Aliens invade the sea

Reading the title, you may be forgiven for thinking that this article is science fiction, perhaps about strange beings living in the depths of the oceans. Well, sorry to disappoint you but in this case the aliens are not from outer space, but instead they are marine plants and animals.

What makes them aliens is that they have been introduced into sea areas where they would not normally live. As examples, microscopic Japanese algae have recently been floating round the North Sea and giant, meter-long Pacific crabs are now roaming off the Norwegian coast.

And although these animals might not be as shocking as an alien invasion, as we will see in this article, they could be one of the biggest threats that the world’s oceans face in the 21st Century.

There are two main routes through which animals move to new sea areas; one is when they are moved for aquaculture or display purposes, the so-called intentional species introductions, and the other is when they are accidentally moved in, e.g. ships’ ballast water.

As a result many species have been and continue to be moved from one ocean to another: a good example being the Pacific Oyster Crassostrea gigas which is cultivated in many European coastal waters.

The movement of marine species for cultivation has generally brought positive benefits to the economies of many coastal communities worldwide, but unfortunately there is a downside. Some introduced species have established themselves in the wild and pushed local marine life out of their usual niches, while others have brought disease organisms and parasites with them that have proceeded to infect or parasitize local marine life.

As a result of these species movements, especially the unintentional introduction of pest and pathogens, species have, in some cases, compromised future production and ecosystem health because appropriate risk-reduction measures (e.g. quarantine) were not undertaken.

Tropical algae in the Mediterranean Sea
One infamous example of an introduced species that has had a huge effect on an ecosystem is the aquarium-bred strain of the tropical alga Caulerpa taxifolia in the Mediterranean Sea. This is thought to have entered the Mediterranean via Monaco Aquarium, where it was used as an attractive-looking algae in the tanks.

Caulerpa taxifolia

In 1984 a small patch of Caulerpa was spotted growing on the seabed outside the aquarium. The patch measured a mere square meter but by 1989 this “small” patch had expanded to cover up to 8,000 square meters, and by 1997 the alga covered approximately 44,5 million square meters of seafloor along the north coast of the Mediterranean.

While Caulerpa is not a threat to human health, it has a major impact on marine ecosystems as it displaces native vegetation including seagrass beds and drastically reduces marine biodiversity. Caulerpa infestations have negatively impacted tourism, commercial and recreational fishing, and recreational activities such as SCUBA diving.

Pacific crabs colonise the Norwegian coast
Another example of a controversial species introduction is the red king crab (Paralithodes camtschaticus) which can grow to a length of 1.6 metres. This crab, which is normally found in the North Pacific, was introduced into the Barents Sea by Russia in the period 1961-1969. The crab was very successful in its new home and created a new and valuable fishing resource in the region. It then started to spread out westwards and Norwegian scientists were surprised when it suddenly began to appear in large numbers in their waters, in the early 1990s.

Red king crabs can grow to a length of 1.6 metres

The crabs are still migrating westwards along the coast of northern Norway and local divers have reported that scallop-beds (Chlamys islandica) and flatfish populations along this coast are being reduced due to predation by the red king crab.

A long-term monitoring programme was launched in 2001 to assess the possible impact of these crabs which can live for up to 25 years. This is a particular concern over scallop beds as a single mature crab has been found to consume an impressive 400-700 g of scallops in 48 hrs!

Ballast water introductions

The introduction of harmful aquatic organisms to new environments via ships’ ballast water and other vectors, has been identified as one of the four greatest threats to the world’s oceans.

Ever since ships began crossing oceans they have been accompanied by plants and animals that have hitched a ride by attaching themselves to the hull. This form of transport was reduced with the development of effective anti-fouling paints, which make it more difficult for marine life to attach to boats. But since then, another form of ship-travel for marine life has arisen with the increasing use of seawater as ballast in ships.

Cargo vessels, when not fully laden with cargo, need to take on ballast. Originally ballast tended to be stones or sand, but since the advent of iron/steel ships there has been a shift to the use of water as ballast.

