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
to the native barnacles and found that it was able to survive environmental stressors better than the natives. It was able to tolerate a greater salinity range and wider environmental temperatures: higher temperatures than southern barnacles and lower temperatures than northern barnacles. It also has a particularly high tolerance to being covered in silt which can quickly build up in estuaries (Gomes Filho, 2007). They have a shorter breeding period of 14 weeks compared to the native barnacle s 12 months, and coupled with a longer breeding season, this invasive species appears to have a range of advantages that help it establish and spread throughout the habitats of our native barnacles (Harms, 1999). These attributes would explain the great success of A. modestus once it reached Europe - by 2002 it was recognised as the most common barnacle species in Belgium even though it had only arrived 50 years earlier (Kerckhof, 2002).
Another advantage is their ability to rapidly exploit freshly available substrates, including artificial materials (Bracewell et al., 2013). Barnacles in general are often the first animal to colonise artificial surfaces (Taormina et al., 2020) but A. modestus in particular appears very successful in colonising plastic. Since the early records of it on plastic waste in European estuaries (Harms, 1990) this barnacle has become a prevalent fouling species observed on both beached and floating plastic waste (Barnes & Milner, 2005; Garcia-Vazquez et al., 2018; Miralles et al., 2018; Rech et al., 2018). This prevalence has serious implications for trying to control their spread, given the abundance of plastic litter on our shores (Nelms et al., 2017) which provides additional settlement opportunities within native barnacles’ habitat. This latest example comes from the discovery of a dense population on a discarded piece of plastic debris on a beach in Suffolk.
On the 5th of July 2023, during low spring tides at Walberswick Beach, Suffolk (ca. 52.3139° N, 1.6728° E) a colony of Austrominius modestus was found on the handle of what appears to be a discarded ground stake (Fig. 1), consisting of a metal spike with a plastic handle. This item was tangled in a net that had washed up on the beach and was wrapped around the remains of wooden pilings. The colony included adults and newly settled juveniles on the plastic handle. Also present on the handle were the
bryozoans Conopeum reticulum (which favours sheltered bays and estuaries) and Electra pilosa, which are also quick to exploit artificial substrates (Ryland, 1967). These bryozoans had overgrown the barnacles, indicating that A modestus was the first to settle. An additional taxon on the handle were specimens of the keel worm Spirobranchus sp. which were too small to identify to species. Patches of the barnacle colony were concentrated on the inner surfaces or within corners, with any exposed surfaces almost free of fauna (Fig. 2). This pattern suggests the complex habitat provided by the angled, three-dimensional plastic provided sheltered conditions and favourable water flow that could have enhanced the growth of the colony (Wilson et al., 2021).
This record is another example of marine litter facilitating the establishment and growth of non-indigenous species (NIS) in the UK, a rapidly developing problem. Although a small colony on a single piece of plastic debris, this is a small-scale example of a wider pattern that is happening across the UK. Human activities are inadvertently providing new settlement opportunities for NIS, as we introduce artificial surfaces to the marine environment (Katsanevakis et al., 2007; Elliott & Birchenough, 2022). Unplanned negative results of our economic activities, such as Abandoned Lost and Discarded Fishing Gear (ALDFG) can provide novel platforms for settlement and the further spread of NIS. A subset of these NIS, such as Austrominius modestus, are classified as invasive species if they display traits of rapid spread and competition with native species (Gittenberger et al., 2023). In these cases, the major environmental stressors of pollution and invasive species form a combined threat to native biodiversity (Ricciardi et al., 2021). Effective solutions to prevent, or at least mitigate, the impacts of marine litter on our shores exist (see Madricardo et al., 2020), but greater public awareness of the dangers of discarded plastic waste needs to be encouraged.
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P. J. Barry CEFAS
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