Arkhipkin et al.: Distribution, stock structure, and growth of Berryteuthis magister 



25 



Discussion 



Statolith and gladius microstructure with 

 validation of growth increments 



Statoliths of all previously investigated species usu- 

 ally had one or two kinds of growth increments, usu- 

 ally referred to as first- and second-order growth in- 

 crements. Growth increments of only one kind occur 

 in ommastrephids: Illex illecebrosus (Dawe et al., 

 1985), Illex argentinus (Rodhouse and Hatfield, 1990; 

 Arkhipkin, 1993), and Nototodarus sloanii (Uozumi 

 and Ohara, 1993). Growth increments of two types 

 were found mainly in loliginids: Loligo opalescens 

 (Spratt, 1978), Photololigo edulis (Natsukari et al., 

 1988), and Loligo vulgaris (Natsukari and Komine, 

 1992); and in gonatids: Gonatus fabricii (Kristensen, 

 1980). The first-order growth increments or "rings" 

 in loliginids and gonatids and the growth increments 

 in ommastrephids have been validated as daily in- 

 crements, whereas an interpretation of the second- 

 order growth increments has not been consistent in 

 different studies. Some authors (Spratt, 1978; Kris- 

 tensen, 1980) have assumed them to be of fortnightly 

 or monthly origin; others have not been able to find 

 any definite pattern in their deposition and have 

 treated them like first-order increments (Natsukari 

 et al., 1988; Natsukari and Komine, 1992). In this 

 investigation, it was revealed that unlike statoliths 

 of loliginids and ommastrephids, ground statoliths 

 of B. magister exhibit three kinds of growth incre- 

 ments: namely, of the first, second, and third orders. 

 Previously, Natsukari et al. (1993) noted only two 

 kinds of growth increments inB. magister statoliths. 

 We revealed that the "second-order growth incre- 

 ments" were grouped into distinct "bands" of the third 

 order, very similar in structure and appearance to 

 loliginid "second-order bands." 



Natsukari et al. ( 1993 ) hypothesized that the first- 

 order growth increments ("fine low contrast rings") 

 in B. magister were deposited daily. However, the 

 authors did not validate the periodicity of "fine rings" 

 directly. Their suggestion that there were two differ- 

 ent modes of year classes for B. magister encountered 

 in the southwestern part of the Sea of Japan, as as- 

 sumed earlier (Yuuki and Kitazawa, 1986), seemed 

 to be rather speculative without some direct verifi- 

 cation (e.g. by statolith ageing techniques). 



Squid, like fishes, have physiological processes that 

 are related to diel rhythms (Campana and Neilson, 

 1985) reflected in periodical changes in metabolic 

 processes and resulting in different rates of deposi- 

 tion onto various hard structures (e.g. gladius and 

 statolith). We found that within the inner layer of 

 the gladius there were regular well-defined laminae 



that were not grouped into any other growth zones 

 or bands. The similarity in the number of growth 

 increments observed within the gladius with the 

 number of "second-order growth increments" in sta- 

 toliths corresponded to the same internal rhythm in 

 the squid organism. Previous research on gladii (e.g. 

 Sthenoteuthis oualaniensis [Arkhipkin and Bizikov, 

 1991]) has suggested that this kind of internal 

 rhythm is diel, and therefore we consider the "sec- 

 ond-order growth increments" of B. magister sta- 

 toliths as deposited daily. However, this method could 

 not be applied to some cold-water species (e.g. 

 Moroteuthis ingens [Arkhipkin and Bizikov, 1991]); 

 therefore it had to be confirmed by another indepen- 

 dent validation method. As in the studies by Uozumi 

 and Ohara (1993) and Uozumi and Shiba (1993), we 

 compared differences between mean ages of succes- 

 sive well-defined modes of frequency compositions 

 with elapsed days between samples and found a good 

 correspondence between them. Therefore, the nature 

 of the "second-order growth increments" within B. 

 magister statoliths and consequently that of incre- 

 ments within the inner layer of their gladii were con- 

 firmed. The statolith ageing technique, however, was 

 found to be more effective because its actual produc- 

 tivity (about 120 ground statoliths per week by one 

 skilled technician) was far higher than that of the 

 gladius technique (40-50 cross sections, respectively). 

 Thus, statoliths were more efficient tools in investi- 

 gations of age, growth, and stock dynamics of B. 

 magister. 



It is noteworthy that the "second-order" growth 

 increments observed inS. magister statoliths ("heavy 

 rings" of Natsukari et al., 1993) correspond to the 

 first-order daily increments of other squid species, 

 whereas the distinct "third-order" bands that we 

 found in B. magister statoliths correspond to the sec- 

 ond-order bands of loliginids and other gonatids. 

 "Fine rings" or "first-order increments" revealed in 

 B. magister statoliths obviously were of a subdaily 

 nature like those observed earlier in Sepioteuthis 

 lessoniana (Jackson, 1990) and in other fish species 

 (Campana and Neilson, 1985). 



The starting point for increment counts remains 

 unclear. In ommastrephids, growth increments are 

 deposited after hatching (Balch et al., 1988), whereas 

 loliginid hatchlings have a few dozen growth incre- 

 ments within their statoliths at hatching, with a 

 prominent check occurring outside the nucleus 

 (Natsukari et al., 1988). Statolith size at hatching 

 for B. magister is unknown, therefore we assumed 

 (after Natsukari et al., 1993) that the first promi- 

 nent check lying outside the nucleus was the "natal" 

 ring. If this assumption turns out to be incorrect, and 

 the nucleus in B. magister appears at hatching as in 



