Repeated measurement of MO2 in small aquatic organisms: a manual intermittent flow respirometer using off-the-shelf components

Measurement of rates of oxygen consumption (M_O2) in small aquatic embryos or larvae (< 1mm) in response to altered environmental conditions has traditionally been challenging. Here, using modifications of a commercially available fluorescent optode flow-through cell (FTC: PreSens™ FTC-PSt3) and routine laboratory supplies (syringes, stopcocks, tubing), we have constructed a manual intermittent flow respirometer (MIFR) that allows measurement of M_O2 in small numbers of individuals when sequentially exposed to different environmental conditions (e.g. changes in seawater pH) through a gravity-driven media replacement perfusion system. We first show that the FTC can be used in 'static' mode while incubating small numbers of embryos/larvae contained within the planar oxygen sensor (POS) chamber with Nitex filters. We then demonstrate the use of the MIFR by exposing larval echinoderms (Fellaster zelandiae, Evechinus chloroticus, Centrostephanus rodgersii) to seawater equilibrated with elevated CO₂, and measured M_O2 during acute and chronic exposure to hypercapnia. This MIFR method will allow investigators to address questions regarding the respiratory physiology of small aquatic animals, such as the thresholds for metabolic depression in embryonic and larval forms.

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