Q3 - Shell Drilling Predation

Theoretical 2 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Invertebrate Body Plans II Foraging & Anti-Predator Behavior

A fictional marine bivalve mollusk ‘sea carpet bag’ is a prey of carnivorous sea snails. The sea snails drill through bivalve shells to inject poison which relaxes the bivalve muscle-adductor. Muscle-adductor relaxation allows bivalve shells to be opened and the hunting snail eats the sea carpet bag.

Scientists divided the surface of sea carpet bag shells into four areas with equal size:

A - anterior area
B - basal area
C - central area
D - distal area

A representative set of 100 dead sea carpet bags was analyzed. The number of holes drilled by hunting snails in different areas, and their combinations, are shown in the Table.

Shell diagram showing the four drilling areas A, B, C, D

Drilling Position	Number of shells
Single hole
A	23
B	5
C	6
D	25
Pairwise combination
AB	4
AC	4
AD	16
BC	1
BD	3
CD	5
Triple combination	2 (1 ABD and 1 ACD)
No holes	6

True or False?

Q3.1. Hunting snails prefer to drill the anterior end of sea carpet bags and avoid the distal end of shells.

Q3.2. The sea carpet bag probably has two muscle-adductors, the first in area A and the second in area D.

Q3.3. Hunting snails are one of the major predators of sea carpet bags.

Q3.4. From the data, we could conclude that if the first drilling does not result in bivalve opening, hunting snails prefer to repeat their attack on the same sea carpet bag.

Q3.5. A possible explanation to the data is that some hunting snails could be drilling sea carpet bags which are already dead.

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Question reproduced from IBO 2023, Theoretical Paper 2, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF