Aplysina insularis is a large sponge and consists of one or more cylindrical tubes united at the base.

Slender tendrils and larger rope-like projections may also occur, and in areas with high levels of sedimentation, these may exceed the tubes in size.

There are no silicaceous spicules in the wall but it is strengthened with a network of fibres forming a hexagonal or circular mesh pattern.

[2][5] If bruised or removed from the water, this sponge turns dark purple or black, exuding a substance that can stain a person's skin, leaving a mark that is difficult to eradicate.

These include the sponge cardinal fish (Phaeoptyx xenus) and several species of neon goby (Elacatinus spp.).