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Explanations for answers to sponge/coralline quiz:

The alga shades the sponge from harsh effects of sunlight.  Possibly, but there is no evidence that sunlight is harmful to the sponge. Halichondria often grows in full exposure to sunlight, perhaps to enhance production and transfer of photosynthetic products from the many green-algal cells that reside within its tissues.

The alga provides protection from desiccation. Possibly. This is the view favoured by the author of the study, but no cause-and-effect data are available. Palumbi 1985 The Amer Nat 126: 267.

The alga provides protection from nudibranch-predators of the sponge. No, there is no evidence for this.  Few animals eat coralline algae, but this is thought to owe mainly to their inedible and hard-to-digest properties – not to the presence of chemical defenses that might provide “second-hand” benefit to a sponge growing nearby.

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