GANNON, A. T.*; GREENAWAY, P.; HENRY, R. P.; Birmingham-Southern College, AL; Univ. of New South Wales, Sydney; Auburn Univ., AL: Relative importance of oxygen and carbon dioxide to ventilatory rate in land crabs

Crabs that exhibit a range of terrestrial adaptation were exposed to elevated ambient carbon dioxide (hypercapnia) and oxygen (hyperoxia) levels to assess the relative magnitude of their ventilatory response. The most terrestrial crabs, Robber Crabs (Birgus latro) and Red Crabs (Gecarcoides natalis) did not show significant ventilatory response to hyperoxia up to 40% oxygen. Both of these species had a hypercapnic ventilatory response (HCVR) of +1400% upon exposure to 10% carbon dioxide. When hypercapnia was combined with hyperoxia the ventilatory response was the same magnitude increase as for hypercapnia alone. Yellow crabs (Geograpsus crinipes), a less terrestrially adapted species also had no significant response to hyperoxia but a significant although smaller magnitude HCVR of +250%. When hyperoxia and hypercapnia were combined Yellow Crabs had no significant change in ventilatory rate. The bimodal-breathing Blue Land Crab, Cardisoma guanhumi, had an HCVR of +700% when acclimated to breathing air, but only +100% when acclimated to breathing water. The least terrestrial crab, the Horn-Eyed Ghost Crabs (Ocypode ceratophthalmus), also had no significant response to hyperoxia. However these Ghost Crabs increased ventilation in response to 5% carbon dioxide, but at 10% carbon dioxide, ventilation was not significantly greater than control values. There was no change in ventilatory rate of Ghost Crabs when hypercapnia and hyperoxia were combined. The most terrestrially-adapted crabs showed great sensitivity to carbon dioxide that was unabated by the presence of excess ambient oxygen, but the relative importance of carbon dioxide as a ventilatory trigger decreased in the less terrestrially-adapted crabs.