Dwarf mussel


Common Name(s) Bisexual mussel, Dwarf mussel 

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

Description/How to recognize 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 500km of the west 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 150µm (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 large 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 as a result they easily create refugees 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.


If there is any interesting evolutionary information include it here (eg. Age of the group, fossil history)

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.

Siyasanga Miza

SANBI, BAMO-TRR2 (Marine Programme)

Official Common Name Bisexual mussel, Dwarf mussel

Scientific Name and Classification Semimytilus algosus

Kingdon- Animalia

Phylum - Mollusca

Class - Bivalvia

Order – Mytiloida

Family – Mytilidae

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    2 May 2018
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