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MISSION DISPATCH 9
June 11, 2005 |
Ron Douglas | San Clemente Basin (off the coast of S. California)
The basic question for many biologists is determining 'what' an animal does (as well
as 'why' is does so and 'how'). While this is relatively easy to answer for most terrestrial
and shallower water animals, it is much harder for many animals inhabiting the deep ocean.
Ideally one would want to make detailed observations of natural behaviour in the wild as well
as perform controlled experiments in the laboratory. Unfortunately neither of these is feasible
for most deep-sea animals, such as cephalopods and fish, as it is impossible to keep most of them
alive for extended periods of time after capture. Furthermore, observation from submersibles
and ROVs, although immensely valuable in some respects, is unlikely to result in seeing natural
behaviour due to the presence of bright lights and/or noise. Much of the knowledge of the
lifestyle of deep-sea animals thus derives from predictions based on detailed structural and
functional observations of some aspects of their biology
One of the prime requirements for any animal's survival is its ability to sense information
within its environment and to act accordingly. The ability to perceive such environmental
stimuli underlies virtually everything an animal does; for example, it's ability to feed,
avoid predators and to reproduce. Not surprisingly therefore most animals, including those
in the deep-sea, have well developed eyes. Although, the deep ocean is often regarded as
a lightless world, this is far from the truth. Not only does some sunlight penetrate the
upper 1000m of the clearest oceans, but most animals also produce their own light (bioluminescence).
Terrestrial and surface water animals experience a wide range of light intensities during
a 24hr period (up to 12 log units). However, individual photoreceptors only respond over
a range of about 3- log units. Animals have therefore evolved a whole variety of structural,
biochemical and electrophysiological mechanisms for adjusting the sensitivity of their eyes.
One of the most obvious of these is the pupil response. Although small invertebrates and fish
do not generally have a pupil response, they are thought to be widespread among cephalopods
(octopus, squid, cuttlefish & vampire squid). In fact the cuttlefish, Sepia officianalis,
has the fastest pupil response of any animal. Whether the same is true of open ocean cephalopods
is unclear, although one would predict that animals living in the relatively dimly lit depths
would lack the ability to change the size of their pupil. Surprisingly, during this cruise we
have observed that rather than being only linked to an animal's habitat, the presence or absence
of a pupil response is at least partially related to their phylogeny. Thus, pupil mobility is
present in many octopuses and cuttlefish but appears to be absent in squid. Most surprisingly
a slow, but significant, pupil response was observed in some deep living octopuses, where
variations in light level are much reduced. Thus, unbeknown to us, these animals occasionally
might visit brighter surface waters, or the response of the pupil serves some other purpose
than adjusting the eyes' sensitivity.
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