546 



Fishery Bulletin 88(3). 1990 



Figure 3 



Early growth of laboratory- reared specimens of sandfishes in fresh 

 condition. For Arctoscopu^ japonicus, average ( • ) and ranges (line) 

 are shown in TL mm (after Maekawa 1985); for Trichodon trirhodon, 

 individuals (O) are shown in SL mm (after Marliave 1981). 



teeth on both jaws. It is clear that these precocious 

 characters are mostly concerned with active swimming. 

 Fast swimming, coupled with large sizes, would not 

 only facilitate successful feeding, but would play an 

 effective role in lowering predation pressure. Slight but 

 distinct size and development differences at hatching 

 between the two species, therefore, are among the 

 most important aspects of their early life histories. 



(b) Orders of fin ray ossification of the two species 

 can be derived from Table 2, although the available size 

 series of T. trichodon (Marliave 1981) is somewhat 

 limited. Except for the precocious caudal and pectoral 

 fins in T. trichodon, all other fin meristics are formed 

 in smaller specimens of A. japonicus. The entire size 

 ranges for fin development are 14.9-19.0 mm in A. 

 japonicus versus 13.0-30.0 mm in T. trichodon. Pos- 

 sible sequence of fin ossification is caudal-pectoral- 

 anal-2nd dorsal- 1st dorsal-pelvic for both species. 



(c) Larvae of both species are sparsely pigmented 

 and lack extensive melanin on the lateral side of the 

 tail. Pigmentation is generally heavier in T. trichodon 

 than in ^4. japonicus. In particular, earlier development 

 of distinct spots along the anterior dorsal margin of 

 T. trichodon is remarkable because this character 

 first occurs after transformation into juveniles in A. 

 japonicus. 



(d) According to the conventional definitions of early 

 developmental stage, transformation from larval to 

 juvenile stages occurs at about 20 mm in A. japonicus 

 versus about 30 mm in T. trichodon. However, T. tri- 

 chodon is distinctly more advanced than A. japonicus 

 in everj' early-life-history trait, except formation of fins 

 other than the caudal and pectoral. 



Major contrasts in their ontogenetic properties may 

 be summarized as follows: A. japonicus has a distinct, 

 but truncated, metamorphosing phase (20-30 mm SL) 

 closely corresponding to the change from pelagic to 

 demersal life (Minami and Tanaka 1985, Okiyama un- 

 publ.); in contrast, T. trichodon has a less distinct meta- 

 morphosing phase (30-50 mm SL) without any eco- 

 logical shifts other than possible decrease of schooling 

 tendencies (Marliave 1981, Bailey etal. 1983), although 

 it starts burying itself in the sand at 55-60 mm SL (J.B. 

 Marliave, Vancouver Aquarium, P.O. Box 3232, Van- 

 couver, B.C., Canada, pers. commun., April 1989). 



Contrast In life history strategy 



Notwithstanding the close systematic relationship be- 

 tween A. japonicus and T. trichodon, there are many 

 life history features unique to each species, particularly 

 with respect to the reproductive styles as discussed 

 earlier. The terms and duration of their early develop- 

 mental stages are shown in Figure 4, revealing that 

 the egg stage of T. trichodon is almost four times long- 

 er than that of A. japonicus; however, the time when 

 the larvae of both species appear is restricted to the 

 spring seasons. The mechanism of this synchronized 

 hatching is unknown, but plausible advantages are ap- 

 parent. It is obvious that spawning substrates play an 

 important role in producing these contrasting patterns 

 of development. Perhaps the specialized mode of egg 

 deposition unique to A. japonicus has evolved in close 

 association with the abundant substrates represented 

 by Sarga.'<sum spp., whereas, as suggested Marliave 

 (1981), most plant substrates would have been too tran- 

 sitory to act as an egg deposition site for T. trichodon. 



