolith accretion in fish (Wright, 1991). Although a 

 close correlation often exists between fish somatic 

 growth and metabolic rate in early life-history stages, 

 intrinsic or extrinsic constraints on somatic growth 

 may affect this relationship and result in a disasso- 

 ciation between otolith and somatic growth (Wright, 

 1991). Recently, Lombarte and Lleonart (1993) pro- 

 posed that otolith growth in fish may occur under 

 dual regulation: overall shape is genetically deter- 

 mined whereas otolith size is governed by environ- 

 mental factors. Several workers have also suggested 

 that temperature plays a particularly important role 

 in determining otolith growth, primarily through the 

 effect of temperature on metabolic rate (Wright, 1991; 

 Bradford and Geen, 1992). In order to demonstrate 

 a link between otolith and somatic growth rates the 

 age-independent variability in the relationship needs 

 to be assessed (Hare and Cowen, 1995). The mecha- 

 nisms governing statolith growth in S. lessoniana 

 remain to be determined but may possibly be related 

 to the high plasticity of somatic growth and a fixed 

 growth trajectory of the statolith. Ontogenetic be- 

 havioral adaptations may also play a key role. None- 

 theless, there are important considerations for age 

 and growth studies based on the morphological fea- 

 tures of statoliths; whereas many squid are known to 

 grow continuously throughout their life (Jackson and 

 Choat, 1992; Jackson, 1994), growth ofstatoliths in our 

 study appears to approach a final asymptotic size and 

 shape. Statoliths grow with the age of the squid, but 

 statolith accretion may respond differently to environ- 

 mental factors than to growth of somatic tissue. 



The breakdown in the relation between statolith 

 total length and age in older S. lessoniana individu- 

 als may be attributed to variable accretion rates in 

 the statoliths of adult squid, possibly in relation to 

 environmental conditions. Varying aragonite accre- 

 tion within statoliths could bring about fluctuations 

 in daily increment widths that ultimately lead to 

 differences in overall length and weight of similar 

 aged statoliths. These differences are likely to be 

 more detectable in the older, larger statoliths of adult 

 squid. Alterations in otolith increment widths lead- 

 ing to different sized otoliths of the same age have 

 been shown for several fish species in response to 

 both biotic and abiotic factors (e.g. Eckmann and Rey, 

 1987;Sogard, 1991; Burke etal., 1993). Further work 

 on other fish species has shown that changes in in- 

 crement width may lag or be unrelated to changes in 

 somatic growth (Molony and Choat, 1990; Milicich 

 and Choat, 1992). Variable statolith increment 

 widths, that may be attributable to environmental 

 conditions, have been shown in at least one squid 

 species (Abralia trigonura, Bigelow, 1992), and one 

 cuttlefish (Sepia hierredda, Raya et al. 1994). 



Modifications of otolith shape and daily increment 

 structure in fish have been attributed to environmen- 

 tal changes or fiuctuations in individual fish physi- 

 ology (Morales-Nin, 1987; Nishimura, 1993; Wright, 

 1993; Tzeng and Tsai, 1994). As some fish migrate, 

 modifications are often necessary in order to meet 

 unique requirements for balance, orientation, and 

 navigation ( Blaxter, 1988 ). A transformation of otolith 

 shape may occur, resulting in discontinuities and 

 secondary growth structures (Sogard, 1991; Hare and 

 Cowen, 1994). Statoliths are the major sensory struc- 

 ture responsible for balance and orientation of squid 

 (Budelmann, 1990). The shape and structure of the 

 statocyst chamber itself are important to the swim- 

 ming performance and sensory perception capabili- 

 ties of cephalopods (Williamson, 1991). Statolith 

 shape and structure is, therefore, also likely to af- 

 fect the response of this unique and complex sensory 

 organ. Although S. lessoniana does not display dis- 

 tinct life-style changes or habitat shifts, a transition 

 from the observed juvenile habitat, where it com- 

 monly shelters among floating surface debris, to an 

 adult lifestyle that is typically more reef-associated, 

 may involve behavioral adaptations that lead to dif- 

 ferential growth of statoliths. Knowledge of the 

 mechanism by which this growth occurs and how it 

 can be modified during ontogeny will be critical to 

 understanding how statoliths grow. 



Most growth of adult statoliths in this study oc- 

 curred in the dorsal and lateral dome regions, pro- 

 ducing a more rounded and bulkier form than that 

 of the juveniles. The negative allometric growth in 



