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.     

Vortex formation and foraging in polyphenic spadefoot toad tadpoles

Animal aggregations are widespread in nature and can exhibit complex emergent properties not found at an individual level. We investigate one such example here, collective vortex formation by congeneric spadefoot toad tadpoles: Spea bombifrons and Spea multiplicata. Tadpoles of these species develop into either an omnivorous or a carnivorous (cannibalistic) morph depending on diet. Previous studies show that S. multiplicata are more likely to develop into omnivores and feed on suspended organic matter in the water body. The omnivorous morph is frequently social, forming aggregates that move and forage together and form vortices in which they adopt a distinctive slowly rotating circular formation. This behaviour has been speculated to act as a means to agitate the substratum in ponds and thus could be a collective foraging strategy. Here we perform a quantitative investigation of the behaviour of tadpoles within aggregates. We found that only S. multiplicata groups exhibited vortex formation, suggesting that social interactions differ between species. The probability of collectively forming a vortex, in response to introduced food particles, increased for higher tadpole densities and when tadpoles were hungry. Individuals inside a vortex moved faster and exhibited higher (by approximately 27%) tailbeat frequencies than those outside the vortex, thus incurring a personal energetic cost. The resulting environmental modification, however, suggests that vortex behaviour may be an adaptation to actively create and exploit a resource patch within the environment.