A symbiotic relationship is an intimate association between different species. It takes a significant time to arise and occurs through co -evolution. Symbiotic relationships can be:
 obligate – the association is necessary for at least one of the organisms to survive
 facultative – the association is beneficial for at least one of the organisms but is not essential for survival.
Parasitism
Parasitism is an interaction between organisms of different species in which one organism
(the parasite) benefits and the other (the host) is harmed. This can happen through physical damage to the host or through stealing nutrients from the host.
‘Parasites were collected from 107 harbour seals off t he coast of Schleswig-Holstein,
Germany between 1997 and 2000. A total of 77 seals were infected with stomach worms and 4 were infected with sucking lice.’
Question 1
(a) Which parasite is an ectoparasite and which is an endoparasite? (2)
(b)What percentage of the seals from this study was infected with stomach worms and what percentage was infected with sucking lice? (2)
Question 2
‘A fungal parasite of amphibians has been linked to dramatic decline in population numbers. The fungus grows best in moist environments and results in a thickening of the skin of an amphibian.’
(a)Why are almost all amphibians at risk of contracting this fungus? (1)
(b)Why would a thickening of an amphibian’s skin result in illness and possibly death? (2)
‘Some amphibians are resistant to the fungus but can still be carriers.’
(c)Why may this be an advantage and a disadvantag e to conservation biologists? (2)
Mutualism
Mutualism is an interaction between organisms in which both organisms benefit.
Question 3
‘One of the best known examples of a mutualistic relationship is that between the clownfish and the sea anemone. Both benefit from the interaction; the clownfish is protected from predators by the stinging tentacles of the anemone, and the sea anemone is p rotected by the clownfish from anemone-eating butterfly fish and also receives nutrients from the clownfish waste.’

(a)Is this association facultative or obligate? (1)
(b) Do you think that this association will be short or long term? Why? (1)
Question 4
‘Coral reefs are one of the most productive of all ecosystems supporting around 25% of all marine species. Reef-building coral is formed from the secretion of a calcium carbonate skeleton by the coral polyps. Within the cells of a coral polyp live microsc opic algae that provide over 90% of a coral’s nutrients as reefs often form in areas of very low nutrient concentration.’
(a)Why is coral reef growth limited to areas of clear, relatively shallow water? (2)
‘Corals provide the algae with protection and a constant supply of carbon dioxide without which the algae cannot survive. Temperature change is one of the environmental stresses that can result in the depletion of algae numbers which results in the ‘bleaching’ of coral.
The coral will die unless the algae recover, leaving only the calcium carbonate skeleton.’
(b) Is this relationship facultative or obligate?
(c) Why are conservationists concerned about climate change?
(1)
(1)
(2) (d)Name two more environmental stresses that may affect coral reefs.
Question 5
‘Coral reefs are often referred to as ‘the cleaning stations’ of the oceans. They are home to animals such as cleaner shrimps and cleaner wrasse. These organisms remove parasites from ‘clients’ visiting the stations and often enter the mouths o f otherwise carnivorous species.’
(a) What kind of symbiotic relationship if this an example of?
(b) Why do you think this?
(1)
(2)
Question 6
Fill in the grid using ‘benefits’, ‘negatively affected’ or ‘unaffected’ to show the effect on each organism in the relationship.
Relationship Effect on organism 1 Effect on organism 2
Parasitism
Mutualism

Question 7
‘Many fish go through life with at least a low level of intestinal or stomach worm infection with no effect on their health.’
(a) What symbiotic relationship is this an example of? (1)
Question 8
‘Sea anemones often inhabit shells of hermit crabs. In one study predators of hermit crabs were presented with hermit crabs either with or without associated sea anemones. After 12 trials with and 12 trials without the results are shown below.’
(a) Describe the trend of the results. (2)
(b) If sea anemones are filter feeders what kind of symbiotic relationship do you think this association represents? Why? (2)
Rejection Rate of Hermit Crab by Predator
9
8
7
6
5
4
3
2
1
0
With anemone Without anemone
Lehnert, K., Raga, J.A. and Siebert, U. (2007) Parasites in harbour seals ( Phoca vitulina ) from the German Wadden Sea between two Phocine Distemper Virus epidemics. Helgoland
Marine Research , 61 , 230–245.
McLean, R.B. and Mariscal, R.N. (1973) Protection o f a hermit crab by its symbiotic sea anemone Calliactis tricolour. Cellular and Molecular Life Sciences , 29 , 128–130.