Jackson and Yeatman Phenotypic plasticity in Photolohgo sp 1 



63 



tropical pelagic cephalopods (e.g. Idiosepius pygmaeus, 

 Jackson, 1989; and Loliolus noctiluca, Jackson and 

 Choat, 1992) do show sexual dimorphism with fe- 

 males obtaining a larger size and greater age than 

 males. However, this is in contrast to the congener 

 Loligo chinensis, males of which obtain a greater 

 length and age (Jackson and Choat, 1992). The eco- 

 logical or genetic factors contributing to a larger body 

 size in either males or females are currently unclear. 

 The wide range in age and length at maturity ob- 

 served in Photololigo sp. 1 off northwestern Austra- 

 lia is considerably different from that of the summer 

 population of L. chinensis in shallow water off 

 Townsville, North Queensland. Jackson ( 1993 ) found 

 that maturation in L. chinensis was more closely re- 

 lated to size than age and that individuals reached 

 maturity rapidly over a relatively restricted size 

 range regardless of age. However, Jackson ( 1993 ) also 

 found that several of the larger and older females 

 were very immature and thus some individuals did 

 delay maturation. Photololigo sp. 1 is a deepwater 

 species captured at depths <100 m whereas L. 

 chinensis off Townsville is a shallow water species 

 (generally <20 m and often in <10 m), thus the re- 



productive tactics of these two squid species may be 

 a response to the two very different environments. 



Considerable variation in size at maturity has also 

 been documented in other loliginids. Photololigo 

 edulis in Japan shows considerable variability in size 

 at maturity both geographically and seasonally. 

 Natsukari and Tashiro ( 1991) have reported mature 

 individuals of P. edulis as small as 52 and 59 mm 

 ML for males and females, respectively, whereas the 

 more typical size for maturation is between 150 and 

 200 mm ML, with some individuals remaining im- 

 mature until >300 mm ML. Loligo vulgaris reynaudii 

 off South Africa can mature as small as 90 and 100 

 mm ML (males and females, respectively) whereas 

 immature males and females can be as large as 250 

 and 180 mm ML, respectively (Augustyn et al., 1992). 

 Loligo gahi also shows a considerable range in size 

 at maturity, with mature males ranging in size from 

 approximately 71 to >300 mm ML and mature fe- 

 males ranging in size from approximately 98 mm to 

 >220 mm ML (Hatfield et al., 1990). An extreme ex- 

 ample has also been recorded by Dunning et al. ( 1994) 

 who found that all males of the cuttlefish Sepia 

 pharaonis collected in the Gulf of Carpentaria, Aus- 

 tralia, were mature over a length range of 34 mm to 

 173 mm ML. These examples of phenotypic plastic- 

 ity are much more extreme than what has been docu- 

 mented for Photololigo sp. 1 in this study. However, 

 by attributing age to individuals, one can consider 

 time-specific aspects in variation in size at maturity. 



Considerable phenotypic plasticity has also been 

 observed in some freshwater fishes. Mann and 

 McCart (1981) noted the presence of two forms of 

 Coregonus sardinella in a small homogenous lake in 

 northern Canada. The presence of two such morpho- 

 logically distinct forms was somewhat surprising 

 because of the lack of environmental heterogeneity. 

 The reasons for such plasticity were not discerned, 

 although there was some evidence of spatial segre- 

 gation and temporal segregation in spawning. Mann 

 and McCart (1981) also suggested that such varia- 

 tion may not be genetically fixed but may rather be 

 a result of different environmental conditions dur- 

 ing early development. An even more marked degree 

 of phenotypic plasticity has been observed in the arc- 

 tic char, Salvelinus alpinus, in Thingvallavatn, the 

 largest lake in Iceland. Arctic char occurs in four 

 forms, two planktonic feeders and two benthic feed- 

 ers (Sandlund et al., 1992). The morphological plas- 

 ticity in this species appears to be predominantly due 

 to niche separation based on habitat and feeding 

 strategies possibly related to ontogenetic or popula- 

 tion divergence. 



The phenotypic plasticity observed in Photololigo 

 sp. 1 does not appear to be displayed in distinct forms 



