Temperature effects on the dynamics of gonad and oocyte development in captive wild - caught blacklip ( Haliotis rubra ) and greenlip ( H . laevigata ) abalone

Wild-caught blacklip (Haliotis rubra) and greenlip (H. laevigata) abalone were held from spent condition at 12oC, 14oC, 16oC or 18oC and routinely sampled to examine gonad development. Descriptors of gross structure included the Visual Gonad Index (VGI) and the Modified Gonad Bulk Index (MGBI). Oocyte Diameter Ratio (ODR) and oocyte volume (based on an ellipsoid) were used as descriptors of ovarian microstructure. For each species, the rate of increase in the VGI, MGBI and oocyte volume of animals held at different temperatures were used to estimate the Biological Zero Point (BZP), the critical temperature below which no development occurs. BZP estimates derived from the daily increase in VGI and oocyte volume were similar (7.8oC and 7.6oC for blacklip abalone; 6.9oC and 6.8oC for greenlip abalone, respectively), but those based on the increase in MGBI were up to 1.8oC lower (6.0oC and 5.7oC, for blacklip and greenlip abalone, respectively). The mean MGBI, in terms of gonad volume per gram of shucked animal weight, ranged from 5–68 mmg and 5–58 mmg for blacklip and greenlip abalone, respectively. The ODR indicated that oocyte shape was highly variable in oocytes < 90μm diameter in both species. Above 90μm, ODR values increased proportionally with oocyte size, indicating a transition in shape from elliptical to round. Ranges for mean oocyte volume for blacklip and greenlip abalone were 0.15–1.4 x 10 μm and 0.02–1.83 x 10 μm, respectively. The pattern of oocyte growth relative to temperature for both species is illustrated using tables of standardized residuals. Determination of the BZP for blacklip and greenlip abalone enables the calculation of the Effective Accumulative Temperature (EAT; the cumulative difference between the water temperature and the BZP, calculated daily) for gamete maturation of these species. This in turn facilitates predictive and deductive estimates of the completion of this process (when water temperature is known) in either natural or artificial (i.e. culture) environments.