Jackson: Cephalopod reproductive investment 



269 



individuals captured under similar environmental 

 conditions. 



Idiosepius pygmaeus 



The pattern of gonad maturation in tropical squids and 

 sepioids becomes more complex when the pattern of 

 seasonal maturation of /. pygmaeus is also considered. 

 As with L. chinensis, individuals of /. pygmaeus which 

 grew during a cooler time of the year also reached a 

 greater age. However, gonad maturation was modified 

 in a different way compared with L. chinensis. For ex- 

 ample, in both seasons males (including mature indi- 

 viduals) had similar-sized testes at an age of 30^40 d 

 suggesting that a minimal gonad size can be reached in 

 just over a month. Nevertheless, as the cooler-season 

 males continued to grow, the gonad continued to reach 

 an appreciably greater size, for the same-sized indi- 

 vidual. Thus, environmental constraints (such as tem- 

 perature, food availability, or light levels) produced a 

 different allometric gonad-soma relationship in this spe- 

 cies which was not apparent in L. chinensis. 



The fact that /. pygmaeus has a short lifespan and 

 rapid growth may account for the lack of females with 

 mature ova. It is possible that ovum maturation could 

 take place very rapidly just before egg deposition. 

 Therefore, it could be difficult to capture females with 

 mature oocytes unless it was just prior to egg deposi- 

 tion. Alternatively, females with ripe ova may not have 

 been near the water surface and therefore not avail- 

 able for sampling. 



Idiosepius pygmaeus appears to be employing a 

 'trade-off in its seasonal reproductive tactics, that is, 

 benefiting from one process bought at the expense of 

 another (Begon et al. 1986). During the cooler period 

 of the year, growth is slowed (presumably as a result 

 of metabolic responses to temperature), therefore 

 lifespan is increased as a necessity, since individuals 

 take longer to reach adult size. As a result, /. pygmaeus 

 appears to change its tactics by partitioning a greater 

 amount of energy into gonads over the longer lifespan. 

 Although a longer time-period is taken to reach matu- 

 rity, there would be a reproductive advantage in that 

 possessing larger gonads would increase reproductive 

 output. A similar situation has been shown to exist 

 with teleost fishes. Stearns (1976) has provided evi- 

 dence from several species of teleosts, showing that 

 the ratio of ovary weight to body weight (as a measure 

 of reproductive effort ) increased with age. 



The fact that this phenomenon occurs in /. pygmaeus 

 and not the larger L. chinensis may be due to the need 

 for /. pygmaeus to maximize its reproductive chances 

 because of the greater nearshore habitat variability, or 

 because of the greater constraints this species faces 

 due to its small body size. 



Acknowledgments 



I would like to thank J.H. Choat for comments through- 

 out this project, C.H. Jackson for critically reading the 

 manuscript and assisting with figures, and the crew of 

 the research vessel James Kirby for specimen collec- 

 tion. Manuscript comments by two anonymous review- 

 ers and L.L. Jones were greatly appreciated. This 

 research was supported by grants from the James Cook 

 University of North Queensland Research Funding 

 Panel. 



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