XII. Depth Related Patterns
We also see that different types of marine animals exhibit
different depth-related patterns. Visually-cued predators like fishes and
crustaceans show a rapid decrease in metabolism and muscle enzymatic activity
with depth. Gelatinous species, including jellyfish and ctenophores, which
do not depend on their visual sense for predation, do not show a decrease
in metabolic rate or enzymatic activity with depth.
Recent work on gelatinous organisms by Eric Thuesen and Jim Childress of the University
of California, Santa Barbara, has shown clearly how different the depth-related
patterns in respiration are between visual and non-visual predators.
I've emphasized that if we measure respiration rate, as well as enzymatic activity,
we see a strong correlation. I want to take a side trip here and talk about
the uses to which biochemical methods can be put to get a handle on the
physiological status of natural populations. Here is the logic. We've seen
that enzymatic activities can provide an index of oxygen consumption rates.
These in turn are an index of physiological status. In other words, if we
measure the rate of oxygen consumption of an organism and plot that against
the amount of ATP generating activity in a gram of muscle, we see a very
strong correlation. But what if we want to get some sense of the physiological
status of fish that we can't keep alive for respiration measurements? Let's
say we wanted to get an estimate of respiration rate for deep sea fishes
that are very difficult to bring up alive. As mentioned earlier, a lot of
these organisms are very fragile. They're difficult to bring up in good
shape. We have to get a lot of our evidence about what these organisms are
like through indirect methods. Video measurements are wonderful for some
purposes; indirect biochemical analysis work very well, too. If we know
how respiration rate and muscle enzymatic activity co-vary, we can simply
measure enzymatic activity and use this value to predict the rate of oxygen
consumption.
 One
organism that we've used in studies that employ enzymatic indices of physiological
state is Sebastolobus alascanus, known as the idiot fish. This photo
taken on the sea floor shows this fish in its normal habitat. It's a commercially
important species that occurs at a number of depths and is found within
the oxygen minimum zone. We collected a number of these fish using set lines
and held the fish for several months in the laboratory. Some were fed; some
were starved. What we observed was that the fed fish had higher rates of
respiration and higher levels of enzymatic activity in their muscles than
the starved fish. Furthermore, when we compared freshly-collected fish from
the field with the lab-maintained fish, we found evidence that the field
fish were living under food-limiting conditions. Both respiration rate and
enzymatic activity in the field-collected fish were similar to values obtained
for starved fish. These data suggest that, down in the oxygen minimum zone,
 food
may be very limiting for this species, at least at certain times and in
certain places. With this "calibration curve" between respiration
rate and enzymatic activity, it should now be possible to sample hundreds
or thousands of these fish from different regions to get a sense of how
well they are doing physiologically. Rapid and inexpensive measurements
of enzymatic activity should provide strong evidence about the nutritional
status and respiration rates of the specimens. These data could be of some
value to fisheries scientists who are concerned about the status of commercially
important species.
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