Ghost crabs on a treadmill: Oxygen Uptake and Haemocyanin Oxygen Affinity
W.J. Van Aardt
Ghost crabs Ocypode ceratophthalmus were exercised on a specially constructed treadmill. At a running speed of 13,3 cm s-1, most crabs ran for 2 h before getting fatigued. At this speed the oxygen consumption rate (MO2) was measured in time intervals for a total of 52 min. For exercised crabs the MO2 values are about eight times higher (28,5 ± 5.0 mmol l-1kg-1h-1 compared with the values (3,5 ± 0,4 mmol l-1kg-1h-1) for resting crabs. One hour after the exercising bout, the MO2 values were not back to normal resting values. The l-lactate levels in the haemolymph increase to 20,7 (± 4,8) mmol l-1 after 20 min exercise, whilst the pH decreases from 7,91 (± 0,05) to 7,71 (± 0,08). For resting crabs pre-branchial PCfe (venous) is 35,9 (± 5,6) mmHg, total carbon dioxide concentration (Ceo, tot.) 22,6 (± 2,6) mmol l-1, l-lactate 0,68 (±0,5) mmol l-1; haemocyanin oxygen capacity (Cmax/Hcyo) 1,50 (± 0,11) mmol l-1; haemocyanin concentration 99,3 (± 21) mg ml-1and haemolymph oxygen content 4,11 ml O2 100 ml-1haemolymph. Haemolymph from resting crabs has P50 values of 4,5 (± 1,0) mmHg, while for exercised crabs it is 10,0 (±0,8) mmHg. Gel-chromato-graphed haemolymph increased the oxygen affinity of the haemocyanin to 1,30 (± 0,1) mmHg, an indication of the presence of oxygen affinity modulators in the haemolymph. A decrease in the pH of the haemolymph has a greater effect on oxygen affinity compared with the effects from either CO2 or l-lactate. Hill-plot analysis of both native and gel-chromatographed haemolymph reveal that co-operativity (n) of the oxygen-binding sites in the haemocyanin sub-units increases to a value of 4 with a corresponding decrease in oxygen affinity. This effect has the result that large amounts of O2 can be suddenly released by the haemocyanin to tissues such as muscle and ganglia for immediate mitochondrial use.