Question Set 1
All questions are based on the following diagram, which shows an experiment
where a cage was established with a block of dead coral on the bottom, placed so
there was about a cm of space between the bottom of the block and the bottom
surface of the sea bed. The cage had a mesh size that allowed coral larvae to swim
through the mesh and settle anywhere on the coral block. The investigator
manipulated densities of Diadema antillarum within each cage, so that it varied from
cage to cage, and then the investigator counted coral larvae that had settled and
established small juvenile corals on top and on the side of the coral block.
Questions:
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1. What is Diadema antillarum and how does it feed on the bottom? D.
antillarum is a sea urchin and it feeds on the bottom by scraping hookshaped parts of its Aristotle’s lantern on hard surfaces.
2. What was the effect of varying Diadema antillarum density on coral settling?
Be quantitative about your answer. (i.e., a change of some specific amount). A
hundred-fold increase in Diadema density resulted in an approximate 6-fold
decline in settlement.
3. Why do you think the investigator focused on the proportion on the upper or
lateral surface of the coral block? This may require some thinking, but the
logical conclusion is that the urchin could only feed in those areas of the
block.
4. What do you think the value of the Y value of the graph would be if there
were no Diadema antillarum, given the results you see on the plot? This is
just approximate but it looks like better than 99 percent.
This is the source of the paper: Sammarco, P.W. 1980. Diadema and its
relationship to coral spat mortality: Grazing, competition, and biological
disturbance. J. Exp. Mar. Biol. Ecol. 45: 245-272.
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Question Set 2:
An investigator worked on a suspension-feeding bivalve mussel named Musculista
senhousia. He looked at a dense bed of this bivalve and measured a few features of
the bivalves after a few months, at various distances from a pipe from which water
flowed with a rich supply of suspended algae. Keep in mind that a higher gonad
index means that the bivalve was making more reproductive tissue. The water plus
algae flowed over the bed of mussels and measurements of individual mussels were
done at varying distances from the inflow of algae-rich water from the pipe.
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Answer these questions:
1. How does a marine mussel feed? Marine mussels draw in water by ciliary
beating on the gills. Water is drawn into the mantle cavity and particles are
captured by cilia on the gills, which transfer the particles along ciliary tracts
to the palp and then to the mouth. Digestible particles in the gut are broken
down and material passes through the gut wall and absorbed.
2. What seems to be happening to the bivalves with increasing distance from
the inflow pipe? In several respects we can see that the mussels are growing
more slowly and making a smaller gonad if they are more and more
downstream from the pipe opening.
3. Why do you think the three factors are all changing in the same way with
distance from the inflow pipe? Because they all reflect a response to
nutrition.
4. State a hypothesis that explains the change illustrated in the figure. The
change in size and gonal index means that mussels upstream are removing
phytoplankton, which depletes the water for mussels living further
downstream.
5. How might you test this hypothesis? You can measure anything you want in
the tank or perform any experiment you want. There are several possibilities,
but one might be to remove the mussels that are closer to the pipe opening,
which should provide more food downstream. Another possibility would be
to put an inflow pipe with food-rich water on the downstream side, so those
mussels are now closer to a food source.
This is the source of the paper:
Allen, B.J., Williams, S.L. 2003. Native eelgrass Zostera marina controls growth and
reproduction of an invasive mussel through food limitation. Marine Ecology
Progress Series, v. 254:57-67.
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