Lesson Plan 7: Oyster Diseases – Oysters get sick too!
Adapted from Project PORTS: Promoting Oyster Restoration Through Schools Curriculum Guidecopyrighted 2007 by L. Calvo & Rutgers University.
Charting the Course:
In the following activity students will become shellfish biologists and examine Dermo (P. marinus) disease dynamics at three New York City experimental reef sites. Students will correlate their disease data observations with environmental conditions at each site.
Objective/Students will be able to:
-identify two common oyster diseases
-correlate environmental conditions with disease prevalence
-interpret data
Background:
Like any organism on earth oysters too are susceptible to disease. The two diseases that plague U.S. East coast oysters are Dermo (Perkinsus marinus) and MSX (Haplosporidum nelsoni). Although these diseases are quite harmful to oysters causing mass mortalities they do not affect humans.
The first disease MSX is caused by a protozoan parasite that is thought to have been accidentally introduced into U.S. east coast waters by ships coming from Asia. MSX was first reported in the Delaware Bay in the 1957. The Delaware Bay, like NY Harbor, is an estuary in southern New Jersey about 130miles south of New York City. The Delaware Bay still has an active oyster fishery which was severely impacted due to disease outbreaks. In the 1950s MSX caused severe mortalities of oysters in the Delaware Bay predominantly in areas of high salinity (generally above 15ppt) killing off up to 95% of oysters over a 2-3 year period. The second disease Dermo is also transmitted by a protozoan. This disease commonly occurred in the southeastern U.S. waters where water temperatures are warm all year round. Occasionally this disease was found in the northeast but the cold winter waters kept the disease at bay. However in the 1990s significant oyster mortalities along the NE coast were observed that were attributed to a northward expansion of the Dermo disease. Most likely increasing water temperatures coupled with milder and drier winters associated with climate change caused Dermo to spread north. Today both diseases pose a threat to oyster restoration efforts in New York City waters.
It is important for restoration scientists to understand how disease affects their efforts to help revive the once thriving oyster populations. Scientists monitor the experimental oyster reefs throughout the year to record the prevalence of these diseases. A sample of 20-30 oysters are taken every month and shipped to a disease testing laboratory. At the laboratory, shellfish biologists analyze the oyster tissue samples using a special diagnostic assay to determine infection rates.
Scientists understand that disease is a natural part of any ecosystem, and plays an important role in balancing out populations. While these diseases can never be entirely eradicated, the hope is that New York City oysters will build a natural resistance to these diseases as has been observed in the Delaware Bay. By developing their own resistance NYC oysters will have overcome one more hurdle that prevents them from taking hold and flourishing in these waters once again.
Procedure/Warm Up
Start up a class discussion about possible threats to oysters. Past and current threats include over harvesting, water pollution, dredging, sedimentation, natural oyster predators, low dissolved oxygen levels and disease. As restoration scientists investigate best methods to employ in trying to restore oyster reefs to our local waterways, they are encountering some of the challenges preventing oyster populations from expanding. Current concerns are degraded water quality such as low dissolved oxygen levels and heavy metal contamination, lack of adequate habitat (dredging in the past has removed habitat on the estuary bottom for oysters and altered underwater current dynamics) and the spread of oyster diseases.
Dermo disease data, temperature and salinity data from three of the experimental oyster reefs is provided for this exercise. Dermo prevalence varies seasonally and annually in response to varying environmental conditions. The disease prevalence increases during the summer and the fall in response to warm water temperatures and then declines in winter and spring in response to cooler water temperatures. Salinity also affects the distribution of the parasite within the estuary. Perkinsus marinus prevalence is higher in areas with higher salinity (>15ppt). The disease tends to be more prevalent in drought years when river flows are reduced and salinities increase in the estuary compared to wet years where more fresh water will dilute the brackish estuary waters.
Activity:
The students are shellfish biologists working at the renowned Marine Animal Disease Laboratory at Stony Brook University. Restoration scientists in New York City have commissioned the shellfish biologists to investigate disease prevalence at the NYC oyster reefs and to advise scientists on how best to move forward with their restoration efforts.
Students will be given disease, temperature and salinity data for three experimental oyster reefs; Hastings reef, Soundview Reef and Govenors Island reef for the year 2012. Graph paper should be handed out or if students are proficient in Excel this exercise can be completed electronically. Students can be divided into groups or work individually.
Students should graph their data and answer the following questions:
1) Compare the temperature at the three sites.
2) Determine the maximum and minimum temperature for the year and indicate the month in which they occurred.
3) Compare salinity at the three sites. Which site had the lowest? Which site had the highest?
4) Determine the range of salinity at each site.
5) Compare P.marinus prevalence at the three sites, which site had the highest and which had the lowest.
6) Determine at what time of the year disease was at its highest infection rate.
7) Based on their analysis of the data students should speculate on why they saw salinity and disease differences between the sites and why temperature varied little between sites.
8) Discuss the relationship between temperature and disease levels.
9) Predict which experimental oyster reef is going to be most impacted by disease.
10) As shellfish biologists what recommendations might students make to the restoration scientists?
Modifications:
Students can take home question sheet as a homework assignment. Student groups can discuss their results and answer questions amongst themselves and present their findings to the class.