The open ocean - A Fascination for Fish: Adventures of an Underwater Pioneer - page 267

have now kept for several years a number of mature eighteen-inch-long
filetail catsharks ( Parmaturus xaniurus ). Like their feline namesakes,
they have green reflective eyes for seeing in almost total darkness. We
suspect they use their sensitive eyesight to find bioluminescent prey.
Searching the canyon walls at depths of around fifteen hundred
feet and using the manipulator arm of the ROV, Gilbert worked with
MBARI scientists to pick up rocks with soft corals attached to them.
These strikingly beautiful mushroom soft corals ( Anthomastus ritteri ),
with clusters of eight-tentacled polyps covering their globe-shaped
body, did very well in our refrigerated aquariums, despite their del-
icate appearance, and thrived on blended krill shakes and brine
shrimp.
One of the strangest creatures we collected was a predatory tunicate
( Megalodicopia nians ). Its nearest relatives are shallow-water attached
animals that make their living by filtering plankton from the water.
Rather than filter-feeding, this tunicate, which lives attached to the rocky
bottom of the canyon floor, lies in wait for small prey to enter its open
hood, to be captured in a manner similar to that of the carnivorous
plant the Venus flytrap. Unfortunately, although these tunicates did
well for a few months in our refrigerated tanks, they then declined and
died. Gilbert set about figuring out why.
One significant feature of the deep sea o¤ California is a zone be-
tween 1,600 and 2,600 feet, known as the oxygen-minimum layer, where
the oxygen concentration is extremely low: as little as one-tenth of that
found near the surface. We suspected that this tunicate was adapted
to living under these very low concentrations, and our system, being
exposed to atmospheric air and 100 percent saturated with oxygen, was
actually toxic to it.
Gilbert therefore set up a special life-support system with low oxy-
gen. It stripped oxygen out of the water by bubbling nitrogen through
a small contact tower connected to the refrigerated recirculating
water system. The water surface of the tank was covered with a plastic
sheet to minimize the absorption of oxygen from the air. This oxygen-
stripping system worked well, and Gilbert began to have success with
the predatory tunicate. They were now staying alive and healthy for
over two years, which was probably their normal life span.