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MISSION DISPATCH 4 • 08/29/01 Today's Weather - images courtesy of NOAA & RSMAS Dispatch by Rebecca Johnson - HARBOR BRANCH Oceanographic Institution Latitude: 42°26'38"N • Longitude: 69°45'37"W Yesterday afternoon, while the sub was down on its first dive, a big shark cruised past the ship about 15 meters off the stern. We couldn't see much of the shark's body except its gleaming, 
triangle-shaped dorsal fin slicing through the water. But one glimpse of that fin was enough
to spark an involuntary shiver and conjure up images of nasty, jagged teeth. Imagine being in
the water with a shark that's on the prowl for its next meal...
Now imagine you're a copepod, one of those tiny creatures we've been bringing up in the MOCNESS tows by the thousands every day. You're swimming along, minding your own business, when WHAM! You're suddenly caught fast, as if clenched in the fingers of some unseen hand. As tiny spears pierce your body, your fate is sealed. You've blundered into the feeding net of Nanomia cara, a nearly invisible curtain of doom draped through an expanse of deep ocean water. For a copepod, Nanomia is as deadly as a large, hungry shark might be for you or I. Visiting Scientist Per Flood peers into an aquarium in one of the ship's cold rooms, essentially a walk-in refrigerator. Suspended in the water beyond the glass is a Nanomia colony, gliding 
effortlessly from side to side. Its long tentacles trail out behind it like the gauzy veils of
a dancer in some eerie underwater ballet. Yet for all its graceful looks, Nanomia is a voracious
predator. After casting its tentacle net - which may spread out a meter or more in the water - copepods and other
hapless victims unknowingly swim into its fatal embrace.
Nanomia's tentacles are studded with nematocysts, minute stinging cells that pierce and entangle small planktonic creatures that rub against them. Once snared, prey are moved into the siphonophore's many "stomachs" (gastrozooids) and digested. Last night Sarah and Marsh sat up until the wee hours of the morning examining the stomach contents of Nanomia colonies. More specimens were just brought aboard, collected on the last dive.  What are we finding in these stomachs? Nanomia seems to be dining primarily on Calanus finmarchicus, as expected. But there's much we haven't discovered. For instance, what other prey species are on Nanomia's menu? Does it eat more during the day or at night? Does it feed in shallow or deep water? Answers to these questions will help us better understand the importance of these gelatinous predators in the ocean food chain.  | |
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