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THE MAINE EVENT: SUMMER 2002 Predation by Gelatinous Zooplankton in the Gulf of Maine Dispatch 4 Wednesday, July 03 2002 Latitude: 40°17.408"N • Longitude: 68°06.792"W Dispatch by Heather Holberger, HARBOR BRANCH Oceanographic Institution A major goal of this mission is to determine the role and impact of the physonect siphonophore Nanomia cara in pelagic food webs. This gelatinous animal can occur in great numbers in the Gulf of Maine and the deep-water canyons south of Georges Bank. When abundant, Nanomia cara may consume millions of copepods, thereby competing with juvenile fishes and marine mammals that also feed on copepods. In order to estimate how much food siphonophores consume, we are studying their feeding behavior. Unfortunately close up, detailed observations of prey capture and ingestion by Nanomia is not yet possible in the natural environment, so we improvised the best we could. We use special samplers on the JOHNSON-SEA-LINK (JSL) manned submersible to collect living colonies of Nanomia. Collected animals are placed in the cold room on the ship. A temperature of 6°C is maintained in this laboratory, and red light is used for illumination 
because the colonies cannot detect this wavelength. We let the colonies
adjust to the new environment for a few hours [which also gave them time to
digest any prey that was in their gastrozooids (=stomachs)] before we
started feeding experiments.
Copepods (pictured left), the prey for the experiments, were caught by towing a half-meter plankton net at the surface immediately after we collected Nanomia. Next, a few dozen individuals of a single species of copepod were sorted from the catch and quickly stained with a vital dye called neutral red. This procedure made it easier to see the 3 mm-sized copepods that were consumed by the gastrozooids of the colonies. After 15 minutes in the stained water, the copepods are removed and placed in the tanks with the siphonophore colonies. The interactions of predator and prey were observed continuously for the first half hour then checked every fifteen minutes after that until all the prey were gone. 
We observed their rates of ingestion and digestion and so far, we have
learned that it takes anywhere from 5 to 60 minutes for a colony to ingest a
copepod and approximately 2 1/2 hours to digest one. Also, the gastrozooids
may ingest more than one copepod at a time. If a tentacle entangles two
copepods then both of them are hauled to the gastrozooid and ingested
simultaneously.
Further observations are needed to answer other questions such as: Do the colonies fill their stomachs immediately after the previous catch has been digested? Or, do they wait awhile before feeding again with a given stomach? Or do they refill the stomach even before the previous catch is completely digested? Quantification of ingestion and digestion rates in relation to the questions stated will allow us to estimate the impact Nanomia cara has on copepod abundance and, inevitably, on other pelagic fauna whose diet is mainly copepods. Do you have a question for the researchers on this ocean expedition? Why not ask a scientist @Sea? 
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