The sea walnut is a species of ctenophore or better know as a comb jelly and they are terrorizing seas such as the Black sea, the Caspian Sea and the Baltic sea destroying populations of fish as they grow in numbers.
They are a species which came from the western Atlantic coastal waters such as North and South America, but due to travelling within ballast water, purposeful introduction and the natural movement of the ocean they have invaded new areas and are taking over.
These beautiful species are bioluminescent and when light hits their cilia it is refracted, and they seem to glow many different colours. They, unlike jellyfish, have no stinging cell within their tentacles. However, their tentacles do have colloblastos cells which means if they catch their prey it isn’t able to escape and is stuck to the tentacle.
They have become such a problem in areas because of the rate they are able to reproduce. The sea walnut is a hermaphrodite which means they can self-fertilise and produce offspring on their own. Within the Caspian Sea it has been recorded that 2-3,000 eggs and made every day and due to the eggs developing within 20 hours of being laid the species population is able to boom rapidly.
The devastating effect that the sea walnuts had on the Baltic sea was the first instance of their invasion and it occurred during the 1980’s. Travel within ballast water of merchant ships is thought to be the main contributor to their introduction. Their rein of terror in this space of water led to a dramatic decline in the population of anchovy, this is because the sea walnuts would eat their eggs and they also competed with the anchovy for other food sources. Due to the decline in food available the population of the species dropped again.
Within the Caspian Sea the reduction of sprat was the problem. Again, the Sea Walnut destroyed all the eggs, and this reduced the population of sprat perilously. Without the sprat to depend on sturgeon and larger mammals such as seals started reducing in population. The problem with this area of water was that the Sea Walnuts were introduced by the Unified Deep Water System of European Russia which means people were the reason for the loss of mammals and sturgeon.
The last area to be discussed is the Baltic seas, this introduction of the Sea Walnut was thought to be either due to natural movement of the jellies floating through the water or transportation in ballast water. Because it is such a large are of water however it is unknown what damage they have caused to the area.