Critter of the Week: Stegnaster inflatus

Stegnaster inflatus (Hutton, 1872) is a sea star endemic to New Zealand waters. It is widespread and found from the shoreline down to approximately 35 m depth.

It has highly variable colouring and can be red, orange, pink, yellow, olive green, etc. Its common name is the "ambush sea star", which alludes to its feeding habits.

Stegnaster inflatus (Hutton, 1872), the ambush star, a Wellington local sea star and New Zealand endemic can be seen in the aquaria at the Island Bay Marine Education Centre. [Edward Howard, Island Bay Marine Education Centre]

Research Subject: 

For a long time Stegnaster had been seen raised on its arm tips in the field, but it wasn’t until 1974 that its feeding behaviour was actually observed and described in a paper by Roger Grace from the University of Auckland

This picture of Stegnaster inflatus in ambush mode was captured by one of our keen volunteers, Edward Howard, in the aquarium at the Island Bay Marine Education Centre. [Edward Howard, Island Bay Marine Education Centre]

In his paper, Roger provides a sequence of drawings showing how Stegnaster seals its prey under its body. The ambush star closes its trap within a second sealing the edges of its disc and arms to the substrate. It then gradually reduces the raised space underneath itself by progressively lowering more of its disc. While the prey struggles it is directed toward the mouth in the centre of the disc by spines.

Sequence of Stegnaster inflatus trapping and eating an amphipod on the wall of an aquarium. The stippled areas of the seastar are where it is pressed firmly against the aquarium glass. Credit: Figures 2-8 from Grace, 1974.

Once trapped in the centre, the ambush star extrudes it stomach over the prey. If the prey is small it will be drawn inside the mouth along with the stomach, but if it is too big, digestion will start inside the stomach while it is still outside the body! If you are trying to imagine what an extruded stomach looks like, check out the imaged of the closely related cushion star below. 

A close up of the closely related common cushion star (Patiriella regularis) extruding its stomach against the wall of an aquarium. [Edward Howard, Island Bay Marine Education Centre]

Our friend Chris Mah has also written about the ambush seastar in his Echinoblog.