When ships load up with seawater ballast in one area, they also take in a sample of the local marine life. When the ballast water is discharged at the ship’s destination, the marine life that has survived the journey is abruptly dumped into a new ecosystem. Often the animals will quickly die in their new environment, but sometimes they are able to fit into a niche or out-compete the local marine organisms and this is when they can cause problems to local ecosystems.

North American jellyfish in the Black Sea
A striking example of what can happen as a result of ballast water transport is the North American comb jelly Mnemiopsis leydii, which was most probably introduced into the Black Sea and Sea of Azov with ship’s ballast water in the early 1980s.

Luckily for the jellyfish, it entered the Black Sea at a time when the combined effects of eutrophication and overfishing had removed the main plankton-eating fish, leaving an open niche for the jellyfish to exploit. The jellyfish fed voraciously on large amounts of zooplankton—including the young of plankton-eating fish such as anchovies, which were particularly hard hit. In fact, by 1994 the anchovy fishery had almost disappeared.

But then in 1997 another “alien” comb jellyfish (Beroe ovata) got into the Black Sea—probably from the Mediterranean—and this species tipped the balance by feeding on the previously introduced American comb jelly. By 1999, the American comb jelly Mnemiopsis leydii had decreased and the Black Sea ecosystem was showing signs of recovery.

Globetrotting Asian whelks
Sometimes it is difficult to definitively prove that a species has moved as a result of either travel with ships (in ballast water or attached to the hull) or through transport for aquaculture—or both. This is the case with the large Asian gastropod mollusc, Rapana venosa, which is currently found in the Black Sea and Mediterranean, Chesapeake Bay USA, the Brittany coast of France and Rio del Plata, Uruguay and Argentina: quite a distribution for a mollusc normally at home in Asian waters.

Rapana venosa can reach a size of 17cm

There are no major predators of adult Rapana venosa in the Black Sea and U.S. waters and the population has become very abundant and destructive to native marine life: it has been responsible for the decimation of native oyster, scallop, and mussel populations.

Although ballast water introductions are now the dominant form of introduction of alien species, the imminent ban on TBT in antifouling paints may increase dispersal of species on hulls as in pre-TBT years.

Recent surveys of non-native species in the North Sea and certain areas of Australia and USA suggested that historically, the number of non-native species likely to have been introduced by hull fouling of ships is greater than ballast water introductions.

However, it is important to note that in some cases it remains unclear whether the species could have arrived as adult individuals in ship's fouling or as larvae in the ballast water. Both avenues need to be addressed to minimize the risk of transfer.

ICES and alien species
ICES has a strong interest in biological invasions as reflected by the establishment of the Working Group on Introductions and Transfers of Marine Organisms (WGITMO) and the Study Group on Ballast and Other Ship Vectors (WGBOSV).

WGITMO deals with intentional introductions for e.g. aquaculture purposes, and through a risk assessment process and quarantine recommendations it works towards the reduction of unintentional introductions of invasive and deleterious species. SGBOSV focuses on unintentional species introductions with e.g. ballast water and hull fouling of ships (www.ices.dk).

Updates and new information regarding first records of non-indigenous species in 2000–2002 were brought to the attention of SGBOSV at its 2002 meeting and are listed in the meeting report. Species of special concern include the red king crab and Rapana venosa.

There is also the ICES Code of Practice on the Introductions and Transfers of Marine Organisms 2003. This gives recommended procedures and practices to reduce the risks of detrimental effects from the intentional introduction and transfer of marine (including brackish water) organisms.

The Code is aimed at a broad audience since it applies to both public (commercial and governmental) and private (including scientific) interests. In short, any persons engaged in activities that could lead to the intentional or accidental release of exotic species should be aware of the procedures covered by the Code of Practice. It can be downloaded at:

www.ices.dk//reports/general/2003/Codemarineintroductions2003.pdf

For further information on alien species in the sea please contact:

Dr. Stephan Gollasch (GoConsult)
Bahrenfelder Str. 73 a
22765 Hamburg, Germany
Tel +49 40 390 5460
Fax +49 40 360 309 4767
SGollasch@aol.com

ICES
H. C. Andersens -
Boulevard 44-46
DK-1553
Copenhagen V
Denmark
Tel: +45 3338 6700
Fax: +45 3393 4215
info@ices.dk