Habitat and distribution of the red decorator crab, Schizophrys aspera, a cryptic crown-of-thorns seastar predator

Cryptic species and their interactions are challenging to describe owing to the difficulties in observing and sampling their populations. Such methodological hurdles are critical to resolve, especially when important interactions involving poorly described species are detected. The red decorator crab, Schizophrys aspera, is a newfound predator of the corallivorous Pacific crown-of-thorns seastar (CoTS; Acanthaster sp.). We discuss the Indo-Pacific distribution and taxonomy of S. aspera and provide characterization of its cryptic population at sites around Heron Island, Great Barrier Reef, that differ in CoTS densities. Most S. aspera (>95%) were found under coral rubble pieces atop existing rubble, associated with large pieces and chasmic interstices. The three smallest individuals (carapace width: 5-11 mm) were found under rubble overlying sand. Mean density of S. aspera was 0.8 +/- 0.2 ind. 100 m(-2), which varied among sites. Areas with lower records of CoTS had higher densities of S. aspera (p = 0.002; R-2 = 0.25), which justifies evaluation of the distribution and impact of this species in context of CoTS outbreaks. We present a method to survey cryptic decapods on coral reefs, along with microhabitat characteristics to help predict S. aspera on reefs prone to CoTS outbreaks and its capacity to act as a natural top-down control mechanism.