Abstract. —Seasonal variability 

 in gonad growth was investigated for 

 the tropical cephalopods Loligo 

 chinensis and Idiosepius pygmaeus. 

 Statolith ageing techniques enabled 

 a comparison of gonad growth in re- 

 lation to both individual size and 

 age. Age analyses revealed that male 

 and female individuals of L. 

 chinensis matured earlier during the 

 warmer summer period than in the 

 cooler winter period. These prelimi- 

 nary results suggested that matu- 

 rity was governed more by individual 

 size rather than age. Analysis of sea- 

 sonal change in the gonadosomatic 

 index (GSI) revealed that L. 

 chinensis gonad tissue accounted for 

 the greatest percentage of body 

 weight in the month of October. The 

 trend in the nidamental gland/ 

 mantle length index closely paral- 

 leled the trend in GSI values for fe- 

 male individuals of L. chinensis, 

 while growth of the nidamental 

 gland and hectocotylus closely par- 

 alleled growth of the gonad. The sea- 

 sonal variation in reproductive in- 

 vestment for Idiosepius pygmaeus 

 followed a different pattern com- 

 pared with L. chinensis. Slower 

 growing cool-season (spring) indi- 

 viduals lived longer and had com- 

 paratively larger gonads than their 

 warm-season (autumn) counterparts, 

 despite no difference in body size be- 

 tween the two seasons. Idiosepius 

 pygmaeus thus appeared to be em- 

 ploying a 'trade-off in its reproduc- 

 tive strategy by partitioning a 

 greater amount of energy into go- 

 nad tissue over a longer lifespan dur- 

 ing the cooler period of the year. 



Seasonal variation in reproductive 

 investment in the tropical loliginid 

 squid Loligo chinensis and the 

 small tropical sepioid Idiosepius 

 pygmaeus 



George D. Jackson 



Department of Marine Biology, James Cook University of North Queensland 

 Townsville 48 11 . Queensland, Australia 



Present address: Department of Zoology, University of Western Australia 

 Nedlands, Perth, Western Australia 6009 



Manuscript accepted 10 December 1992. 

 Fishery Bulletin, U.S. 91:260-270 1 1993 1. 



In reviewing cephalopod reproduc- 

 tion, Mangold ( 1987) has emphasised 

 that there are at least as many open 

 questions as there are established 

 facts, and that there are thus many 

 gaps in our knowledge as well as con- 

 tradictory statements. Recent re- 

 search (e.g., Hanlon et al. 1989, 

 Rodhouse & Hatfield 1990, Jackson 

 & Choat 1992) is revealing that 

 cephalopod lifespans are considerably 

 shorter than many estimates made 

 over the last several decades. This 

 past confusion has apparently led 

 to a poor understanding of the re- 

 productive tactics of cephalopods. 

 Clearly, any ideas regarding the 

 lifespan of an organism will influence 

 ideas regarding reproductive events 

 in the individual. 



Statolith ageing techniques have 

 the potential for resolving some of 

 the discrepancies in our understand- 

 ing of the reproductive tactics of 

 squids and sepioids. By analyzing in- 

 dividual age and maturity status, 

 age-at-maturity and time-specific 

 schedules of gonad growth can be con- 

 structed. The focus of this study was 

 to consider seasonal variation in age- 

 at-maturity and reproductive invest- 

 ment in the tropical loliginid squid 

 Loligo chinensis and the small sepioid 

 Idiosepius pygmaeus. Statolith age- 

 ing techniques have been applied to 

 both /. pygmaeus (Jackson 1989) and 

 L. chinensis (Jackson 1990a, Jackson 



& Choat 1992). Analysis of seasonal 

 samples for both of these species has 

 revealed that there was considerable 

 seasonal variation in growth (Jack- 

 son & Choat 1992). This study was 

 therefore undertaken to see if there 

 was any seasonal variation in gonad 

 growth or seasonally-induced varia- 

 tion in age-at-maturity in these two 

 species. 



Loligo chinensis is common in 

 North Queensland waters and can be 

 captured by bottom trawls through- 

 out all months of the year. The 

 species is sexually dimorphic, with 

 males growing longer than females. 

 In North Queensland waters, males 

 are commonly encountered up to 

 -180 mm dorsal mantle length 

 (DML), while females are common up 

 to ~120mmDML. Hatchling size for 

 L. chinensis is -1.4 mm (unpubl. 

 data). In the summer, L. chinensis 

 reaches adult size in ~100d, while 

 winter growth is slower with adult 

 size reached by 140-170 d (Jackson 

 & Choat 1992). 



Idiosepius pygmaeus is a common 

 neritic cephalopod which is found in 

 surface waters in mangrove, estua- 

 rine, and breakwater habitats (Jack- 

 son 1989). This species, however, is 

 only common in nearshore surface 

 waters between March and Novem- 

 ber, with few specimens observed 

 over the summer months (December- 

 February) (Jackson 1992). Idiosepius 



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