Official common name: Bisexual mussel, Dwarf mussel

The bisexual mussel, Semimytilus algosus, occurs in two types of different forms, where one has a thick yellow smooth shell commonly found on emergent rocks. The other type has a thinner uniformly brown shell and is mostly found on compact platforms (Caro and Castilla 2004).

Description/How to recognise a…

Commonly, the bisexual mussel is small, elongated, relatively fat and smooth with a brown-green shell, the maximum growth is 50mm. Most specimens are hermaphroditic, having a male gonad on one side and female one on the other. Mussels have small rope-like tentacles they use to attach to hard substrates or to each other in colonies (Caro and Castilla 2004; Picker and Griffiths 2011).

Getting around

Semimytilus algosus moves by secreting thread-like silky fibres that the animal uses to attach to substrates (Ayala et al., 2006).


According to a study by (Ayala et al., 2006), the attachment behavior they observed showed that S. algosus distinguish between substrates it wants to attach to through interpreting the biological, chemical and physical signals it receives from that particular substrate (Ayala et al. 2006).


In South Africa the geographic range of S. algosus is recorded on the West Coast from Groenriviersmond in the north of Bloubergstrand in the south. Its native distribution is in the Pacific Coast of South America, where it is recorded from the intertidal and subtidal rocky shores and as a fouling species in scallop aquaculture facilities (De Greef et al. 2013).


Semimytilus algosus occupies the lower intertidal part of the rocky shores along 500m of the western coastline (Bigatti et al. 2014).


S. algosus is a normally populous filter-feeding bivalve (Altamirano-Chovar et al. 2006).



S. algosus is reproductively active throughout the year. In its native range, the larval development is completed in about 27 days and late in its development it reaches a maximum size of 150 µm (Bigatti et al. 2014).

Family life

Semimytilus algosus tend to be numerically dominant and form dense mussel beds (Bigatti et al. 2014).


Friends and foes

Due to their dominant numbers, they easily exclude other species from the subtidal areas of the rocky shore (Bigatti et al. 2014).

Smart strategies

The size of S. algosus is typically small and as a result they easily create refuges for other small species that do not normally occur within mussel beds (Bigatti et al. 2014).

Poorer world without me

In its native range, S. algosus is collected and used for human consumption. Also, in Chile it is reported to be grown in aquaculture farms (Bigatti et al. 2014).

People & I

*Any observations of this species can be uploaded on to iSpot in order to feed into the current project to map the distribution of marine alien species in South Africa.

Conservation status and what the future holds

Semimytilus algosus is a non-indigenous species, listed as an invasive in The National Environmental Management: Biodiversity Act (NEMBA), Act No. 10 of 2004 under category 1b; which refers to species that must be controlled according to the NEMBA.

Scientific classification

Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Mytiloida
Family: Mytilidae
Genus: Semimytilus
Species: S. Algosus
Author: Gould, 1850

References and further reading

  • Altamirano-Chovar, C., Rudolph,  a, Sepúlveda, R.D., 2006. Differential sensitivity to human influence in juvenile Semimytilus algosus (Gould, 1850) (Mollusca: Mytilidae) from four coastal sites in south-central Chile. Bull. Environ. Contam. Toxicol. 77, 171–8. doi:10.1007/s00128-006-1047-2
  • Ayala, C., Clarke, M., Riquelme, C., 2006. Inhibition of byssal formation in Semimytilus algosus (Gould, 1850) by a film-forming bacterium isolated from biofouled substrata in northern Chile. Biofouling 22, 61–8. doi:10.1080/08927010500533122
  • Bigatti, G., Signorelli, J.H., Schwindt, E., 2014. Potential invasion of the Atlantic coast of South America by Semimytilus. Bioinvasions Rec. 3, 241–246.
  • Caro, A.U., Castilla, J.C., 2004. Predator-inducible defences and local intra- population variability of the intertidal mussel Semimytilus algosus in central Chile. Mar. Ecol. Prog. Ser. 276, 115–123.
  • De Greef, K., Griffiths, C.L., Zeeman, Z., 2013. Deja vu? A second mytilid mussel, Semimytilus algosus , invades South Africa’s west coast. African J. Mar. Sci. 35, 307–313. doi:10.2989/1814232X.2013.829789
  • Picker, M., Griffiths, C.L., 2011. Alien & invasive animals: A South African perspective. Struik Nature, Cape Town.

Author: Siyasanga Miza
SANBI, BAMO-TRR2 (Marine Programme)

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