Question
The worm Branchiobdella italica lives on the external surface of the freshwater crayfish Austropotamobius pallipes. A study was carried out in a river in central Liguria, north-western Italy, of the range of sizes of B. italica found on adult A. pallipes.
a. Describe the body length frequency of the B. italica worms collected in this study. [1]
b. The relationship between A. pallipes and B. italica is mutualistic.
A. pallipes feeds on algae and another worm, B. exodonta, lives inside A. pallipes as a parasite. State the trophic level of B. exodonta in this food chain. [1]
c. Distinguish between mutualism and parasitism, providing another example of mutualism and another example of parasitism. [2]
▶️Answer/Explanation
a. The body length frequency of B. italica worms shows a normal distribution, with most worms having medium length, and fewer worms being either very short or very long.
b. B. exodonta is a secondary consumer because it feeds on A. pallipes, which is a primary consumer (feeds on algae). This places B. exodonta at the third trophic level.
c.
- In mutualism, both organisms benefit from the relationship.
- In parasitism, only the parasite benefits, often harming the host.
Examples:
- Mutualism: Nitrogen-fixing bacteria in the roots of legume plants (both benefit: bacteria get nutrients, plant gets nitrogen).
- Parasitism: A tapeworm living in the human intestine (tapeworm benefits by feeding, human may suffer from nutrient loss).