Why do deep spongeshave silicaceous and calcareous spicules?

Deep sea sponges have spicules made up of SILICA mostly. Because the concentration of silica is low on water surface level, as we go deep down the concentration increases and thus they mostly have siliceous spicules.

Also some sponges synthesize silica enzymatically.

These spicules can be upto 3 m long and provide mechanical features. 

 

Hope this helps. Then do give a thumbsup ! 😊👍