AUSTRALIAN BARNACLE
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OCCURRENCE OF THE INVASIVE AUSTRALIAN BARNACLE AUSTROMINIUS MODESTUS (CIRRIPEDIA, BALANOMORPHA, ELMINIIDAE) ON PLASTIC DEBRIS AT WALBERSWICK PETER BARRY This note describes the discovery of an invasive barnacle growing on plastic waste on a beach in Suffolk. The barnacle Austrominius modestus (Darwin, 1854) (formerly known as Elminius modestus) was first described by Charles Darwin in his monograph on the sub-class Cirripedia. Darwin was working on British Museum specimens originally collected in Australasia. For this species, he gave a type locality of ‘New South Wales; Van Diemen’s Land; New Zealand’ (Darwin, 1854). Within a century however, live specimens had started to appear in European harbours. This barnacle’s distribution has since made a progressive spread outwards from the first detection on the south coast of England to many other coastlines in Northwest Europe (Crisp, 1958; Allen et al., 2006;). The first detection of Austrominius modestus in the United Kingdom was from a pontoon in Chichester Harbour, West Sussex, England in September 1945 (Bishop, 1947). Bishop suggested that the species had arrived attached to the hulls of vessels transiting between the UK and Australia during the period 1940–1943, in support of the war effort. Records from the late 1940s and early 1950s charted the barnacle’s spread beyond Chichester Harbour, with first reports from the Netherlands in 1946 (Boschma, 1948), France in 1950 (Bishop, 1954) and Ireland in 1957 (Beard, 1957). In the Low Countries, initial reports began with observations of the barnacle attached to other shelled animals (mussels harvested in Ostend in 1950 and a crab in 1952), or attached to a fisheries vessel’s hull (Leloup & Lefevere, 1952) and even a piece of a shipwreck. Debris that washed ashore delivered the first specimens to Dutch naturalists in 1946, before further populations were confirmed growing on harbour walls in 1947 and then in Belgian harbours in 1952. From these small populations, the species has spread throughout the littoral zone of Northwest Europe and is currently expanding northwards throughout Scandinavia (Glenner et al., 2021). Worldwide, the barnacle has spread from Australasia to South Africa and Japan (Kerckhof, 2002). This barnacle is very distinctive and easy to identify: the most obvious feature is the four plates instead of the regular six found among our native barnacles. The body has a flattened, low profile with rounded ridges and an overall shape that Southward (2008: 86) described as a ‘sinuously octoradiate outline’. Instead of the regular hard basal plate, it has a membranous base so that when it is prised off a rock, it leaves behind no calcareous plate and the brown flesh inside is visible through a transparent membrane. The size is normally less than 10 mm in diameter and the white plates are sometimes tinged light grey. The squat profile and rounded ridges make this species very distinctive and easy to separate from native barnacles (Fig. 1) Austrominius modestus is primarily an intertidal species, and populations are usually found along sheltered coastlines and in estuaries. It is often found mixed in among our native barnacle Semibalanus balanoides and extends down into the shallow subtidal where it is found alongside another native species, including Balanus crenatus (Southward, 2008). Harms (1999) conducted experiments comparing this Trans. Suffolk Nat. Soc. 59 (2023)