An initiative of Earthwatch Institute

  1. Giant1_keith_davey Giant Rock Barnacle by Keith Davey
  2. Giant2_elvira_poloczanska 30-60mm (height) & 25-30mm (diameter) by Elvira Poloczanska

Giant Rock Barnacle

Austromegabalanus nigrescens


  •  Barnacles are small invertebrates that live inside hard circular or pyramid-like structures made from calcium-carbonate.
  •  Giant rock barnacles are the largest type of barnacle and are comprised of six large plates that are white to light green in colour, and have top to bottom (transverse) grooves on them. They have a distinctive bright blue body (mantle) inside.
  • They can be found singly or in groups
  • Size:  30-60mm height and 25-30mm diameter





What to Observe

·         Search area for 30 minutes and record under the following categories:

  • Abundant - found easily with little searching
  • Frequent - found with minimal searching
  • Rare - only 1 or 2 individuals found with intensive searching
  • Not found - not present during search





ClimateWatch Science Advisor

Giant rock barnacles are under increasing stress due to ocean acidification which can weaken their calcium carbonate shells and reduce body condition. This makes them more prone to disease, predation and low reproduction. Increasing water temperature as a result of climate change will likely affect their abundance and cause a southward shift in their distribution. The giant rock barnacle was absent from Tasmania in the 1950s, but now is found along the north-east coast. Scientists think that warming temperatures have allowed it to settle and thrive on Tasmanian shores.

When To Look

Throughout the year

Where To Look

  • Common on rocky shores, in the low tide level. Usually found attached to steep rocks in areas of high wave exposure.
  • Geographe Bay WA, SA, north and eastern TAS, NSW and Southern QLD. 

Giant Rock Barnacle distribution map - GBIF

Giant Rock Barnacle distribution map - GBIF



Davey, K. (1998). A Photographic Guide to Seashore Life of Australia. New Holland Publishers Australia Pty Ltd.

Edger, GJ. (2008). Australian Marine Life. The Plants and Animals of Temperate Waters [2nd Ed]. New Holland Publishers Australia Pty Ltd.


  1. Search Species

  1. What Else?

    Barnacles can resemble limpets as both are round, pyramid-shaped and cream in colour. Barnacles can be distinguished from limpets by their shells, which are always made up of multiple plates.  Limpets always have shells that consist of only one singular plate. Limpets are also free moving whereas barnacles are always fixed to a hard surface. Giant rock barnacles can be distinguished by their size, as they are taller than they are round and have similar shape to a volcano. They can be found singularly or in a group.

  1. Did You Know?

    Barnacles are crustaceans and are related to crabs and shrimps.

    Barnacle larva are free swimming and live in the plankton layer and when they are old enough they return to the rocky shores where they find a spot and cement their heads to the rock and then grow their shell around their body.

    Many barnacles are hermaphrodites (have both male and female sex organs) and can self-fertilize but tend to rely on a neighbour to fertilize their eggs.