Semimytilus patagonicus

Semimytilus patagonicus is a small mussel of the family Mytilidae, originating along South America's Pacific seaboard and living on wave-exposed rock ^[1]. For years it was known as Semimytilus algosus, a name that, according to a 2021 study, yields to the one holding priority ^[2],[3]. It is hermaphroditic and breeds throughout the year in its native range ^[4],[1]. Outside that range it has become invasive, for example on the west coast of South Africa ^[1].

The shell is elongate and typically mussel-shaped, with a nacreous interior, a hinge without teeth and a compact ligament ridge; outside it is smooth, with irregular concentric rings from the umbo to the posterior edge ^[1].

In central Chile two morphotypes are described: one with a thick, smooth shell on compact rocky platforms, and one with a thinner, ringed shell on emergent rocks ^[5]. In 20 large specimens from a fishing vessel that reached Argentina from Callao, shell length was 19.5 to 49.5 mm, averaging 38.8 mm ^[1]. In 120 specimens collected near Lima the mean was 32.3 mm ^[6].

The native range runs from Manta, Ecuador, to the Gulf of Arauco, Chile, at about 37 degrees south ^[1]. It is recorded on intertidal and subtidal rocky shores, and as a fouling organism on scallop culture structures ^[1].

It is a common organism on wave-exposed rocky shores of the South American Pacific ^[1]. On the west coast of South Africa it has colonised the lower intertidal along about 500 km ^[1]. The type material against which the species was compared comes from Callao, Lima ^[1].

Hermaphrodites predominated among the 20 mature mussels from that vessel: 14 had hermaphroditic gonads, 1 had only male and 5 only female gonads ^[1]. A genetic study showed that, although hermaphroditic, the species probably has doubly uniparental inheritance of mitochondrial DNA, with two mitochondrial genomes that differ by 34.7 % ^[4].

In its native range reproduction takes place all year, with recruits present in every month and a summer peak ^[1]. Larval development is completed in about 27 days and the pediveliger metamorphoses at a maximum size of 150 micrometres ^[1].

In the laboratory, with 72 individuals kept for 21 days at 17, 20, 23 and 26 degrees, mortality was 0 % at 17 degrees in all treatments, and the filtration rate fell as microplastic concentration rose ^[7].

In central Chile, the mussel's predators include the crab Acanthocyclus gayi and two snails, Nucella crassilabrum and Concholepas concholepas ^[5]. Experiments with waterborne cues showed that the presence of predators, above all the crab, increases shell thickness ^[5].

When it forms dense beds the species excludes others from exposed subtidal zones, and in South Africa it occupies the lower intertidal along about 500 km, where non-native species dominate some rocky shores ^[1]. A fishing vessel that had been anchored for one month in Callao in 2012 carried reproductively active specimens to Puerto Madryn, Argentina, in its sea chests ^[1].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

In a bioassay with 120 mussels from Miraflores, near Lima, an anionic detergent whose active ingredient was sodium dodecylbenzene sulfonate gave a median lethal concentration of 34.95 mg per litre at 72 hours ^[6]. The study therefore classed the risk to the mussel as harmful, and proposed it as a tool for assessing the acute toxicity of detergents in the sea ^[6].

Exposure to plastic particles smaller than 125 micrometres at 0.125 and 0.250 mg per litre lowered its filtration rate, and mortality reached 67 % in one treatment ^[7].

In Peru the species is gathered from the wild for human consumption, and in Chile it has been raised in limited quantities at some aquaculture sites ^[1]. It is also a fouling species in scallop aquaculture facilities ^[1]. Its economic effect in South Africa, where it is invasive, had not yet been evaluated ^[1].

The studies cited here that used Peruvian material come from the Lima coast: mussels were collected at Miraflores ^[6], and the type material it was compared with comes from Callao, a harbour sheltered by a breakwater with sandy and rocky shores ^[1]. Look for it on wave-exposed intertidal rock ^[1].