Effect of reproduction on escape responses,metabolic rates and muscle mitochondrial properties in the scallop <Emphasis Type="Italic">Placopecten magellanicus</Emphasis>

In scallops, gametogenesis and spawning can diminish the metabolic capacities of the adductor muscle and reduce escape response performance. To evaluate potential mechanisms underlying this compromise between reproductive investment and escape response, we examined the impact of reproductive stage (pre-spawned, spawned and reproductive quiescent) of the giant scallop, Placopecten magellanicus, on behavioural (i.e., escape responses), physiological (i.e., standard metabolic rates and metabolic rates after complete fatigue) and mitochondrial capacities (i.e., oxidative rates) and composition. Escape responses changed markedly with reproductive investment, with spawned scallops making fewer claps and having shorter responses than pre-spawned or reproductive-quiescent animals. After recuperation, spawned scallops also recovered a lower proportion of their initial escape response. Scallop metabolic rate after complete fatigue (VO_2max) did not vary significantly with reproductive stage whereas standard metabolic rate (VO_2min) was higher in spawned scallops. Thus spawned scallops had the highest maintenance requirements (VO_2min/VO_2max). Maximal capacities for glutamate oxidation by muscle mitochondria did not change with reproductive stage although levels of ANT and cytochromes as well as cytochrome C oxidase (CCO) activity did. Total mitochondrial phospholipids, sterols and the proportion of phospholipid classes differed only slightly between reproductive stages. Few modifications were detected in the fatty acid (FA) composition of the phospholipid classes except in cardiolipin (CL). In this class, pre-spawned and spawned scallops had fairly high proportions of 20:5n-3 whereas this FA in reproductive-quiescent scallops was threefold lower and 22:6n-3 was significantly higher. These changes paralleled the increases in CCO activity and suggest an important role of CL on the modifications of CCO activity in scallops. However, mitochondrial properties could not explain the decreased recuperation ability from exhausting exercise in spawned scallops. Shifts in maintenance requirements (VO_2min/VO_2max) and aerobic scope (VO_2max − VO_2min) provided the best explanation for the impact of reproduction on escape response performance.     

Allozyme heterozygosity and escape response performance of the scallops, <Emphasis Type="Italic">Argopecten purpuratus</Emphasis> and <Emphasis Type="Italic">Placopecten magellanicus</Emphasis>

Multilocus allozyme heterozygosity (MLH) has been positively correlated with growth in some marine bivalves and was suggested to facilitate swimming activity in pectinids. Using two highly mobile scallops, Placopecten magellanicus and Argopecten purpuratus, we examined escape response performance and morphometric characteristics as a function of allelic variability at metabolic loci. Ten allozyme systems were used for A. purpuratus and 7 for P. magellanicus. In each species, the morphometric characteristics and escape response parameters were analyzed separately using principal components analysis (PCA) and the scores of the major principal components were related to allozyme heterozygosity. In both P. magellanicus and A. purpuratus, positive correlations were found between MLH and morphometric parameters, but escape response parameters were only positively linked to MLH in P. magellanicus, and then weakly. The hypothesis that MLH improves fitness of pectinids by increasing the capacity to escape predators is not supported.