Ficopomatus enigmaticus - Australian tubeworm

The Australian tubeworm F. enigmaticus occurs in waters with varying salinities ^[2], ranging from fresh to salt waters with a salinity of more than 35 PSU ^[3] and possibly even 55 PSU ^{[4, 5]}.

Its original range is unknown. It is believed to be native to the southern hemisphere ^[2]. However, it is considered as introduced in Australia ^[2], along the east coast of South America ^[6], South Africa and the East Asian coast ^[7]. The individuals found along the coast of Indonesia and India – which used to be considered its native range – appear to belong to a different species ^[8].

First observation in Belgium

The Australian tubeworm was first observed in August 1950 in the port of Ostend. The worms were found on the hull of the ‘Hinders’, the former research vessel of the Fisheries Service ^[8].

Spreading in Belgium

The species has been reported on hard substrates in the ports of Ostend and Nieuwpoort ^[9]. Since 2015, it is also found in the Sea Scheldt ^[10]. These findings are often solitary animals or small and fragile reefs with fewer than 1,000 individuals ^{[11, 12]}. Since 2021, the Australian tubeworm has also been occasionally observed in the Zeebrugge marina ^[13].

Spreading in neighbouring countries

The first European sightings (1921) of the Australian tubeworm came from Caen, in the (French) English Channel area. At the time, the worm was already quite common at less than 30 metres depth. It was found attached to plant stems, wood, stones and shells ^[14]. One year later, individuals were observed in the port of London ^[2]. In brackish coastal areas in southern England, the Australian tubeworm still forms compact reefs with more than 1,000 individuals per m² ^{[2, 12]}. The species is also permanently established in Ireland ^{[2, 12]}.

The first Dutch individuals were found in 1968 in Lake Veerse and the Canal through Walcheren (Zeeland) ^[15-18]. Although the species used to be common in Lake Veerse, it disappeared when salt water entered the lake. Nevertheless, the species may still occur near places where fresh water flows into the lake. The worm is found in the rest of Zeeland’s inland waters, in harbours with fresh water inflow and, since 1991, also in the North Sea Canal ^[16].

In France and Spain, the Australian tubeworm is found in brackish estuaries along the Atlantic coast and the Mediterranean area ^{[8, 16, 19]}. There, the worm created gigantic reef structures up to four metres in diameter and two metres in height very quickly after its introduction ^[20]. In Germany, the species was first reported in 1975 in Emden, close to the Dutch border. The age of these worms was estimated at two to three years, which suggests that their introduction to Germany may have been earlier than 1973 ^[21].

Great Britain is generally considered its northernmost boundary to which the species can reproduce and maintain a population. However, some transient populations have been observed in Danish waters, in addition to some established populations in industrially heated waters ^{[22, 23]}

The Australian tubeworm is thought to have spread across the world’s seas by attaching itself to the hulls of cargo ships or via the transport of its larvae in ballast water ^[7]. This assumption is supported by the fact that the species is always in the vicinity of shipping routes ^[24]. However, it is also possible that specimens got transported via commercial mollusc shells ^[2].

This worm is not as dominant in Belgium as in warmer regions. The reefs built in temperate regions (reefs with a diameter of up to 20 cm) are only a fraction of those found in warmer regions ^[7] (such as the Mediterranean, see above ^[20]). This suggests that the species achieves optimal growth conditions at higher temperatures.

One of the reasons why the Australian tubeworm thrives in our regions is its high salinity tolerance, which is ideal for the colonisation of estuaries. Additionally, estuaries offer an abundant food supply that allows this bristle worm to grow more quickly and reproduce fast. The absence of competitive species and predators also contribute to its success ^[2].

The species is sensitive to low temperatures and finds it difficult to thrive in northern Europe, except for areas with artificially elevated water temperatures ^[2]. For example, this animal will not continue to expand its tube at a water temperature below 7°C ^[25]. A temperature above 18°C is required for optimal growth and reproduction ^{[26, 27]}.

The Australian tubeworm can survive in fresh and salt water but prefers brackish water ^[6]. The species reproduces best in water with a salinity between 10 and 30 psu ^{[26, 27]}. Finally, the amount of food in the water column, the flow velocity and the water depth influence the presence of this polychaete ^[6]. The species is mainly found in shallow to medium-deep waters and prefers a low dynamic environment ^[26].

From an economic point of view, many consider the Australian tubeworm to be a pest species due to the reefs it builds on ship hulls and port infrastructure. These reefs can be removed by scraping them off, but this is labour intensive ^[2]. Antifouling paints are being used to prevent or limit fouling. However, such paints are expensive ^[28] and can damage the ecosystem ^[29]. Because this worm needs a hard substrate to develop, it is not recommended to dump hard objects in places where the species is found ^[30].

Indigenous species benefit from the large-scale presence of this worm in (semi-)closed waters – such as harbours – because, as a filter feeder, it improves the water quality. The Australian tubeworm removes suspended particles from the water column and provides more oxygen and fewer nutrients, which particularly benefits benthic organisms ^[2]. However, when the filter-feeding tubeworm is present in high numbers, it can reduce the amount of plankton present, leaving less food for indigenous species ^[2]. Furthermore, extremely high numbers influence local water currents (hydrodynamics) and sedimentation properties ^[6].

Many tube-building worms exert a strong influence on their environment. Similar to the native honeycomb worm reefs, a very typical fauna can be found in and around the tubes of the Australian tubeworm. Their tubes form a distinct habitat, which is why this species is an ecosystem engineer ^[31].

Examples of animals that live in association with the tubes of the Australian tubeworm are the amphipods (Amphipoda) Leptocheirus pilosus, Monocorophium insidiosum and Melita palmata, and the ragworm Hediste diversicolor, a bristle worm (Polychaeta) ^[32]. Several native species can benefit from the habitat structures formed by a non-native ecosystem engineer. The fact that the effect of introducing an exotic species is never unequivocally positive or negative got demonstrated in Argentina. There, it was found that Australian tubeworm reefs provide an important hiding place for an indigenous crab, which in turn catches other indigenous species ^[33].

Ficopomatus enigmaticus is a bristle worm that lives in hard limestone tubes. It is up to 2 mm in diameter and 40 mm long. The tube has a trumpet-shaped ending where the mouth is located ^[16]. The worm forms bulky, reef-like aggregations that can be up to 7 metres in size. In the early stages, the tubes are white, but later, they turn brown. The worm itself grows to a length of 20 to 25 mm. They have a ‘crown’ of 12 to 20 so-called radioles that are used for filtering ^[34].

The Australian tubeworm belongs to the sessile, tube-building worms (Serpulidae). Typical of this group is the lid or operculum, used to close the tube when the worm retreats ^[35]. The operculum’s shape is used to identify the different species. In the case of the Australian tubeworm, it is rather clubbed or mushroom-shaped and has black spines.

There are two different types of tube-building worms. Some species use mucus to glue sand grains and small shell fragments together to form the tube. Examples of this type are the common sand mason worm Lanice conchilega or the less common Ross worm Sabellaria spinulosa. On the other hand, there are species that, like the Australian tubeworm, secrete calcium to form the tube ^[35]. Another example of this second type is the indigenous Christmas tree worm Pomatoceros (Spirobranchus) triqueter.

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