Life History Parameters and Elasticities of the Bat Ray, Myliobatis californica

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Parameter Female Male Ref.
Largest bat ray (Diskwidth and mass) 1.22 m; 1.40 m; 1.8 m, 95.3 kg 0.915 m, 12.5 kg; 1.02 m (calculated from mass) 16.8 kg 2, 3, 4, 6
Size at maturity ~0.88 m DW ~0.60 m 6
Smallest mature . . .
Largest immature . . .
Size at which 50% are mature 0.88 m DW (n = 108) . 6
Age at maturity 5 yr (~ 6yr according to VBGF) 2 -3 yr 6, 7
Mating Premating position: Smaller ray (male) has tail flexed dorsally at 90 deg. and right clasper erected at 45 deg. Male repeatedly bumped underside of female in attempts to insert clasper into her cloacae. 8
Ovulation June -July . 6
Size of ovulated eggs 22 - 28 mm diameter (est. mass 5.6 -11.5 g) 6
Gestation time 9 - 12 months . 6
Embryo development aplacental viviparity, trophonemata of uterine wall produce "mother-milk" after egg-yolk has been used up 5
Litter size 2 - 12; 2 -7 (# of pups may increase with size of mother) 1, 6
Birth size 0.26 - 0.35 m DW; 0.274 - 0.305 m DW (0.21 kg) 1, 2, 6, 7
Smallest free-swimming 0.22 m DW, sex unknown (a) 6
First year growth 0.122 m DW (calculated from VBGF) 0.143 m DW (calculated from VBGF) 7
VBGF parameters (b)
MycVBGF.gif 		DWoo = 1.587 m , k = 0.10 yr-1 (5ln2/k = 34.8 yr), DWo = 0.294 m DWoo = 1.004 m, k = 0.23 yr-1 (5ln2/k = 15.1 yr), DWo = 0.305 m 7
W - DW equation (c) W(kg) = 18.6* DW^3.199(m) W(kg) = 15.9*DW^3.096(m) 6, 7
Life history table:
Elasticity estimates based on alpha = 6.0 yr, Assuming Abar/alpha = 1.5 (thus Abar = 9.0 yr)		 E(m)  = 1/Abar = 1/9 =  0.111;
E(JS)/E(m) = alpha = 6.0, E(JS) = 0.667;
E(AS)/E(m) = Abar-alpha = 3.0, E(AS) = 0.333;
Sum or ratios = Abar;
Sum of elasticities = 1 + E(m) = 1.111

Mollet 2003 (AES Manaus)
Normalized elasticities:
E(fertility) = E(m)  = 10.0%;
E(juvenile survival) = E(JS) = 60.0%;
E(adult survival) = E(AS) = 30.0%


1) Herald 1953; 2) Herald et al. 1960; 3) Miller and Lea 1972; 4) Eschmeyer et al. 1983; 5) Wourms 1981; 6) Martin and Cailliet 1988a; 7) Martin and Cailliet 1988b; 8) Tricas 1980.

a) Unusually small free-living specimen according to Martin and Cailliet 1988a. Perhaps a runt?
b) The von Bertalanffy growth function (VBGF) in the from DW(t) = DWoo - (DWoo - DWo) exp(-kt) is used. DW(t) is diskwidth at age t. The 3 parameters are DWoo = asymptotic diskwidth), DWo = diskwidth at birth, and k is a rate constant with units of reciprocal time. 5 ln2/k (i.e. 5 half-lives) gives an estimate of longevity.
c) The pre-exponential constant gives the weight of a 1 m DW bat ray if meter is used as length unit.

Additional references