As humans transport goods all over the planet we also unintentionally transport animals and plants to places that they do not belong. We call these animals and plants non-native or alien species. If conditions are right for the non-native species they can become established and outcompete our own native species for food and habitat. This is when they are called invasive species and could have a negative impact on our native species sharing the same habitat. This is bad news considering all the other pressures on our wildlife.
How do they travel such great distances?
One of the major transporters of marine non-native species are the large goods ships that travel from one side of the planet to the other, taking on ballast water in various ports and ejecting the water at their destination. Ballast water aids the huge ships to balance. At ports, as containers are removed from the ship, ballast water is taken on to keep the whole vessel evenly balanced. The problem is that the water in ports often contains tiny floating animals that are the offspring (or larvae) of mussels, crabs, clams and other invertebrates. These larvae get sucked into the ballast tanks and survive onboard until ejected at the destination port, which is sometimes on the other side of the planet. These animals would not normally have reached these far off destinations naturally.
Aquariums and aquaculture, or the farming of aquatic plants and animals, are another two major contributors towards the invasive non-native species spread. Shellfish farms import juveniles to grow and breed from but these can often escape captivity or have other species attached to them. The Manila clam (Tapes philippinarum) from the Indo-Pacific region was introduced for farming in the south of England in 1989, but has since escaped! Of all mollusc farming in the world, the Manila clam makes up an astounding 25% and this is because the species can grow quickly and reproduce in great numbers. It is also very hardy and has started to spread in the south of England and is breeding with one of our own native species. To learn more about Invasive Non-Native Species (INNS) in Wales check out the Wales Biodiversity Partnership INNS pages.
A third, less well-known method of transportation of non-native species is by rafting – or attaching to floating items. Numerous bivalves (eg. mussels, cockles, oysters) have crossed the Atlantic Ocean attached to bait buckets, buoys, crates and other sturdy plastic items. They wash ashore usually after particularly violent storms and are then stranded with the rest of the marine litter. We call these bivalves ‘rafting bivalves’. They attach to their ‘raft’ using byssus threads or cement, depending on the kind of bivalve. Byssus threads are produced by a special gland in the foot of the animal to allow the shell to anchor onto hard surfaces such as rocks. You may have seen this with mussels on our rocky shores. Oysters and other similar bivalves use a special cement to glue themselves onto hard surfaces and so they are also able to attach to the plastic rafts. I am especially interested in learning more about marine bivalve shells that attach to ocean plastics and then wash ashore on our beaches and have started to add them to our Marine Bivalve Shells of the British Isles website.
To find out more about Rafting Bivalves check out next week's blog.
