Disease X is a potentially fatal disease found in sea urchins, with symptoms such as body deflation, body lesions, and discoloration. The cause of disease X has yet to be determined, and several hypotheses have been tested.
One such study investigated the possible connection between the presence of viruses known as densoviruses and disease X in several species of sea urchin.
Some of the results of the study are shown in the graph below.
State the viral load of species 1 when symptoms are not present.
Compare and contrast the relationship between viral load and disease X symptoms in the different species studied.
Use the graph in part (a) to deduce the impact of densoviruses on disease X.
Aquatic organisms have a thin layer of water surrounding their bodies known as the diffusive boundary layer (DBL). Organisms exchange substances with their environment via this layer, and its composition is different to that of the surrounding seawater.
Another study looked at the impact of the composition of the DBL on the development of disease X. Two types of organic matter were added to the water surrounding sea urchins of species 2:
- Organic matter in the form of lab-grown algae.
- Seawater containing normal levels of organic matter.
The sea urchins were assessed over 10 days for signs of disease X lesions and the water in the DBL was tested for bacteria. Note that the control contained no additional organic matter.
Describe the formation of disease X lesions over the testing period.
Discuss any observations on the growth of disease X lesions and the abundance of bacteria in the DBL shown in part (d).
As part of the same study, the researchers looked at the impact of reduced oxygen levels in the DBL on lesion growth in a fourth species of sea urchin.
Their results are shown in the graph below.
Calculate the number of individuals that would have lesions after 10 days with depleted oxygen levels in a population of 23.
Suggest how depleted oxygen might lead to the increase in growth of disease X lesions shown in part (f).
A student read the results described in parts (d) and (f) and concluded that disease X is caused by an increase in organic matter and a decrease in oxygen levels in the DBL of sea urchins.
Evaluate the student’s conclusion.
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