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Genevieve Trafelet-Smith - Marine Ecology Lab

High School Teacher - Old Mill High School

Introduction


Ctenophores (Mnemiopsis leidyi) are commonly found throughout the Chesapeake Bay and its tributaries such as the Rhode River. Eastern Oysters (Crassostrea virginica) are considered a keystone species in the Chesapeake Bay because they provide habitat and food for many different organisms. Restoration efforts have not been able to bring oyster populations back to historic levels. It has been shown that ctenophores feed on oyster larvae, but more information is needed to determine their overall effect on aquatic food webs. Oyster larvae need to survive to mature oysters so that the oyster population can reestablish itself.  If the predation by ctenophores is too great, there will not be enough larvae developing into adults to reestablish the oyster population.  The results from this experiment will help to identify whether ctenophores have a preference for oyster larvae over zooplankton.  The data collected from this experiment will also provide information for future experiments that will involve predation rates of ctenophores on Asian oysters, Crassostrea ariakensis.  
 The main objective of this experiment is to determine if the presence of zooplankton influences the predation rates of ctenophores on oyster larvae. Before the experiment could be conducted, two smaller questions had to be answered. First, whether formalin or Lugol's works best in preserving ctenophores that will have their gut contents analyzed. Second, what is the digestion time for oyster larvae and zooplankton in ctenophores so that the optimum duration of the experiment could be determined.

Materials and Methods


Collection of experimental animals
Ctenophores and zooplankton were collected from the Rhode River, MD using a 0.5m hoop net frame with 202?m mesh.  Ctenophores were collected by trawling at approximately 3 meters of depth until 100 animals were collected.   The duration of each tow was two minutes to reduce the damage to the ctenophores.  Samples were then taken to the wet lab where they were added by to a 30-gallon aquarium filled with filtered sea water. A paddle wheel was used to keep the ctenophores from settling on the bottom and to maintain an reasonable level of dissolved oxygen.  Ctenophores were kept at least 24 hours in filtered water to allow them ample time to clear their guts prior to the experiments. 
Zooplankton samples were collected by trawling at approximately three meters.  Zooplankton samples were poured through a 2mm mesh sieve to remove ctenophores from the sample and taken back to the lab.  Zooplankton samples were left to sit for 1-2 hours in a bucket with an air stone to separate out the dead organisms in order to reduce the number of dead organisms added to the stock tank.  They were then added to the stock tank and bubbled with an air stone.
Veliger oyster larvae, approximately 15 days, old were obtained from Don Meritt of Horn Point Lab-University of Maryland.  The larvae were collected in a coffee filter, wrapped with a saturated paper towel, and put on ice during transportation.  They were rinsed into a bucket of filtered water and allowed to acclimate until they began to swim around (approximately 20 minutes). They were then transferred to an aquarium and maintained 24-48 hours by feeding them a processed algal diet.

Preservative determination
 Zooplankton and ctenophore samples were collected and put into separate stock tanks and the starting density of Acartia was determined.  The ctenophores were then simultaneously added to the zooplankton stock tank and allowed to feed for 15 minutes.  All of the ctenophores were then removed from the zooplankton tank, their lengths measured, and preserved.  Half the samples were preserved in 5% Lugol's and half were preserved in 10% formalin.  In the lab the samples were compared to determine which preservative was the most effective, accurate, and time appropriate.  Both the liquid and ctenophore remnants were analyzed; specifically in the Lugol's samples where the physical structure of the ctenophore had been preserved.  In those cases, the ctenophore had to be broken open under the microscope to view the guts and its contents.

Digestion time
Ctenophore samples were collected the day prior to conducting the experiment and oyster larvae were obtained the morning of conducting the experiment.  The densities of the stock tank were taken before and after the ctenophores were allowed to feed.  The ctenophores were introduced as a group to the oyster larvae stock tank, allowed to feed fro 15 minutes, and then removed into a filtered sea water tank.  At digestion times 0, 30 , and 120 minutes six ctenophores were randomly removed, their lengths measured, and preserved in 50mL of filtered water and 10% formalin.
Ctenophore predation of zooplankton and oyster larvae
 Ctenophores and zooplankton were collected the day before and oyster larvae were obtained two days before conducting the experiment.  Densities of Acartia and oyster larvae tanks were determined and the necessary volumes for addition of each into the experiment tanks were calculated. The target density of Acartia was 10 individuals was per liter.  The oyster larvae density was varied over all the tanks within a range of 0.05 to 10 per milliliter.  Each tank was filled with approximately 72 liters of filtered sea water.  Before the addition of zooplankton and oyster larvae, the equivalent volume of water to be added was removed to keep each tank at the same total volume.  The zooplankton and oyster larvae were allowed to sit for 30 minutes before the addition of the 10 randomly chosen ctenophores from the holding tank.  The ctenophores were allowed to feed for 20 minutes and then removed.  The lengths of each ctenophore were measured before they were preserved in 50mL of filtered sea water and 10% formalin.  The contents of the samples were then enumerated under a microscope.


Results


Preservative determination
Both formalin and Lugol's were effective in stopping digestion in the ctenophores and preserving the samples.  There appeared to be no difference between the numbers of zooplankton observed in regards to the preservative used.  There was a distinct difference in the condition and appearance of the ctenophores depending upon the preservative used.  The samples preserved in Lugol's had shrunk greatly but were whole and had to be dissected to observe the gut contents. This was time consuming and it was difficult to distinguish between copepods and ctenophore tissue. The samples preserved in formalin dissolved into small pieces and could be treated like zooplankton sampled directly from the tanks.  This made observing copepods less time consuming in formalin preserved samples. 

Digestion time
The samples from this experiment are still being processed. Once the samples have been processed and the data analyzed, this section will be completed.

Ctenophore predation of zooplankton and oyster larvae
The samples from this experiment are still being processed. Once the samples have been processed and the data analyzed, this section will be completed.

 

Classroom Applications:
I teach several different high school science courses and the information and experience from this internship can be applied to each course in several different ways. The information in general that I learned about sea nettles, ctenophores, and oyster larvae can be infused into my lessons specifically for Marine Biology. I have learned a lot about the feeding habits of these organisms and this can be applied when food webs are covered. Most textbooks have terrestrial food webs and so this would be a wonderful example of aquatic food webs and an example that is close to the students because it is in the Chesapeake Bay.
One weakness of students in general is being able to analyze data and explain what it means and why that is important. I can give the students some of the data from the experiments and have them analyze what it means. They would have to decide what actions to take based on their interpretation of the data just like scientists do. Having actual data for students to work with makes it more meaningful than when hypothetical data is given; they are more interested in the real data. The students can also make graphs based on the data to practice their graph making skills. 
 If ctenophores are available during an appropriate time during the spring semester, I can collect some ctenophores and have students set up their own experiment similar to the one I conducted. If that is not possible, the students can design their own experiment with some adaptations.

Funding provided by the National Science Foundation - Research Experience for Teachers