Loefer and Sedberry: Life history of Rhizopnonodon terraenovae off tfie soutfieastern United States 



85 



visible for some time after their actual formation until 

 enough new tissue has grown distally to provide the 

 compression and contrast necessary for reliable identifica- 

 tion. In other words, the increments observed in our study 

 first became visible in July, but may have actually formed 

 one to several months earlier. It should be noted that the 

 methods of vertebrae processing and examination followed 

 during our study were more similar to those of Simpfen- 

 dorfer (1993) than to those of Parsons (1985) or Branstet- 

 ter (1987a). These methods may have contributed to the 

 close similarity found in both the physical appearance (i.e. 

 that of "check marks" as opposed to pairs of growth bands) 

 and temporal deposition of increments between our study 

 and that of Simpfendorfer ( 1993). 



We found young of the year R. ter-racnovae with and 

 without a birth mark. This is unusual in that most studies 

 that have documented the presence of a birth mark have 

 found one present in all specimens examined (e.g. Casey et 

 al., 1985; Branstetter, 1987b; Simpfendorfer, 1993). Simp- 

 fendorfer ( 1993) suggested that the "birth" mark in R. tay- 

 lori was probably laid down sometime after birth because 

 he observed the same overestimation of size at birth by 

 back-calculations noted previously in our study. No tempo- 

 ral estimation of the lag between birth and the formation 

 of a birth mark has been published. The young-of-the-year 

 R. terraenovae examined during our study demonstrated a 

 distinct temporal transition from the lack of a birth mark 

 to the presence of a birth mark iFig. 10). The data sug- 

 gest that the birth mark is not actually laid down at birth 

 in June, but approximately one month later in July. This 

 time lag may explain the overestimation of size at birth 

 by back-calculation. It is possible that the mechanism for 

 the formation of the birth mark lies in the switch from 



embryonic to normal somatic growth, which may not occur 

 immediately following parturition. 



The von Bertalanffy growth parameters derived for our 

 study demonstrated differences from those derived for 

 previous studies (Fig. 11). Parsons (1985) estimated an L^ 

 of 709 mm PCL, and Branstetter (1987a) 820 mm PCl" 

 L„ for our study was 745 mm PCL for males, and 749 mm 

 PCL for females. The ^q value produced by Parsons was low 

 at -2.01 yr, whereas the values produced by Branstetter 

 (-0.99 yr) and our study (-0.90 yr for males and -0.94 yr 

 for females) agreed well with the known gestation period 

 of approximately 11 months. Parsons (1985) estimated K 

 by several methods, resulting in values ranging from 0.39 

 to 0.53. The higher values agreed well with the estimates 

 of our study (0.49 for females and 0.50 for males). Brans- 

 tetters ( 1987a) estimate of K was 0.36, lower than that of 

 the current study. 



Yearly growth rate estimates by Parsons (1983b) and 

 Branstetter (1987a) revealed an increase of 133 to 211 

 mm PCL during the first year of life. 94 mm during the 

 second year, 55 mm during the third year, and 16 to 32 mm 

 growth after maturity. We found similar, though slightly 

 higher growth rates: 198 to 202 mm PCL during the first 

 year, 100 to 108 mm during the second, 63 to 69 mm dur- 

 ing the third, and from to 46 mm thereafter. 



Parsons (1985) determined age at maturity by three 

 methods: extrapolation of growth rates to size at maturity, 

 the VBGE, and Holdens method (Holden, 1974). The esti- 

 mates produced by these methods ranged from 2.0 to 3.5 

 for males, and 2.4 to 3.9 for females. Branstetter (1987a) 

 compared his von Bertalanffy-derived estimates to those 

 of Parsons (1985), and found his results in general agree- 

 ment with Parsons" higher estimates. Branstetter (1987a) 



