Aerobic and anaerobic movement energetics of hybrid and pure parental abalone

The underlying mechanisms controlling growth heterosis in marine invertebrates remain poorly understood. We used pure blacklip (Haliotis rubra) and greenlip (Haliotis laevigata) abalone, as well as their hybrid, to test whether differences in movement and/or aerobic versus anaerobic energy use are linked to a purported increased growth rate in hybrids. Abalone were acclimated to control (16 °C) and typical summer temperatures (23 °C), each with oxygen treatments of 100% air saturation (O₂sat) or 70% O₂sat. The experiment then consisted of two phases. During the first phase (chronic exposure), movement and oxygen consumption rates (ṀO₂) of abalone were measured during a 2 day observation period at stable acclimation conditions. Additionaly, lactate dehydrogenase (LDH) and tauropine dehydrogenase (TDH) activities were measured. During phase two (acute exposure), O₂sat was raised to 100% for abalone acclimated to 70% O₂sat followed by an acute decrease in oxygen to anoxia for all acclimation groups during which movement and ṀO₂ were determined again. During the chronic exposure, hybrids and H. laevigata moved shorter distances than H. rubra. Resting ṀO₂, LDH and TDH activities, however, were similar between abalone types but were increased at 23 °C compared to 16 °C. During the acute exposure, the initial increase to 100% O₂sat for individuals acclimated to 70% O₂sat resulted in increased movement compared to individuals acclimated to 100% O₂sat for hybrids and H. rubra when compared within type of abalone. Similarly, ṀO₂ during spontaneous activity of all three types of abalone previously subjected to 70% O₂sat increased above those at 100% O₂sat. When oxygen levels had dropped below the critical oxygen level (P_crit), movement in hybrids and H. laevigata increased up to 6.5-fold compared to movement above P_crit. Differences in movement and energy use between hybrids and pure species were not marked enough to support the hypothesis that the purportedly higher growth in hybrids is due to an energetic advantage over pure species.