JACKSON: LIFE HISTORY OF CEPHALOPOD IDIOSEPIUS PYGMAEUS 



DISCUSSION 



Daily periodicity in ring formation has been shown 

 for both Illex illecebrosus (Hurley et al. 1985; Dawe 

 et al. 1985) and Alloteuthis suhulata (Lipinski 1986) 

 in the North Atlantic, and for juvenile specimens 

 of the pacific market squid, Loligo opalescens, (Yang 

 et al. 1986). The tropical /. pygmaeus has provided 

 further evidence for the one-ring:one-day hypothesis 

 in a nonteuthoid cephalopod and suggests that daily 

 statolith rings are a widespread phenomenon among 

 cephalopod species. 



The fact that the rings were most consistently 

 visible near the lateral dome region of the stato- 

 lith of /. pygmaeus is probably related to the thin- 

 ness in this region, which allows better light 

 transmission. A similar situation has been found 

 in the dolphin fish Coryphaena hippurus. This fish 

 has a complex elongate sagitta. Consequently when 

 mounted in a clearing agent, growth rings are 

 visible only on the otolith's lateral region rather 

 than on the longest axis (Oxenford and Hunte 

 1983). 



Check marks within the microstructure of the 

 statoliths of /. pygmaeus are very similar to stress 

 checks observed in fish otoliths (Pannella 1980; Cam- 

 pana 1983; Campana and Neilson 1985). Stress in 

 fish has been shown to reduce branchial uptake of 

 calcium, resulting in a calcium-poor check structure 

 that is visually prominent compared to the surround- 

 ing daily increments. The visual intensity of a check 

 in fish is often proportional to the magnitude and 

 duration of the stress that caused it (Campana 1983). 

 The fact that a statolith check was often induced 

 when /. pygmaeus was captured and exposed to 

 tetracycline suggests that stress is also a likely cause 

 for statolith check formation. The trauma of cap- 

 ture, confinement in low oxygen conditions during 

 collection along with subsequent staining and tem- 

 perature shock associated with the transfer to 

 aquaria, all could contribute to inducing a statolith 

 check. 



The fact that /. pygmaeus matures at an early 

 age, suggests that it is capable of a number of 

 generations in any one year. This agrees with com- 

 ments by Voss (1983) that small cephalopod species 

 are probably capable of multiple generations per 

 year. 



Although size-related sexual dimorphism is com- 

 mon in cephalopods the condition observed in /. 

 pygmMeus is unusual and approaches one end of the 

 continuum with the exception of species of pelagic 

 octopods with dwarf males (e.g., Argonauta spp.) 

 (Wells and Wells 1977). This study confirms that 



statolith microstructures provide a means for age- 

 ing cephalopods. 



ACKNOWLEDGMENTS 



I would like to thank R. F. Hartwick who provided 

 valuable assistance throughout this research, J. H. 

 Choat for offering useful suggestions and for critic- 

 ally reading the manuscript, N. E. Milward for ad- 

 vice during the project, C. C. Lu for identification 

 of cephalopod specimens, and C. H. Jackson for 

 assistance with field collecting and preparation of 

 figures. 



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P- 

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271 



