Seven-Armed Sea Star

With a diameter of up to 350mm in subtidal forms, the colouration of A. scabra varies from orange through dark red to brown, or light blue to grey. The dorsal surface is covered in speckled plates, from the center of which protrude one or two short, stubby white spines which are specialised ossicles. These spines are blunt on top, becoming sharper and sometimes longer on the sides of the arms. Spines can appear blue in juveniles.

The madreporite is not obvious. Tube feet are a combination of off-white and bright orange.

Intertidal individuals are generally smaller with thicker dermal plates, allowing them greater protection when being tossed about by waves. Despite this they are still found to be more damaged than subtidal individuals.

A common inhabitant of the rocky reef subtidal or the rocky shore intertidal, adult seven-armed seastars have been found down to 150m. A. scabra has unique roaming habits – there is never a permanent home. Therefore, migratory and resident A. scabra in a location all result from random movement. Juveniles are more adventurous than adults in the coastal intertidal - they show considerable movement patterns within the intertidal and shallow sublittoral zones, not venturing beyond 20m in depth.

For A. scabra, individual size relates to diet composition (larger predators eat larger prey). J. C. Town (1981) found a population feeding on species of 60 different genera.

Highest feeding frequencies are shown in individuals in the radius range of 10-29mm, with peaks of activity in the months of May and June. Those in the 50-89mm radius range have an eating frequency more constant than all other sizes. Not only is the frequency of foraging larger in small individuals, they also eat larger portion sizes.

Sexual maturity is achieved when there is a decrease in overall ossicle (endoskeleton) thickness with a simultaneous radial increase to at least 110mm. A. scabra can undergo both sexual and asexual (fissiparous) processes of reproduction. Environmental influences induce gametogenesis and gamete maturation, with water temperature a major factor.

Offspring disperse through the means of larval transport or epiplanktontic drift (drift occurring between the surface and 100m in depth). This free dispersal reduces local intraspecific competition as local currents carry gametes away from the spawn site. Spawning occurs throughout late August and early September, or when plankton richness is at a local maximum. Larvae are planktotropic.