Oxygen Consumption in Crustaceans — VAN Weel et al. 
213 
do so. Consequently, their oxygen consump- 
tion varies (more or less) proportionately with 
the oxygen tension of the surroundings. 
The Crustacea used in these experiments 
obviously belong to the large group of ani- 
mals with an adjustable respiration. They lack 
the ability to regulate (at least to any appre- 
ciable extent) their oxygen consumption when 
the oxygen content of the water is altered. 
The oxygen consumption varies with the oxy- 
gen tension (Maloeuf, 1936) and according 
to Amberson et al. (1924) may even be di- 
rectly proportional to it. Our experiments 
confirm this (Table 1). Phymodius ungulatus 
and Platypodia granulosa seem to consume 
even less oxygen than expected when the 
oxygen content of the water is first reduced. 
After this initial period, however, oxygen con- 
sumption proportionate to the tension be- 
comes apparent. 
It has been claimed that many Crustacea 
do have a regulating mechanism in that they 
increase the rhythm of the beat of the scaph- 
ognathites and pleopods when the oxygen 
content of the water decreases (Munro Fox 
et al., 1933, 1934; Johnson, 1936; Schlieper 
and Peters, 1937; van Heerdt and Krijgsman, 
1939; van Heerdt, 1940). As these structures 
are the propellers by means of which the 
water flow is directed over the gills, an in- 
crease of the beat will bring the gills in con- 
tact with a larger quantity of water from which, 
consequently, more oxygen can be absorbed. 
In such cases some kind of regulation may be 
expected, not a typical adjustment as found 
in our experiments. Although, as mentioned 
before, we have been unable to make exact 
counts, in some cases an increase in the beat 
was noted when the oxygen tension of the 
water started to decrease. This would there- 
fore point toward a regulation; but if so, then 
it must be very weak and is therefore not 
apparent in the direct results. However, when 
the utilization of oxygen (the oxygen con- 
sumed expressed as the percentage of the 
available oxygen in the water) is studied, such 
a weak regulation may eventually be found. 
As long as the utilization remains constant 
when the oxygen tension of the water de- 
creases, it means that there is little or no 
regulation, and the respiration is therefore of 
the adjustable type. When the utilization curve 
rises, it means that there is some regulation, 
that more oxygen is consumed than would 
be the case in the adjustment type of respira- 
tion. A drop of the curve means a decrease 
in oxygen consumption, as compared with 
the adjustment type of respiration. 
Before the results with respect to the utili- 
zation of oxygen can be discussed, the ques- 
tion arises as to what can be the (primary) 
cause of regulation or adjustment in the proc- 
ess of respiration. Considering the simplest 
possibility, this is based on a limiting factor. 
Respiration being de facto an oxidation, the 
limiting factor can be either the oxidizable 
substances or the oxygen available. If there 
is an ample quantity of the former, the amount 
of oxygen reaching these substances must be 
regulated to maintain oxidation at a constant 
level. To increase (or decrease) this level, more 
(or less) oxygen must be admitted. Here oxy- 
gen is clearly the limiting factor. In the case 
of the oxidizable substances being the limit- 
ing factor, the rate of oxidation is determined 
by the amount of such substances made avail- 
able by the organism. 
An objection may be raised in that the 
possibility might exist that there is a com- 
plete tissue oxygen saturation in the organism 
which becomes unsaturated at lower tensions, 
resulting in increased utilization at that point. 
In that case, however, the utilization curve 
will steadily rise with decreasing oxygen ten- 
sion of the water, because the quantity of 
oxygen consumed by the tissues remains con- 
stant. This is not the case (Figs. 1-7). Ad- 
mittedly, the alternative between "fuel” (oxi- 
dizable substances) and available oxygen as 
limiting factors is probably too simple but 
is unavoidable at this time as far too little is 
known of the intracellular respiration proc- 
esses in Crustacea to include them in the 
picture. As long as such processes in this 
