Description: A distinctive barnacle with an outer shell made up of 6 spiny plates on a cup-shaped base. It is relatively small, growing to around 8 mm, and does not attach to a hard substratum, instead it is embedded in the goosebump sponge Dysidea fragilis. The opercular aperture is large and poorly-defined, with a white tergo-scutal flap.

Habitat: Embedded in Dysidea fragilis, often in crevices from the low intertidal to depths of around 50 m.

Distribution: This species has a southern distribution in Britain and Ireland. The current northern limit is south Wales.

Similar Species: Unlikely to be confused with any other species.

Key Identification Features:

• Barnacle that lives embedded in Dysidea fragilis.
• Outer shell made up of 6 spiny plates with a poorly defined opening opercular aperture.

Distribution Map from NBN: Acasta spongites at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Acasta spongites at iNaturalist World Species Observations database.

WoRMS: Acasta spongites at World Register of Marine Species.