Considering that swim-flume or chasing methods fail in the estimation of maximum metabolic rate and in the estimation of Aerobic Scope (AS) of sedentary or sluggish aquatic ectotherms, we propose a novel conceptual approach in which high metabolic rates can be obtained through stimulation of organism metabolic activity using high and low non-lethal temperatures that induce high (HMR) and low metabolic rates (LMR), This method was defined as TIMR: Temperature Induced Metabolic Rate, designed to obtain an aerobic power budget based on temperature-induced metabolic scope which may mirror thermal metabolic scope (TMS = HMR—LMR). Prior to use, the researcher should know the critical thermal maximum (CT max) and minimum (CT min) of animals, and calculate temperature TIMR max (at temperatures −5–10% below CT max) and TIMR min (at temperatures +5–10% above CT min), or choose a high and low non-lethal temperature that provoke a higher and lower metabolic rate than observed in routine conditions. Two sets of experiments were carried out. The first compared swim-flume open respirometry and the TIMR protocol using Centropomus undecimalis (snook), an endurance swimmer, acclimated at different temperatures. Results showed that independent of the method used and of the magnitude of the metabolic response, a similar relationship between maximum metabolic budget and acclimation temperature was observed, demonstrating that the TIMR method allows the identification of TMS. The second evaluated the effect of acclimation temperature in snook, semi-sedentary yellow tail (Ocyurus chrysurus), and sedentary clownfish (Amphiprion ocellaris), using TIMR and the chasing method. Both methods produced similar maximum metabolic rates in snook and yellowtail fish, but strong differences became visible in clownfish. In clownfish, the TIMR method led to a significantly higher TMS than the chasing method indicating that chasing may not fully exploit the aerobic power budget in sedentary species. Thus, the TIMR method provides an alternative way to estimate the difference between high and low metabolic activity under different acclimation conditions that, although not equivalent to AS may allow the standardized estimation of TMS that is relevant for sedentary species where measurement of AS via maximal swimming is inappropriate.
The cephalopod digestive gland (DG) is responsible for enzyme production as well as nutrient and lipid storage. Octopus maya (Mollusca: Cephalopoda) is a holobenthic octopus species with aquaculture potential. To develop a balanced food for the rearing of this octopus, it is necessary to understand its digestive physiology. We performed histological studies on the structural change of the DG (cytological ontogeny) associated with age (from 0 to 30 d posthatching, DPH) and food (postprandial change in 120 DPH juveniles). Early ontogeny of DG was defined in 3 stages: (1) yolk platelets stage (0 to 5 DPH), (2) transition stage (6 to 10 DPH) and (3) heterolysosomes (food reserves) stage (>12 DPH). In Stage 1, the DG had anatomically undifferentiated tubules, but was filled with yolk platelets. The tubular structures developed lumen by 5 DPH. Stage 2 (starting at 6 DPH) corresponds to mixed exogenous and endogenous feeding. At that time, the yolk platelets were gradually consumed until completely exhausted at 9 DPH. At the onset of Stage 3, the DG structure was completely tubular, exhibiting digestive cell microvilli and other cellular features typical to octupus DGs. During exogenous feeding (12 DPH and onward), acidophilic secretory lysosomes, heterolysosomes and some heterophagosomes appeared on DG cells. O. maya has long digestive cycles in which the extracellular and intracellular digestion can take up to 8 h. Although the ecological implications of this information for aquaculture will still have to be proven, results demonstrated that O. maya is an energetically efficient species and thus suitable for rearing in captivity.
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