Jackson Cephalopod reproductive investment 



267 



slower-growing spring individuals did, in fact, eventu- 

 ally produce larger gonads than their autumn counter- 

 parts. 



Discussion 



The biotic and abiotic influences on maturity are com- 

 plex. Factors such as light (day length), temperature, 

 and food availability can all affect rate and age-at- 

 maturity. Gonad development is directly under hor- 

 monal control, and appears to be influenced by the 

 optic gland (Mangold 1987, Boyle 1990). However, the 

 process of maturity is not completely understood and 

 may also be controlled to a certain degree by indi- 

 vidual genetic factors, apart from outside influences 

 (Mangold 1983). This study gives preliminary insight 

 into the maturation process in tropical squids and 

 sepioids based on age information. Statolith ageing 

 techniques should prove useful in providing a time- 

 scale on the squid maturation process and problems 

 encountered with variation in size-at-maturity. 



Loligo chinensis 



Variability in size-at-maturity appears to be a com- 

 mon phenomenon with cephalopods. Hixon (1980) 

 found that some immature Lolligunciila brevis females 

 were as large as other fully-mature individuals. Con- 

 siderable variation in maturity has also been shown 

 to exist in L. opalescens, with females maturing by 

 81 mmDML while other females remain immature un- 

 til 140mmDML (Hixon 1983). Similar discrepancies 

 in size-at-maturity have also been documented for Se- 

 pia officinalis (Boletzky 1983) and Dosidicus gigas 

 (Nesis 1983). Mangold (1983) has also demonstrated 

 that cultured octopuses reared from the same egg mass 

 reached maturity independent of sibling size or age. 

 The delay in maturity for some L. chinensis females 

 for a given length (e.g., Fig. IB, summer) is similar to 

 observations made for Lolligunciila brevis by Hixon 

 (1980). However, in contrast to the other studies cited 

 above, L. chinensis females do not show a wide differ- 

 ence in size-at-maturity. The fact that maturity was 

 more closely related to length than to age suggests 

 that there might be some physical or physiological 

 mechanisms controlling maturation apart from age. 

 Some female cephalopods have been shown to develop 

 eggs only when a minimum threshold in body size is 

 achieved (Mangold 1987). The maximum size for indi- 

 viduals of L. chinensis obtained in this study is smaller 

 than the maximum size recorded for the species, which 

 is up to 300 mmDML (Roper et al. 1984). This sug- 

 gests that the squids in Cleveland Bay may be a pre- 

 cocious population which is maturing near the mini- 



mum threshold in body size for the species. This could 

 therefore account for the restricted size-range at 

 maturity. 



Although male and female L. chinensis were mature 

 in the same age-range in summer, a different situation 

 existed in winter, suggesting that maturity in the fe- 

 male winter population was out of phase with the 

 males. This situation has been noted with Todarodes 

 pacificus in which males reach maturity 3-6 mo ear- 

 lier than females (Okutani 1983). This may not be an 

 ecological constraint, considering the reproductive tac- 

 tics of cephalopods. In many species of cephalopods, 

 males mate with immature females (Mangold 1987) 

 with the females retaining the spermatophores until 

 they spawn. An extended interval between mating and 

 spawning has also been noted in octopods, with up to 

 a 114 d interval between spermatophore transfer and 

 spawning (Boyle 1990). 



Alternatively, the apparent absence of mature fe- 

 males in July may have been due to inadequate sam- 

 pling, since the oldest-aged female captured in winter 

 was 134 d whereas males as old as 173 d were cap- 

 tured. Since maturation was rapid (e.g., females ma- 

 tured in <80d in summer), it is possible that older 

 mature females existed in winter but that none were 

 captured. 



Based on previous work on the effect of temperature 

 on growth rate (e.g., Forsythe & Hanlon 1988 and 

 1989), seasonal differences in water temperature may 

 also account for the lack of gonad development in L. 

 chinensis during winter. As age of field specimens in 

 relation to maturity processes has not previously been 

 considered, only aquarium culture experiments are 

 available for comparison. Richard (1966, cited in 

 Mangold 1987) has shown that males and females of 

 the cuttlefish Sepia officinalis from the English Chan- 

 nel, which were raised at 20°C, attained sexual matu- 

 rity at 7 mo and 140 mmDML, while conspecifics raised 

 at 10°C were totally immature at the same age and 

 were only 50 mm in length. Moreover, Richard (1970, 

 cited in Mangold 1987) also found that females of S. 

 officinalis raised at 18°C had a gonad index of 8% at 

 270 d, while it took 480 d for females raised at 13°C to 

 reach the same value. Other factors, such as seasonal 

 variability in food supply and day length may also 

 influence seasonal differences in maturation rates. 



The fluctuations of the GSI and NGL/ML index in- 

 dicated that ambient environmental conditions (e.g., 

 temperature and food availability) were influencing 

 both the maturity process and the energy L. chinensis 

 partitioned into reproduction. Although individuals 

 were often mature (e.g., the majority of males ana- 

 lyzed) there were time-periods when the ovary and 

 testes accounted for a greater percentage of total body 

 weight. This was consistently recorded for both sexes 



