South African Brown Mussel
Photos: A. Hosie, NIWA
This pest was detected on an oil rig in New Zealand waters in 2007. MAFBNZ actions have, however, reduced the likelihood of its successful establishment in New Zealand to negligible.
An oil rig was defouled (had fouling organisms removed from its below-water surfaces) in Tasman Bay in December 2007.
It was later determined that Perna perna had been present on the oil rig at the time of its defouling. MAFBNZ initiated a response to this detection and in early March 2008 dredged the drop zone beneath the rig defouling sites. Debris from the seabed was removed and disposed of in a landfill. Any suspected Perna perna were removed, and the risk of the species establishing in New Zealand was greatly reduced.
What you can do
Marine farmers in the top of the South Island (Golden and Tasman Bays, Marlborough/Nelson) can keep a close eye on mussel lines and spat gathering areas.
Report any mussels or spat of concern to: 0800 80 99 66
Perna perna is an edible bivalve mollusc belonging to the Mytilidae family. It is native to the tropical and sub-tropical regions of the Atlantic Ocean. It is found in waters off the west coast of Africa and the coast of South America up to the Caribbean.
It is generally a brown/brown-yellow colour and is sometime tinged with green around the margins. It grows to approximately 90 mm long, although can reach sizes of up to 120 mm.
Illustration of left valves from specimens of mussels showing Perna perna and other mussel species found in Tasman Bay
- A. Blue mussel Mytilus sp.from Tasman Bay
- B. Green lipped mussel Perna canaliculus from Tasman Bay
- C. Native green lipped mussel Perna canaliculus collected from Ocean Patriot rig in Taranaki
- D. Perna perna dredged up from defouling site in from Tasman Bay
- E. Perna perna collected from Port Elizabeth, South Africa
- F. Native bearded mussel Modiolus areolatus (without beard) collected from Tasman Bay.
Apart from the colouration, another distinguishing feature is that Perna perna appears to have a straighter and proportionately longer hinge line in comparison to the total length of the shell than the native green lipped mussel. (see illustration below showing hinge line)
- A. Green lipped mussel Perna canaliculus
- B/C. South African brown mussel Perna perna
The blue mussel (Mytilus sp.) may occasionally appear similar to Perna perna due to naturally occurring colour variation. One way to distinguish the genus Mytilus from the genus Perna is by looking at the muscle scar pattern inside the shell. Perna has a divided posterior adductor muscle, giving the appearance of two muscle scars whereas in Mytilus the scar is continuous (see illustralion below).
- A. Perna perna
- B. Mytilus sp
In its native range (South Africa) Perna perna is competitively inferior, being out competed by the European mussel (a Mytilus species), as it is not as resistant to human disturbance (such as harvesting activities) and certain parasites.
It is, however, an invasive species in the Gulf of Mexico and has the potential to be an invasive species in New Zealand. It naturally colonises rocky shores but can also attach to submerged man-made structures, and if it establishes in the Tasman Bay area, could pose a threat to the marine farming industry in the top of the South Island by competing with farmed green lipped mussels. The brown mussel is listed on a world database of invasive species, although it's not known how it would perform in the New Zealand environment.
Habitat and spread
At the site in Tasman Bay where the oil rig was defouled, extensive dredging has collected a very small number of Perna perna, which indicates that the numbers of Perna perna originally on the oil rig was low.
MAFBNZ does not know of any populations of the brown mussel in New Zealand. The only mussels are in the area that has been dredged. If no other populations exist, and the dredging operation removed most of the Perna perna present, then the chance of successful establishment in New Zealand is considered extremely low.
Perna perna prefers rocky shores and grows in the intertidal and shallow subtidal areas. It does not do well in sandy-bottomed areas. It can, however, survive on sand at depths where current movement is weak.
It is not known how Perna perna would perform in New Zealand. The adult brown mussel can tolerate a temperature range of 10 to 30°C and a salinity range of about 15 to 50 ppt. Perna perna does not develop from larvae into spat (baby mussels) at temperatures below 18°C. This indicates that populations in New Zealand may only establish in warmer areas such as the top of the South Island and warmer parts of the North Island.
The mussel uses external fertilisation during its spawning season over the summer months. The two sexes release eggs and sperm to the water during spawning to produce larvae. Fifteen hours after fertilisation the larvae have well-developed hinge teeth and 10-12 days after fertilisation the larvae settle on rocky surfaces. The larvae can travel up to 20km in ocean currents.
Keep boats and equipment clean
As well as the natural spread detailed above, this pest can spread easily on fouled hulls of vessels, aquaculture and other marine equipment, and in ballast water.
Its human-assisted spread can be prevented by:
- ensuring vessel hulls and marine equipment are free of fouling, and regularly treated with antifouling paint
- regularly cleaning hulls in a facility with collection and land-based disposal of fouling material
- minimising the movement of excessively fouled structures from one location to another.
Some useful resources
Further information on Perna perna can be found at:
- Dredging for pest mussels complete in Tasman Bay 20 May 2008
- Good progress on biosecurity mussel dredging in Tasman Bay 14 March 2008
- Ocean floor cleaned as part of Tasman Bay biosecurity operation 27 February 2008
Page last updated: 28 October 2008
Page last updated: 5 November 2008