The elegant, tubular branches of the tube sponge (Kallypilidion fascigera) are easily torn, and so it occurs only on deeper reef slopes, where wave action is minimal. It sometimes grows as a single tube, but it is more often seen as bunches of tubes joined at the base. The tips of the tubes are translucent and slightly rolled in. The color of this sponge is usually pinkish violet, although some specimens are pinkish blue. When this sponge releases sperm, it resembles smoking chimneys.

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Most of the 5000 or more sponge species are marine, although some 150 species life in fresh water. Marine sponges are abundant in all seas and at all depths, and a few even exist in brackish water. Although their embryos are free swimming, adults are always attached, usually to rocks, shells, corals, or other submerged objects.

This orange demosponge, known as the orange icing sponge (Mycale laevis), often grows beneath platelike colonies of stony corals. The large oscula of the sponge are seen at the edges of the plates. Unlike someother sponges, Mycale does not burrow into the coral skeleton and may actually protect the coral from invasion by more destructive species.

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The appearance of the Breadcrumb Sponge (Halichondria panicea) varies from thin sheets to thick crusts and large lumps. On wave-exposed shores, it usually grows under ledges as a thin, green crust, its osculae opening at the tops of small mounds. Its green color is produced by photosynthetic pigments in symbiotic algae in the sponge’s tissues. In deeper, shaded waters, the sponge is usually a creamy yellow. In waters with strong currents, this sponge may cover large rocky areas and kelp stems.

Barrel Sponges (Xestospongia testudinaria, found in tropical waters of the western Pacific) grow large enough to fit a person inside. Their hard surface is deeply ridged, but their rim is thin and delicate. The barrel sponge belongs to the Demospongiae, the largest class of sponges, containing about 95% of sponge species. The skeleton of sponges in this class is made from both scattered spicules of silica and organic collagen called spongin. An almost identical barrel sponge, Xestospongia muta, occurs in the caribbean.