symbioses
Sponges form symbiotic relationships with various organisms, including mutualisms with photosynthetic bacteria/plant cells, and swimming scallops and crabs, and parasitisms on mollusc shells.
NOTE “symbiosis” is a general term used in North America for any two organisms living together. There are several categories of symbiosis described in the scientific literature, but the one most commonly used, commensalism (one partner benefits while the other is unaffected) assumes information about the participants that is usually unavailable. The two symbioses of clearest definition (and most interest) are mutualism, where both partners benefit, and parasitism, where one partner benefits and the other is harmed.
Mutualism
Several west-coast sponges house single-celled plants known as zoochlorellae. The relationship is thought to be mutualistic in that the plants provide products of photosynthesis (glycerol, glucose, amino acids) to the host in return for a place to stay, and the host provides carbon dioxide and ammonia nutrients required for photosynthesis. A common host for zoochlorellae is the “crumb-of-bread” sponge Halichondria panicea which has two growth forms. One is a flat, tough open-coast form that often contains so many zoochlorellae that it appears greenish in colour.
For some reason, the green open-coast Halichondria often has a bad smell. Whether this has something to do with its algal symbionts, or with the presence of defensive chemicals, is not known.
The other form is often found on docks in quiet water conditions, although may occur in open-coast situations. It has a softer consistency and mostly lacks symbionts.
NOTE thought to be similar to green plant cells found in west-coast sea anemones. These are discussed elsewhere in the ODYSSEY: SEA ANEMONES: SYMBIONTS.
What is the explanation for the enhanced growth performance of the nudibranch? Select the best idea from the list below and see explanations of the answers. Ideas from Knowlton & Highsmith 2005 J Exper Mar Biol Ecol 327: 36.
Nudibranchs eat more of the zoochlorella-containing sponge.
The symbionts increase the density of the sponge tissue; hence, providing more tissue per bite for the nudibranch.
The symbionts increase the nutritional content of the food.
