Thomas and Moltschaniwskyi Ontogenetic changes in size and shape of statoliths of Sepioteuthis lessoniana 



637 



Mugiya and Tanaka, 1992 ), indicating a possible dis- 

 association between otolith and somatic growth. 

 Similarly, disassociation between statolith and so- 

 matic tissue growth has been demonstrated in the 

 tropical loliginid squid Loligo chiiieiisis (Jackson, 

 1995), and also in two ommastrephid squids, 

 Todarodes angolensis and Todaropsis eblanae (Lipinski 

 etal., 1993). 



The periodic growth increments in squid statoliths 

 are bipartite structures consisting of a discontinu- 

 ous zone and an incremental zone comprising arago- 

 nite (CaCO.^) crystals (Lipinski, 1986). Specific mor- 

 phological differences in the aragonite crystal struc- 

 ture have been illustrated between statolith regions 

 within several squid species (Lipinski, 1993). Crys- 

 tals of the lateral dome region often display greater 

 variation in shape, size, and orientation than the 

 more homogenous wing crystals (Lipinski, 1993). 

 These variations in crystal structure suggest that 

 accretion is not uniform and that different regions of 

 the statolith may grow at different rates. 



The present study provides an assessment of the 

 growth and shape of Sepioteuthis lessoniana sta- 

 toliths based on the linear measurement of statolith 

 dimensions. The potential for using statolith shape 

 descriptions as a proxy for age estimations and the 

 relation between statolith and somatic tissue growth 

 in these squid are examined. Overall descriptions of 

 individual statolith shape covered a broad length and 

 weight range of squid, providing a basis for compari- 

 son with age estimates determined by using tradi- 

 tional analysis techniques. With the increased use 

 of statoliths in squid age and growth studies, it is 

 important to understand how statoliths grow and 

 how this growth relates to growth of somatic tissue. 



Materials and methods 



Study species and collection methods 



A total of 103 Sepioteuthis lessoniana individuals 

 were captured from waters around the Townsville 

 region of north Queensland, Australia, between Janu- 

 ary and August 1995. Twenty-three juvenile squid 

 (16-43 mm mantle length [ML]) were captured by 

 using purse seines and dip nets. Juveniles often shel- 

 ter in surface waters among floating debris or artifi- 

 cially constructed shade devices and are easily net- 

 ted. Eighty adult squid (82-245 mm ML) were cap- 

 tured from coastal waters at night with squid jigs 

 and by means of light attraction. Individuals were 

 separated into two age groups for these descriptions: 

 juvenile (<60 days) and adult (>60 days). This sepa- 

 ration was based on the techniques used to capture 



Figure 1 



Sepioteuthis lessoniana statolith showing the 

 dimensions measured for morphometric analy- 

 ses. TL = total statolith length; DLL = dorsolat- 

 eral length; VLL = ventrolateral length; LDL = 

 lateral dome length; MW = maximum width. 



the individuals, which may be related to differences 

 in life style and ecology. 



Size measurements 



Mantle length was measured from all Sepioteuthis 

 lessoniana specimens upon capture. Wet weight was 

 not obtained for the majority of individuals because 

 of the inability to weigh animals accurately on a boat. 

 Statoliths were removed and stored in 707^ alcohol, 

 and adult statoliths were later weighed to the near- 

 est 0.01 mg. Weights of juvenile statoliths were not 

 recorded because they were too small to obtain accu- 

 rate measurements. Five dimensions were measured 

 from each statolith following the descriptions of sta- 

 tolith shape by Clarke (1978): total length, dorsolat- 

 eral length, ventrolateral length, lateral dome length, 

 and maximum width (Fig. 1 ). Dimensions were mea- 

 sured from whole statoliths viewed with the ante- 

 rior (concave) side positioned upward by using an 

 Ikegami-290 high-resolution black and white video 

 camera mounted on a compound microscope. Squid ages 

 were estimated by using statolith increment analysis 

 on ground and polished statoliths (Jackson, 1990). All 

 increment counts were made in the dorsal-dome re- 

 gion of statoliths because increment definition was con- 

 sistently clearest in this area. Age estimates for 62 



