JACKSON: LIFE HISTORY OF CEPHALOPOD IDWSEPIUS PYGMAEUS 



and twenty-two one-half hour oblique Tucker Trawl 

 plankton samples taken at a number of stations 

 across the Central Great Barrier Reef Lagoon 

 (Jackson and Hartwick unpubl. data) yielded 

 only 19 /. pygmaeus specimens, even though 

 numerous other cephalopod larvae were captured. 

 In comparison, up to 171 individuals have been 

 dipnetted during one 2 hour collecting trip along 

 1 .72 km of breakwater in the vicinity of Townsville 

 harbor. 



Statolith Structure and Microanatomy 



The statoliths of /. -pygmaeus are complex three- 

 dimensional structures. Because of limited depth of 

 field for one plane of focus, some rings near the 

 nucleus are not discernible when photographed (Fig. 

 lA; classification is after Clarke 1978). Growth rings 

 are most clearly seen in the lateral region near the 

 rostrum. A specimen stained with tetracycline twice, 

 17 days and 8 days before death, showed consider- 

 able statolith growth over a relatively short period 

 of time (Fig. IB). In many instances a prominent 

 discontinuity (check) within the statolith corre- 

 sponded to the time of staining. This feature was 

 particularly useful for subsequent ring counts under 

 the light microscope. A tetracycline stained statolith 

 was selected and photographed under both white 

 light, to identify the daily ring structure (Fig. IC) 

 and UV light to identify the point of staining (Fig. 

 ID) from which the subsequent daily rings were laid 

 down. Further validation evidence is given for six 

 tetracycline stained specimens in which the stato- 

 lith ring sequence could be accurately counted (Table 



1). 



Statolith checks were also observed within the 

 ring sequence of some field-captured /. pygmaeus 



specimens (Fig. 1 A, arrow). Statolith checks are con- 

 siderably more prominent than the other rings due 

 to a greater degree of transparency in the check 

 region producing enhanced \isibility under the light 

 microscope. The degree of enhanced visibility often 

 varied between checks. 



Sexual Dimorphism and Maturity 



This species shows considerable sexual dimorph- 

 ism, with females achieving a much greater size than 

 males (Fig. 2). The size for all male specimens col- 

 lected ranged from 5.8 mm to 10.3 mm and 36 mg 

 to 159 mg in weight, while females ranged from 6.5 

 mm to 17.6 mm and weighed between 67 mg and 

 655 mg. 



The reasons for the marked size-related sexual 

 dimorphism in I. pygmaeus could be ascertained by 

 ageing individual males and females. Females 

 achieve a larger size predominantly by growing at 

 a much faster rate than males and to a lesser ex- 

 tent by growing older than males (Fig. 3). Moreover, 

 weight is an unreliable index of age, particularly in 

 females, as indi\'iduals of similar ages can vary con- 

 siderably in weight (Fig. 3). 



Males mature as small as 6.8 mm and 62 mg and 

 as young as 42 days. In contrast, females matured 

 at around 13 mm and 400 mg and as young as 60 

 days. The largest immature female aged was 59 days 

 and was 339 mg and 13.7 mm. All males examined 

 had spermatophores present although the youngest 

 specimen caught (35 days, 6.1 mm, 40 mg) only had 

 one spermatophore in Needham's sac. 



Based on statolith age analysis, the lifespan of I. 

 pygmaeus is quite short, with the oldest specimens 

 aged being 67 days and 79 days for males and 

 females respectively. 



Table 1, 



-Age validation information for Idiosepius pygmaeus (L: lateral region; R: rostrum; DD: dorsal 



dome). 



267 



