THE USE OF STATOLITH MICROSTRUCTURES TO ANALYZE 

 LIFE-HISTORY EVENTS IN THE SMALL TROPICAL CEPHALOPOD 



IDIOSEPIUS PYGMAEUS 



George David Jackson^ 



ABSTRACT 



Populations of the sepioid Idiosqaius pygmaeus were located in mangrove and estuarine localities in the 

 Townsville region of North Queensland Australia in 1986. This species was small, easy to observe and 

 collect in the field and sexually dimorphic, with females being much larger than males. 



Statolith microstructures of /. pygmaeiis proved to be a useful ageing tool which can be used to 

 interpret life history phenomena in this species. Increments were calibrated by marking statoliths in 

 situ with tetracycline and counting the rings laid down subsequent to marking. This validated the daily 

 periodicity of the observed rings. 



Statolith discontinuities (checks) were occasionally seen within the microstructures of some specimens. 

 These discontinuities appear to parallel similar structures found in fish otoliths. 



Based on statolith analysis, /. pygmaeiis matured at an age of IV2-2 months. Females were larger 

 and grew faster than their male counterparts. Females of similar age were found to vary considerably 

 in size. The estimates of growth rates and longevity for /. pygmaeus suggested multiple generations 

 within one year. 



Pannella (1971) discovered daily growth increments 

 within the otolith microstructure. Subsequently a 

 plethora of information has been obtained from oto- 

 lith microstructural analysis, which has greatly aided 

 studies of fish biology and population dynamics (see 

 Campana and Neilson 1985 for a review of the rele- 

 vant literature). Growth ring analysis has provided 

 a means to evaluate age structures and growth rates 

 in young fishes, and constitutes a powerful tool for 

 population analysis. Similar growth increments have 

 been observed within the statolith microstructure 

 of many cephalopod species (Clarke 1966; Hurley 

 and Beck 1979; Spratt 1979; Lipinski 1981; Kris- 

 tensen 1980; Rosenberg et al. 1981; Radtke 1983; 

 Natsukari et al. 1988) although concentric rings do 

 not appear to be laid down in octopus statoliths 

 (Boyle 1983). Radtke (1983) suggested that squid 

 statoliths are analogous to fish otoliths; continuing 

 research is supporting his view. 



The majority of cephalopod species appear to be 

 short lived and exhibit rapid growth rates (Packard 

 1972; Saville 1987). The ability to age cephalopods 

 is critical to understanding life history phenomena 

 and population dynamics. Despite the presence of 

 statolith microstructures, there have been few at- 



'Department of Marine Biology, James Cook University of North 

 Queensland, Townsville, Queensland 4811, Australia. 



tempts to use this information to develop a picture 

 of demographic events in the Cephalopoda. 



Early in 1986 relatively large populations of the 

 sepioid /. pygmaeus were discovered in mangrove/ 

 estuarine localities in the Townsville region of North 

 Queensland, Australia. This species was small and 

 markedly sexually dimorphic. Idiosepms pygmaeus 

 was readily observed and captured in the field, and 

 it was robust enough to make a useful experimen- 

 tal organism. Moreover its small size suggested a 

 relatively short lifespan, providing the potential of 

 obtaining complete records of age and size specific 

 events. 



Microstructural examination of the statolith of/. 

 pygrnaeus was undertaken. Growth rings were pres- 

 ent and could be used as an accurate means of age- 

 ing. Most similar studies have focused on Northern 

 Hemisphere temperate squids. This is the first study 

 to analyze statolith ring structure and periodicity 

 in a tropical sepioid. 



The research aims of this investigation were two- 

 fold; 1) to validate statolith increments as growth 

 rings and 2) to utilize the growth ring data to inter- 

 pret life history phenomena. 



MATERIALS AND METHODS 



Idiosepius pygmaeus specimens were captured 

 between May and August 1986, and in May 1987, 



Manuscript accepted November 1988. 

 Fishery Bulletin, U.S. 87:265-272. 



265 



