Jackson Cephalopod reproductive investment 



261 



pygmaeus is sexually dimorphic, with females reach- 

 ing much larger sizes than males. In North Queensland 

 waters, males are commonly encountered up to 

 -10 mmDML, while females are commonly encountered 

 between 13 and 18 mmDML. Planktonic hatchlings are 

 ~1 mmDML (pers. observ.). This species has a short 

 lifespan, with maturity reached in <80d (Jackson 

 1989), and exhibits slower growth during the cooler 

 seasons of the year (Jackson & Choat 1992). 



Materials and methods 



Loligo chinensis and /. pygmaeus were captured from 

 tropical waters off Townsville, North Queensland. 

 Preparation and enumeration of statolith growth in- 

 crements for both species were similar to techniques 

 used for Sepioteuthis lessoniana (Jackson 1990b), al- 

 though statoliths of /. pygmaeus were not ground or 

 polished. Individuals of L. chinensis were captured in 

 paired trawl nets (each net had an 11m gape and 

 3.8cm mesh) which were towed for ~20min. The ma- 

 jority of individuals of L. chinensis were captured in 

 Cleveland Bay (19°11'S,146 56'E) in water depth <20 m. 

 Trawling was undertaken generally for one day each 

 month from February 1988 to November 1989, and up 

 to 15 trawls were taken on each sampling date. Indi- 

 viduals of /. pygmaeus were captured by dip-netting 

 along a breakwater east of the Townsville harbor 

 (19 o 15'S,146 o 50'E; see Jackson 1992). Individuals of 

 L. chinensis used in the age analysis were captured on 

 12 January 1989 (summer, n=37) and 13 July 1989 

 (winter, n=21). Individuals of/, pygmaeus were ana- 

 lyzed from autumn and spring. Individuals (n=41) 

 for the autumn sample were captured during four 

 sampling periods over two years: 22 and 23 March 

 1988; 21 and 22 March 1989. Individuals (n=38) for 

 the spring sample were from six sampling trips over 

 two months: 10, 23, 24 August 1988; 7, 20, 21 Sep- 

 tember 1988. The greatest differences in growth rates 

 and population age structure were observed between 

 these two seasonal periods (Jackson & Choat 1992). 



Analysis of reproductive structures 



Specimens of L. chinensis were initially fixed in buff- 

 ered 10% seawater-formalin to preserve the large tis- 

 sue mass and later transferred to 70% alcohol to pre- 

 vent damage to the statoliths. Specimens of/, pygmaeus 

 were preserved immediately in 70% alcohol due to their 

 small body size. Gonads were removed, blotted with 

 paper toweling (L. chinensis) or filter paper (/. pyg- 

 maeus), and weighed. Dorsal mantle length (DML) was 

 measured on both species, and nidamental gland length 



(NGL) and hectocotylus length was measured on 

 female and male individuals of L. chinensis, respec- 

 tively. Measurements were taken with an eyepiece mi- 

 crometer (/. pygmaeus) or with either callipers or a 

 graduated ruler (L. chinensis). Maturity was deter- 

 mined by the presence of mature oocytes in the ovary 

 along with large nidamental glands in females, and 

 the presence of spermatophores in males. 



To discern the pattern of growth for gonads, the 

 nidamental gland, and the hectocotylus, measurements 

 of gonad weight, nidamental gland length, and 

 hectocotylus length were plotted against both mantle 

 length and age. Due to the large amount of scatter in 

 many of the plots (especially with /. pygmaeus) and the 

 complex curvilinear relationship between many of the 

 relationships, regression analyses were not carried out. 



Gonadosomatic and NDL/DML indices 



The seasonal trend in gonad growth was also exam- 

 ined for L. chinensis. Maturity status was determined 

 for 231 individuals from trawl samples between Feb- 

 ruary 1988 and November 1989 (this analysis included 

 data from individuals which were aged from the Janu- 

 ary 1989 and July 1989 samples). For most samples, 

 all individuals within the adult size-range (>100mm) 

 were used in the gonad analysis, except for several 

 summer samples in which a very large number of indi- 

 viduals >300 mm were captured. 



Parameters measured for each squid were dorsal 

 mantle length, body weight, gonad weight, and nida- 

 mental gland length for females. The gonadosomatic 

 index (GSI) was calculated for each specimen as 



gonad weight (g) 



total body weight (g) 



x 100. 



For females, the nidamental gland length/dorsal mantle 

 length (NGL/DML) index was also calculated as 



nidamental gland length (mm) 

 mantle length (mm) 



x 100. 



Analyses of nidamental gland length and hectocotylus 

 length were not carried out for /. pygnmaeus. 



Results 



Loligo chinensis 



Gonad growth with age There were differences in 

 the relationship between gonad weight and age for the 

 two samples of L. chinensis taken within different sea- 

 sonal periods (Fig. 1A,B). The relationship between 

 testis weight and age was similar during both sea- 



