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Life In the Sea: What Grows Up May Stay Down When you think of animal migrations, you probably think of the thousands of birds that fly south every winter. Perhaps you picture groups of gigantic whales traveling thousands of miles across the ocean, or maybe enormous herds of caribou stampeding across Alaska. But there are two other migrations that occur in the ocean, in which animals move vertically up or down in the water column, rather than horizontally across the ocean. One of these occurs on a daily basis, and is called vertical migration. During vertical migrations, fish, shrimp, squid and jellyfish leave the deep, dark waters where they spend the day, and head for shallower waters around sunset to feed. Around sunrise, they head back down to deeper waters where the light is dim, helping to hide them from predators. The other type of up and down movement is called ontogenetic migration. 
Ontogeny is the development of an organism from the embryo to the adult, so animals that undergo ontogenetic migrations
change depth (usually moving to deeper depths) as they mature. This is not a daily movement like vertical migrations,
but rather, it's like moving to a new neighborhood. The youngest and smallest individuals are usually found at the
shallowest depths - in the top 200 feet of the water column. The oldest and biggest adults are usually found at the
deepest depths - from 2,300 to 3,000 feet. Often times, the older organisms that live at the deepest depths during
the day also undergo vertical migrations, coming up into shallower waters at night to feed.
So, why do they undergo both vertical AND ontogenetic migrations? Well, the youngest and smallest individuals are like toddlers - they can't swim very fast, and don't have the strength or energy to undergo nightly migrations. However, being so much smaller than the adults, they're not as easily seen, so they can hang out in shallow, brightly lit waters during the day, and still be fairly difficult to see. As they get bigger, becoming teenagers, they're stronger, faster swimmers, but also more visible to their predators. So, they move down several hundred feet in the water column where there's less light and it's harder to see them during the day. These teenagers start to undertake nightly vertical migrations, because all the good food is up at the shallower depths. As they grow even bigger into adulthood, they're, they have to go even deeper, where there's very little light, so that their predators can't see them during the day. Now they're strong enough swimmers, however, to under nightly migrations covering over 1,500 feet. 
The question we'll be asking is - how do different life history stages of the same species see in the bright light
field in shallow waters, and well as in the deeper dark depths? Some of the fish and squid have adjustable pupils,
just like we have, but that's not an option for shrimp that have compound eyes. Do they have an eye that works o.k.
in both bright light and dim light, but isn't the best design for either condition? Or are there mechanisms in place
that allow for the conversion of an eye designed to function best in bright light to one that functions best in very
dim, a conversion that occurs with a time course of less than a year? The reason we need to answer questions like
this is that many of these crustacean species that undergo ontogenetic migrations are major components of the oceanic
food webs that support the major commercial fisheries. Understanding whether and how they adapt to different light
environments is critical to modeling what effect a change in their light environment (particularly for the shallow
water juveniles, where a change in water clarity may occur due to pollution) may have on their distribution patterns.
In addition, this information will also provide an assessment of their ability to survive if global warming continues
to warm surface waters, pushing them to deeper cooler water. If their eyes are adapted for optimal vision in the
brighter lights available in surface waters, being forced into deeper darker waters will significantly diminish their
ability to find prey. However, if their eyes are designed for a variety of light levels, working o.k. in bright
lights and o.k. in dim lights, but not optimal in either, then being forced to deeper darker waters would not have the
same impact on species survival. We will search for some answers to these and other questions as we explore the waters
off Southern California, so stay tuned...
--Tammy Frank, Ph.D. Department of Visual Ecology Harbor Branch Oceanographic Institution  |
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