| Date | May 2017 | Marks available | 2 | Reference code | 17M.3.HL.TZ1.13 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Distinguish | Question number | 13 | Adapted from | N/A |
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.
Describe the body length frequency of the B. italica worms collected in this study.
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.
Distinguish between mutualism and parasitism, providing another example of mutualism and another example of parasitism.
Markscheme
a. higher frequency of medium length worms
b. shows normal distribution
c. lower frequency at extremes
Allow correct numerical description of these points.
secondary consumer / third trophic level
a. in parasitism only one organism benefits whereas in mutualism both benefit
b. example for both parasitism AND mutualism
Do not allow B. italica or B. exodonta as examples.
eg parasitic: human tapeworms AND mutualism: bacteria in human digestive tract
