| | L i g h t a n d V i s i o n i n t h e D e e p S e a | ||||||||||||||||||||
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Keywords: Neurobiology, vision, vertical migration
(For a review of visual pigments present in shallow
water species, see Goldsmith, 1972; Lythgoe, 1972; Cronin, 1986. For
information on the photosensitivities of deep-sea crustaceans,
see Frank and Case, 1988a,b; Frank and Widder, 1994a,b; Cronin
and Frank, 1996. For information on the visual pigments of deep-sea
fish, see Crescitelli, 1991; Partridge et al., 1992; Douglas and
Partridge, 1997.)
However, there are some interesting exceptions to this rule. Several
species of deep-sea fish possess photophores that emit light primarily
in the far red wavelengths (so far red that we can just barely
see their light if we're completely dark-adapted), and remarkably,
they have several visual pigments, one of which can actually see
this far red light (see the bioluminescence section for information
on the red bioluminescence, and Partridge and Douglas, 1995 for
information on the red-absorbing visual pigments). There are
also several species of deep-sea shrimp that possess two visual
pigments. One pigment absorbs in the blue (approximately 490-500
nm, as expected), and the second pigment, rather than being a
far-red pigment like in the fish, absorbs in the near UV -- about
400 nm (Frank and Case, 1988; Cronin and Frank, 1996). The species
that possess this unusual short wavelength pigment have also been
shown to be behaviorally very sensitive to UV light (Frank and
Widder, 1994 a,b).
What are the possible functions of such a short wavelength pigment? The
first thing to realize is that there is enough near-UV light down
at 600 m (the daytime depth of these species) for these species
to see (see calculation in Frank and Widder, 1994a). One hypothesis
is that since the species with two visual pigments (one at 400
nm, and one at 490 nm) are vertical migrators, they might be using
the change in the color of light at sunset to cue their migrations.
Vertical
migration is the largest animal migration on earth, and occurs
in all the world's oceans and lakes. It is the movement of organisms
up through the water column at sunset to shallower water, and
their movement back down through the water column to deeper water
at sunrise. This migration happens every night, and in some areas,
the schools of migrating organisms are so massive that the layer
can be picked up on shipboard sonars - it looks like the bottom
is rising up. The major reason that vertical migrations occur
is to avoid predators; these species remain in deeper darker water
during the day, and come up at night to feed in the food rich
surface water and hide from their own predators under the cover
of darkness.
Although this is such a huge and widespread phenomenon,
very little is known about the light cues that trigger this massive
migration. What is known is that it has to be more than just
an intensity change; otherwise, organisms might start migrating
during the day on a dark and stormy day, and this doesn't happen. Since
the spectral distribution of light changes at sunset, and there's
a relative increase of short wavelength light with respect to
long wavelength light, we hypothesized that the shrimp that possess
both the short and long wavelength visual pigments might be able
to sense this spectral cue at sunset (Frank and Case, 1988). However,
recent measurements made with an in-situ light meter at 150 m
depth from a submersible indicate that the shift in the spectral
distribution of light that occurs at the surface at sunset is
not seen at this depth (Frank and Widder, 1996). Since the crustaceans
with the near-UV visual pigments start their migrations from 500-600
m depths, they clearly cannot be sensing any changes in the spectral
distribution of light at sunset, and must be relying on other
cues. This does not answer the question of why these particular
species possess this dual visual pigment system, and this does
not appear to be a universal phenomenon amoung deep-sea shrimp. We
have studied 22 other species of shrimp, including those in the
euphausiid, pasiphaeid, penaeid, sergestid, and amphipod families,
and it appears that only these genera of caridean shrimp in the
family Oplophoridae - Systellaspis, Janicella and Oplophorus -
posses this near UV visual pigment in addition to the typical
blue absorbing pigment. As there is no funding to study interesting
but rare phenomena in the ocean, this question will most likely
remain unanswered for a while.
Current research in our laboratory involves determining what
light cues are responsible for cuing these massive vertical migrations. Results
of recent submersible work in the Gulf of Maine indicate that
migration patterns are staggered - i.e. the three species quantified
during this study - a euphausiid shrimp Meganyctiphanes norvegica,
a cydippid ctenophore Euplokamis sp., and two species of caridean
shrimp, Dichelopandalus leptocerus and Pasiphaea multidentata, did
not all start their migrations at the same time (Frank and Widder,
in press). We are currently conducting further studies to
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the surface, which, due to absorption and scattering, is primarily
blue light and (2) bioluminescence, which is also primarily blue
light. Therefore, one would expect deep-sea organisms to possess
visual pigments that absorb more in than blue compared to shallow
water organisms. This has proven to be the case for most deep-sea
organisms.