Digestive and locomotor capacity show opposing responses to changing food availability in an ambush predatory fish

Abstract

Metabolic rates vary widely within species, but little is known about how variation in the ‘floor’ [i.e. standard metabolic rate (SMR) in ectotherms] and ‘ceiling’ [maximum metabolic rate (MMR)] for an individual's aerobic scope (AS) are linked with digestive and locomotor function. Any links among metabolic traits and aspects of physiological performance may also be modulated by fluctuations in food availability. This study followed changes in SMR, MMR, and digestive and locomotor capacity in southern catfish (Silurus meridionalis) throughout 15 days of food deprivation and 15 days of refeeding. Individuals downregulated SMR during food deprivation and showed only a 10% body mass decrease during this time. Whereas critical swim speed (Ucrit) was robust to food deprivation, digestive function decreased after fasting with a reduced peak oxygen uptake during specific dynamic action (SDA) and prolonged SDA duration. During refeeding, individuals displayed rapid growth and digestive function recovered to pre-fasting levels. However, refed fish showed a lower Ucrit than would be expected for their increased body length and in comparison to measures at the start of the study. Reduced swimming ability may be a consequence of compensatory growth: growth rate was negatively correlated with changes in Ucrit during refeeding. Southern catfish downregulate digestive function to reduce energy expenditure during food deprivation, but regain digestive capacity during refeeding, potentially at the cost of decreased swimming performance. The plasticity of maintenance requirements suggests that SMR is a key fitness trait for in this ambush predator. Shifts in trait correlations with food availability suggest that the potential for correlated selection may depend on context.

The relationships between specific dynamic action, nutrient retention and feed conversion ratio in farmed freshwater Chinook salmon (Oncorhynchus tshawytscha)

“…SDA profiles could potentially vary between feed‐efficient and inefficient fish in a number of ways: (a) the duration of SDA; (b) the peak level of SDA; (c) the total energy required to process the meal; (d) the proportion of AS that the SDA peak takes up, known as the percentage reduction of AS (Jordan & Steffensen, 2007); (e) the proportion of the energy consumed via the meal that is then expended on SDA, known as the SDA coefficient (Dupont‐Prinet et al ., 2010; Fu et al ., 2005; Khan et al ., 2015); or a combination of the above. For example, southern catfish ( Silurus meridionalis ) with higher maintenance requirements had a shorter SDA duration (Fu et al ., 2018), indicating that SDA duration and maintenance may be linked. If this is the case, then SDA duration may also be linked to non‐maintenance processes.…”