Molgula manhattensis - Sea grape
If the sea grape M. manhattensis got indeed introduced to Europe, it most likely came from the east coast of North America. However, it is suggested that the sea grape occurring in Europe is distinct from the one found in North America. Because of that, it can be considered another species indigenous to Europe (M. tubifera) [5]. The situation gets even more complicated because a lot of the sea squirts that occur along the southern coast of England [6] and the Dutch Wadden Sea [7] were first identified as M. manhattensis but are actually the indigenous species M. socialis [6, 7]. Recent genetic research (from 2011) confirmed the latter but could not give a decisive answer about whether or not M. manhattensis is indigenous to Europe [8].
First observation in Belgium
The first official Belgian observation and report of the sea grape M. manhattensis dates to the mid-19th century [9]. A massive aggregation of the species was reported – under the synonym Ascidia ampulloides – in Ostend.
A 1762 drawing of an illustration of a sea squirt that grows in the sluices and docks of Dijkwater on the island Schouwen-Duiveland (Zeeland) shows a resemblance to M. manhattensis [5]. If this is indeed M. manhattensis, this means that the species was already present in the Flemish ports of Nieuwpoort and Ostend back then [10]. However, this drawing could also depict M. socialis [11].
Spreading in Belgium
After the first observation by Van Beneden in 1847, the sea grape remained present in the port of Ostend during the following century. In 1934, the population collapsed by an abrupt decrease in salinity caused by the flow of fresh water through the Bruges-Ostend Canal. However, the situation got rapidly restored, and the species – identified as M. tubifera – was observed in large numbers between 1937-1939 [12] and 1952-1953 [13]. In 1960-1961, the sea grape – from then called M. manhattensis – was abundant in the marina of Ostend [14], and in the 1970s in the Sluice Dock of Ostend [15, 16]. In 1999, the sea grape got observed in the Inlet dock and marina of Zeebrugge, the port of Blankenberge and the Sluice Dock of Ostend. Recently, the sea grape got again reported in the marina of Zeebrugge and the Dutch Western Scheldt near Hoedekenskerke [17].
Recent genetic research confirmed that the species living in Ostend is M. manhattensis and not the related, similar-looking European species M. socialis [8].
Spreading in neighbouring countries
In the Netherlands, the earliest observations of M. manhattensis are doubtful. The introduction probably occurred very early. Therefore, it is studied based on literature and museum specimens [5]. The first piece of evidence of the presence of M. manhattensis is most likely the drawing of a sea squirt from 1762 (see above) [2, 5, 18, 19], although it might depict the species M. socialis [11]. Nevertheless, in 1956, the species occurred abundantly in the waters of Zeeland, on several locations in the Wadden Sea, the port of Ijmuiden and Den Helder (North Holland), and in the Zuiderzee (until the damming in 1932) [20, 21]. Towards the end of the 1970s, the sea grape still occurred in the aforementioned locations and was considered a regularly occurring species in the Dutch waters [2, 5]. Both the sea grape M. manhattensis [7] and the similar-looking sea squirt M. socialis [7] occur in the Wadden Sea.
M. manhattensis has been present in the Danish fjords since the 19th century [21], and more recently also on the German island Sylt, near Denmark [8].
The first observations of M. manhattensis in France date back to the 1940s and occurred in the north of Brittany [22]. To this day, the species’ distribution along the French Atlantic coast is limited to the Breton region [8]. Along the British coast, the species was reported as generally occurring in 1970 [23]. However, a lot of these observations may concern the species M. socialis; the European sea squirt that looks very similar to the non-indigenous species [6]. A recent study found no M. manhattensis near the British islands and the Iberian Peninsula.
Outside of Europe and its original habitat (northeast America), the species occurs in Australia since 1967, in Japan since 1972, and in northwest America since 1984. The sea grape now also occurs along the coasts of China [21].
The introduction to Europe happened quite early. The only way that M. manhattensis could have been introduced from North America two centuries ago was by attachment to ship hulls [5].
M. manhattensis attaches itself to all types of hard substrates, making ports – with their numerous artificial surfaces and ships hulls – an ideal environment for them to settle. However, the sea grape also grows on sandy soil, which poses a problem for most other species of tunicates since their inflow opening could get clogged. The sea grape has a large in- and outflow siphon, leaving it unaffected by the presence of sand [2].
The sea grape typically occurs in the sea but is also resistant to lower salinity levels of down to 11 PSU [2]. By comparison, the North Sea has a salinity of approximately 35 PSU. Temperature has little influence on its distribution [4]. The species appears to be tolerant of industrial water pollution [24].
Through attachment on structures in ports or ship hulls, the sea grape might cause damage to the surface on which it grows. As a result, biofouling might lead to economic loss. The prevention of fouling by cleaning or treatment with antifouling paint is expensive [25].
Because of its adaptability, M. manhattensis thrives in unfavourable conditions, like murky or polluted waters or water with a high content of organic matter [8]. This feature helps the species settle in port areas.
The larvae of the sea grape settle in places suitable for juvenile oysters, which causes competition for space [14]. It can also attach itself to shells of the American oyster (Crassostrea virginica), and during certain seasons it is the main biofouling organism attached to this oyster [8].
When transported by humans, M. manhattensis can carry undigested organisms. After all, the sea grape is a very inefficient feeder. The species filters a lot of water, but the food it collects often leaves its body undigested. Particularly harmful algae species prove to be poorly digestible and, consequently, pose a risk to new habitats. When conditions are favourable for the algae, harmful algal blooms might form. These blooms have major impacts on the ecosystem and can be harmful to people (e.g. through infection of cultivated organisms) [26].
Sea squirts are filter feeders. Water, carrying numerous small food particles, flows through the oral siphon. These particles get caught in mucus produced by a gland called the endostyle. Filtered water and waste products leave the sea squirt via the atrial siphon [2].
M. manhattensis has a striking shape. It looks like a grey-green sphere with a diameter of three centimetres and protruding in- and outflow siphons of seven millimetres. The oral siphon has six lobes; the atrial only has only four. The exterior of this sea squirt is full of thin hairs, covered in mud or sand. The sea grape does not form colonies, but individuals often occur together [2]. The specific U-shaped intestine is a feature that distinguishes M. manhattensis from other, similar-looking sea squirts. However, the shape of the intestine is very similar to that of M. socialis. The two species can be distinguished by the number of exits of the spermiduct. M. socialis has only one exit, while M. manhattensis has multiple ones [11].
Sea squirts are hermaphroditic [2]. Sperm gets released into the water column, and the eggs get fertilized within the sea squirt’s body. It can practice self-fertilization, which promotes the colonization of new areas [8]. About ten hours after fertilization, the larvae spawn, after which they get released into the water. They settle on hard substrate within one day, where they develop into adults [27].
The sea grape is a biofouling species on different types of solid substrates (boulders, rocks, seaweeds, wood, ship hulls, etc.), but also loosely attaches itself to sandy soils [23]. This species occurs from the low-water mark to a depth of about 90 metres [23].
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