Ventilatory response to ambient hypo-and hypercapnia in the lugworm,Arenicola marina (L.)

Summary

• 1. The mass specific rate of water ventilation,V W oxygen extraction coefficient,${\text{Ew}}_{{\text{o}}_{\text{2}} }$; mass specific rate of oxygen uptake,$\dot M_{{\text{O}}_{\text{2}} }$; water convection requirement,$\dot V{\text{w/}}\dot M_{{\text{O}}_{\text{2}} }$; mas specific rate of carbon dioxide production,$\dot M_{{\text{CO}}_{\text{2}} }$; and the respiratory quotient between inspired and expired seawater, R, were studied on unrestrained lugworms,Arenicola marina, placed in an artificial burrow and acclimated for 3 h or 25 h to 12 pairs of partial pressure of O₂ ($P{\text{I}}_{{\text{O}}_{\text{2}} }$=80, 160, 500 and 700 Torr) and CO₂ ($P{\text{I}}_{{\text{CO}}_{\text{2}} }$=0.1, 0.3 and 3 Torr).

• 2. For a given acclimation time and$P{\text{I}}_{{\text{O}}_{\text{2}} }$ value, the mean values ofV W,${\text{Ew}}_{{\text{o}}_{\text{2}} }$,$\dot M_{{\text{o}}_{\text{2}} }$ and$\dot V{\text{w}}/\dot M_{{\text{o}}_{\text{2}} }$ were not significantly different in hypocapnia, normocapnia and hypercapnia (Table 1).

• 3. In 25-h acclimated hyperoxic animals, regardless of$P{\text{I}}_{{\text{CO}}_{\text{2}} }$, the mean values ofV _w,${\text{Ew}}_{{\text{O}}_{\text{2}} }$,$\dot M_{{\text{O}}_{\text{2}} }$ and$\dot V{\text{w}}/\dot M_{{\text{O}}_{\text{2}} }$ did not differ from those obtained in 3-h acclimated lugworms. In hypoxia, for the three$P{\text{I}}_{{\text{CO}}_{\text{2}} }$,V W and$\dot M_{{\text{C}}_{\text{2}} }$ were higher in 25-h than in 3-h acclimated lugworms (Table 1).

• 4. The analysis of$\dot M_{{\text{CO}}_{\text{2}} }$ and R values shows that a steady state for CO₂ exchange was obtained (R not significantly different from 1) in the animals acclimated to hypoxia for 25 h, regardless of$P{\text{I}}_{{\text{CO}}_{\text{2}} }$, and in the animals acclimated for 3 h to normocapnic hypoxia and to hypo-and normocapnic normoxia. R was significantly higher than 1 in animals acclimated to hypocapnic hypoxia for 3 h. In the eleven other conditions of$P{\text{I}}_{{\text{O}}_{\text{2}} }$,$P{\text{I}}_{{\text{CO}}_{\text{2}} }$ and acclimation time, R was significantly lower than 1 (Table 1).

• 5. We conclude that under our experimental conditions i) carbon dioxide had no influence on the ventilation and oxygen uptake ofArenicola marina whether the animals were in a steady state for CO₂ exchange or not, ii) variations of$\dot M_{{\text{CO}}_{\text{2}} }$ and R probably reflected the dynamic evolution of CO₂ stores during acclimation to new$P{\text{I}}_{{\text{O}}_{\text{2}} } /P{\text{I}}_{{\text{CO}}_{\text{2}} }$ conditions, iii) in hypoxia, regardless of$P{\text{I}}_{{\text{CO}}_{\text{2}} }$, an increase of the acclimation time from 3 to 25 h led to a significant lowering of the critical oxygen partial pressure. These results confirm that$P{\text{I}}_{{\text{O}}_{\text{2}} }$ is by far the most important factor regulating ventilation in the lugworm as in other water breathers.