Giant Rock Barnacle Michael Marmach/Museums Victoria

Giant Rock Barnacle

Austromegabalanus nigrescens

Did You Know?

  • Barnacles are crustaceans and are related to crabs and shrimps
  • They are hermaphrodites (have both male and female sex organs) and can self-fertilize but tend to rely on a neighbour to fertilize their eggs
FactBox Image

Barnacles are small invertebrates that live inside hard circular or pyramid-like structures made from calcium-carbonate. They are distinguished by their size: they are taller than they are round and have similar shape to a volcano. They can be found singularly or in a group.

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.

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.

Size

30 - 60 mm height and 25 - 30 mm diameter.

 

Species: WhatToObserve Image

What to Observe

Search area for 30 minutes and record the following:

  • 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

Climate Adaptations

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 will likely affect their abundance and cause a southward shift in their distribution.

They were absent from Tasmania in the 1950s, but are now found along the north-east coast. Scientists think that warming temperatures have allowed it to settle and thrive on Tasmanian shores.

Species: WhenAndWhere Image

When and Where

When To Look

  • Throughout the year

Where To Look

  • Geographe Bay in Western Australia, South Australia, north and eastern Tasmania, NSW and Southern Queensland
  • Common on rocky shores, in the low tide level
  • Found attached to steep rocks in areas of high wave exposure
Species: WhatElse Image

What Else?

Similar Species

Limpets can resemble Barnacles as both are round, pyramid-shaped and cream in colour. Limpets can be distinguished by their shells which consist of only one singular plate and they are also free moving whereas barnacles are always fixed to a hard surface.