Ferreira and Vooren: Age, growth, and structure of vertebra in Galeorhmus galeus 



29 



Figure 20 



Von Bertalanffy growth curves for male and female Galeo- 

 rhinus galeus and observed lengths-at-age. 



vertebrae and the ring formation identified. This tech- 

 nique is not subject to problems derived from the 

 presence of marginal connective tissue (Stevens 1975), 

 and a large number of thin rings near the margin can 

 be counted. 



From observation of margins of vertebrae, we can 

 conclude that ring formation probably occurred dur- 

 ing the period June-September. The high percentages 

 of ring formation observed in the sample indicated that 

 the process probably extends over a larger period than 

 observed in the current study, covering perhaps half 

 of the year. The growth band would be formed during 

 the remaining period, and the rings would thus be an- 

 nual marks. However, as vertebrae were examined 

 only for four-month periods (June- September), the 

 need for a proper validation remains, as emphasized 

 by Beamish and McFarlane (1983). The hypothesis that 

 two rings could be formed during a year was disre- 

 garded, due mainly to the results of Grant et al. (1979). 

 These authors estimated a longevity of 40 years for 

 Galeorhinus australis ( = Galeorhiyius galeus, Com- 

 pagno 1984) in South Australia, from mark-recapture 

 data. Several individuals were recaptured after periods 

 of more than 25 years at large, and 10% of the recap- 

 tures occurred 15 years after release. This evidence 

 supported the hypothesis of one ring per year, which 

 also led to the conclusion that individuals can attain up 

 to 40 years of age. 



Casey et al. (1985), observing a large number of rings 

 near the margin, suggested that if these could be in- 

 terpreted as annual marks, the ages determined for 

 several species of sharks may have been underesti- 

 mated. The present results agree with this hypothesis, 



and provide evidence that the school shark is a long- 

 lived, slow-growing species. 



The conclusion that the rings are translucent and 

 strongly calcified, while the growth bands are optical- 

 ly opaque and less calcified is in agreement with the 

 fact that the rings are stained dark by the silver nitrate 

 technique (Stevens 1975, Cailliet et al. 1983). Cassel- 

 man (1974, 1983) concluded that the translucent zone 

 in calcified tissue of fish is more heavily mineralized 

 than adjacent opaque zones and that calcium content 

 is directly related to translucency. 



In reference to the mode of calcification of cartilage, 

 Moss (1977) wrote that "something radically different 

 occurs in shark cartilage, making it certain that there 

 can be no unitary description of vertebrate cartilagi- 

 nous calcification." Hoenig and Walsh (1982) described 

 the occurrence of vascularized cartilage canals in 

 calcified vertebrae of several species of sharks. Many 

 canals were found to contain blood cells, and they sug- 

 gested a nutritive role for these canals. However, dif- 

 fusion through the matrix is impossible after mineral- 

 ization and in mammals; for instance, calcification of 

 endochondral bone causes cellular degeneration and 

 death of chondrocytes, because the isolated cells can- 

 not maintain a normal metabolism (Robbins 1975). In 

 the school shark, however, chondrocytes remain alive 

 after calcification, as is evident from their normal, lipid- 

 free appearance in calcified zones. Interchange be- 

 tween cells and vascular elements is probably sustained 

 after mineralization by the canaliculi that were ob- 

 served permeating the intercellular matrix. 



The growth of crystals within a preformed organic 

 structure is the basic mode of skeletal formation 

 (Weiner 1984). Narrow uncalcified matrix areas ob- 

 served at the edge of school shark vertebrae show that 

 development of hyaline cartilage precedes the process 

 of calcification. In addition, evidence of interstitial 

 growth indicates that the tissue still remains uncalcified 

 for a while after the appositional growth. In the verte- 

 brae of embryos only the cone area was calcified. 

 Among juveniles some uncalcified zones occurred also 

 in the intermedialia. During the adult phase, the cone 

 and intermedialia display the same mineralization pat- 

 tern, with corresponding opaque and translucent alter- 

 nate zones. This is evidence that the calcification, even 

 when occurring at different times, follows the same 

 pre-established rules. Weiner (1984) suggests that the 

 organic matrix performs active, specific roles in this 

 process. Growth of hydroxy apatite crystallites occurs 

 in the space between collagen fibrils and perhaps within 

 them (Glimcher in Kemp 1984). Therefore, regions 

 of organic matrix formed during slow-growth phases 

 would offer more space for crystal growth when ex- 

 posed to the calcium and phosphate ions which form 

 the mineral crystallites. Such regions, e.g., the rings, 



