The reef cave dwelling ultraconservative coralline demosponge Astrosclera willeyana Lister 1900 from the Indo-Pacific - Micromorphology, ultrastructure, biocalcification, isotope record, taxonomy, biogeography, phylogeny

Astrosclera willeyana LISTER 1900 is a pyriform-half spherical, predominantly bright orange colored, coralline demosponge with a mean size of about 20 mm in height and maximum head diameter. The habitat of Astrosclera is generally restricted to cryptic and light reduced environments of the Indo-Pacific, found mainly in reef caves, but sometimes also in the dim-light areas of cave entrances and overhangs, where it is green colored at the side towards the light. Caves of Indo-Pacific coral reefs were divided into four major facies zones, named 1 to 4 with decreasing light intensities. Astrosclera occurs in reef caves on a carbonate basement in Zone 2, 3, and 4, reaching maximum abundance in Zone 3 and the proximal part of Zone 4, but rare in the distal, very dark areas of Zone 4. Other abundant coralline sponges in reef caves are Spirastrella (Acanthochaetetes) wellsi and Vaceletia crypta. Astrosclera is the most common coralline sponge throughout the studied sites of the Indo-Pacific.