Barnacle; Giant Rock - Busselton Jetty
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Barnacle; Giant Rock

 

 

Occurrence at the Busselton Jetty:  The giant rock barnacles dominate the intertidal region of the jetty piles.  When exposed at low tide, they seem dormant. Once the water rises they fascinatingly and consistently flick out their feeding appendages, cirri, to capture plankton to feed.  On numerous occasions, when the water is calm, the long penis has been observed extending out and inserting into another individual to exchange sperm.

Austromegabalanus nigrescens
Giant Rock Barnacle

The giant rock barnacle is the largest of the Australian barnacles growing up to 6 centimetres tall and are often found attached to rocks and reefs near the low tide level .  Their distinguishing bright blue mantle is surrounded by six rigid calcareous plates, and two protective shields, the scuta and terga, which seal the entrance to the shell.  The outer shell is often eroded and encrusted with marine algae and organisms.   Other barnacles may be observed growing on giant rock barnacles, these are not young giant rock barnacles but rather a different species of barnacle Tesseropora rapax.

Barnacle larvae are free swimming and resemble the larvae of other crustaceans.  As the barnacle larvae mature, they settle head first on a suitable surface, such as timbers at the top of the piles.  Their head ‘cements’ on to the hard surface, their jointed legs become modified feathery feeding tentacles known as “cirri”, and their external skeleton becomes a series of hard plates that surround the body.  Giant rock barnacles are filter feeders, and feed by flicking out the cirri through the plates, sifting plankton from the water column before drawing them back inside the shell to the mouth.  Water is also drawn in during this process to prevent desiccation during exposure at low tide. 

Barnacles are hermaphroditic, whereby they possess both male and female reproductive organs.  Barnacles have the longest penis of any animal in relation to its body size, the penis extends out through the top of the shell in search for a nearby mate to exchange sperm.  This is an evolutionary trait designed to maximise the abundance of the species.  Barnacles often form clusters, however if they are separated by some distance they do have the ability to self fertilise.

Giant rock barnacles occur on southern Australian shores from Geographe Bay, WA to northern NSW and eastern Tasmania.

References:

Briedahl, Harry. (1997) Australia’s Southern Shores.  Environment Australia, Victoria

Edgar, G. (1997) Australian Marine Life: The Plants and Animals of Temperate Waters, Australia

Morrison, S., Storrie, A.  (1999)  Wonders of Western Waters.  Department of Conservation and Land Management, Western Australia

Morrison, S and P., Storrie, A.  (2003)  Beneath Busselton Jetty.   Department of Conservation and Land Management, Western Australia