NOTE McFarlane et al : Use of neural arches in age determination of Hexanchus gnseus 



863 



Figure 2 



A stained neural arcli from a 207-cm (total 

 length) female bluntnose sixgill shark. Note that 

 the staining occurs along the outer third portion 

 of the arch. Five bands are indicated by dots. 



Discussion 



The nature of the staining observed on the neural arches 

 suggests that these structures have a potential use in the 

 age determination of elasmobranchs. In sixgill sharks, the 

 bands were distinct and appeared at regular intervals. 

 The number of bands per neural arch increased with total 

 length of the shark, suggesting their potential for age 

 determination. The banding occurred only on the outer 

 portions, indicating that the calcium was deposited after 

 the proximal portion of the arches developed, probably 

 after birth. The methodology used in our study for the 

 staining of calcium deposits in neural arches offers some 

 promise for age determination of sixgill sharks. Future 

 research is required to refine the method by determin- 

 ing sectioning methods and thickness, optimum staining 

 times, or perhaps alternative solution concentrations. For 

 example, it might be more effective to cut neural arches 

 into thin sections prior to soaking in silver nitrate rather 

 than to soak larger portions and then section them. 



Calcium deposits have been observed in the neural 

 arches of several elasmobranchs (Cailliet, 1990). For ex- 

 ample, Peignoux-Deville et al. (1982) provided an exten- 

 sive description of these deposits for dogfish {Scyliorhinus 

 canicula). However, age determination research for elas- 

 mobranchs has focused on the calcium deposits in the ver- 

 tebral centra. Use of the neural arches as aging structures 



may provide an alternative age determination method for 

 species in which the vertebral centra are poorly calcified 



Valid age and growth information is fundamental to stock 

 assessment and management. However, Cailliet (1990) 

 lists only 39 species of elasmobranchs for which there are 

 published or ongoing age verification studies. As interest 

 and concern in commercial and recreational fishing of 

 elasmobranchs increases, the need for age determination 

 techniques becomes more pressing. Researchers need to 

 investigate alternative methods for aging elasmobranchs, 

 and we suggest that neural arches may prove to be useful. 

 It is important to note that our observations on calcium 

 banding in sixgill shark neural arches are preliminary and 

 are for immature sixgill sharks only. We stress that funda- 

 mental to all aging determination studies is the validation 

 of the method (Beamish and McFarlane, 1983), and future 

 work is required to determine if the bands observed in our 

 study are formed on annual, or regular, intervals. 



Acknowledgments 



Bill Andrews and Grady O'Neill provided technical assis- 

 tance in producing the photographs. Maria Surry pro- 

 duced the schematic diagrams of the vertebral columns 

 and neural arches. 



