Question

7 discuss the ecological benefits that the presence of sponges may have on aquatic ecosystems.
8 why were living sponges placed in ca²⁺/mg²⁺ free seawater overnight? hypothesize about the outcome of that exercise if this step had been omitted.

Explanation:

For question 7, sponges can filter water, providing cleaner water for other organisms and serving as a habitat and food source. For question 8, living sponges were likely placed in $Ca^{2+}/Mg^{2+}$ - free seawater to study their dependence on these ions for processes like skeleton - formation. If omitted, the results related to ion - dependence might be confounded or inaccurate.

Answer:

1. Sponges filter water, improving water quality for other aquatic organisms. They also serve as habitats and food sources for various species.
2. Sponges were placed in $Ca^{2+}/Mg^{2+}$ - free seawater probably to study their reliance on these ions. If this step was omitted, results about ion - related functions in sponges could be inaccurate or confounded.