Bugula neritina - Brown bryozoan
The origin of this species has not yet been definitively determined. Currently, this bryozoan has a broad global distribution in temperate, subtropical, and tropical waters [3]. It is likely native to the Northwest Atlantic Ocean (North America) [4,5].
First observation in Belgium
The Brown bryozoan was first observed in Belgium in 1999, with numerous specimens found on the hull of a sailing yacht moored in Mercatorhaven in Ostend [6].
Distribution in Belgium
After the initial observation in Mercatorhaven in 1999, the species was next found at this location on test panels in August 2021 [3]. However, in 2010, the Brown bryozoan was already observed within the fouling community on the research vessel RV Belgica in the inner port (achterhaven) of Zeebrugge. In November 2019, a specimen was also seen on a floating pontoon in the marina of Zeebrugge, where an established population has since been recorded [3].
Distribution in neighbouring countries
The Brown bryozoan was first observed in Plymouth, United Kingdom, where it was presumably introduced between 1904 and 1912 [7]. Subsequently, large colonies were also observed in Millbay Docks [8]. In the mid-20th century, the organism was spotted in various locations in southern England and northern France (Brest and St-Malo) [9,10]. During the 1950s-1960s, it was abundant in a dock in Swansea, South Wales, heated by discharged cooling water from a power plant, where the water temperature was about 7 to 10 °C above ambient temperatures [11-13]. However, by the end of the 20th century, the species was no longer found in the wild in southern England [14]. In Swansea, the species likely disappeared after the reduction (1960) and final cessation (1976) of electricity production, when summer temperatures likely fell below 20°C, possibly preventing reproduction. However, details regarding the disappearance of this organism in British waters between circa 1970 and 1999 are lacking [15]. The Brown bryozoan was again commonly found in the marina of Perros-Guirec, Brittany, in 1998, a location that is not artificially heated [6]. The species was subsequently rediscovered in the United Kingdom in 2004 during a survey of marinas in southern England [16], and was also found in Guernsey [17].
The species is common in the ports of the Mediterranean Sea [10,18]. Additionally, reports have been made of its occurrence in Aveiro, Portugal [19], and in the Azores. The species was even found on the remote island of Tristan da Cunha in the South Atlantic Ocean [15]. In April 2007, several colonies were detected in Burghsluis, Netherlands (Eastern Scheldt), where they were already common by August of the same year [20]. A year later, they were also found in the Roompot marina near Wissekerke [21]. Since 2015, colonies have been found scattered throughout the Eastern Scheldt, as well as in the Grevelingenmeer and in certain locations along the Dutch North Sea coast (Katwijk-Noordwijk, IJmuiden), often attached to barnacles (Amphibalanus amphitrite) [22-25]. The origin of the colonies along the North Sea coast remains unclear [3].
The most likely vector responsible for the multiple introductions of the Brown bryozoan is shipping (attachment to hulls - fouling) [6].
The Brown bryozoan, like Bugula stolonifera (another type of bryozoan), is highly tolerant to pollution and varying salinity. Both species are frequently found together [6]. The connection to artificially heated waters is not definitive and is sometimes questioned, considering documented occurrences outside such zones [15,26].
The Brown bryozoan is typically found in the intertidal zones of harbor areas and bays, attached to natural substrates such as rocks and seaweed, as well as a variety of artificial substrates including wharves, ship hulls, buoys, and other submerged structures [27]. Additionally, an Australian study demonstrated that both larvae and adult organisms exhibit high resistance to dissolved copper, an active agent in various anti-fouling paints, thereby enhancing the species' potential to establish on treated ship hulls [28].
The larval stage lasts only a few hours, after which the organism attaches to a substrate. If no suitable substrate is available, certain larvae can swim for more than 24 hours; otherwise, half of the larvae attach within 2 hours [29]. The short duration of the larval swimming stage suggests that natural dispersal via prevailing currents is unlikely [30-32]. However, the species does exhibit a cosmopolitan distribution, indicating its high adaptability to diverse environments, as well as temperature and salinity variations ranging from polyhaline to euhaline waters (18-30 psu) [33]. Additionally, the Brown bryozoan possesses a survival technique known as quiescence, during which colonies withdraw [34]. This phase occurs at various times of the year and appears to be dependent on water temperature, with populations in cooler areas withdrawing in winter and those in warmer areas withdrawing in summer months [35].
The Brown bryozoan is a fouling organism, which means its presence can incur costs for its removal. It is well-known for fouling aquaculture structures, particularly attaching to oyster shells. Additionally, the species settles on algae, established bryozoan colonies, and other organic materials. Apart from potentially overgrowing other organisms, the species can also compete for space and food resources [33].
Furthermore, small colonies can attach to the walls of ballast tanks or floating materials within ballast tanks [36]. In some ports, the Brown bryozoan is considered one of the most problematic fouling organisms, indicating its ability to proliferate rapidly [6]. The primary factor influencing the spread of this species is the availability of suitable substrates for larval attachment [29].
Currently, the Brown bryozoan has a very localised distribution in our region, and there is no evidence yet of significant ecological impacts [37]. Measures against this species are not yet established.
The Brown bryozoan typically forms dark red-purple or purple-brown (sometimes dull dark red) upright bushy, branching plumose colonies up to 15 cm tall. The branches are formed from 2 series of zooids with a membranous frontal surface, where the zooids are large and proximally narrow [3,38]. There are no spines present, but the free upper corner of the outer zooids protrudes slightly [6,38]. The polypide has 20 to 24 tentacles. It is the only Bugula species lacking avicularia. A more detailed description can be found in the literature [3,38].
Scientific research has revealed that the vulnerable larvae of the Brown bryozoan possess a chemical defense against vertebrate and invertebrate predators, utilizing bioactive metabolites known as 'bryostatins' [39-41]. These larvae depend on a microbial symbiont (Candidatus Endobugula sertula) for the production of these chemical compounds [40,42,43]. However, this defense mechanism is no longer present in mature colonies [39-41].
Bryostatins are complex polyketides discovered for their anti-cancer activity in the 1970s [44]. Based on clinical trials, bryostatins have shown promise in treating various types of cancer [42,45].
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