I recently went to the beach and saw many small, clear jelly balls on the ground. I was told not to touch them, but I knew they weren’t dangerous because I recognized them. These were sea gooseberries, a small species of marine animal.
The sea gooseberry is zooplankton and belongs to a group of marine animals called comb jellies, or Ctenophores. They are also known as sea walnuts or Venus’s girdles. Unlike jellyfish, they are not dangerous and don’t contain any stinging cells.
The sea gooseberry lives in the cold waters of the Pacific and Atlantic. They resemble a ball of jelly the size of a gooseberry when fully grown.
Greek in origin, Ctenophores means comb bearers as they have eight rows of combs running down the jelly-like body. These combs are cilia, similar to fused tiny hairs and beats in rhythm together. The rhythms propel the sea gooseberry forward with its mouth in front. They can also reverse or change direction by reversing or changing the direction of the beats.
A sea gooseberries mouth is at the top and leads to a central digestive cavity which also houses the sex organs. They have two long tentacles which come out from each side of the body.
They can orientate in the water by using a balancing organ at the end opposite the mouth. The organ used for balance is made up of a solid calcareous ball. The ball is called a statolith and is found on four cilia. A change in the body’s position causes the statolith to push firmly against one or more of the cilia.
This increases the rate at which the cilia beat. The higher beat is then transmitted to the cilia on the adjacent comb rows, which join in the same beat. This allows the sea gooseberry to reposition itself in the water.
Ctenophores are pelagic, which means they spend their entire lives in the ocean floating around. However, they can also be found washed up on shore in their hundreds or even thousands. This usually happens if they have been caught in a storm or a current.
If you see a sea gooseberry in the sea, they are usually found with their mouth uppermost with the two tentacles trailing below. There are adherent cells on the long tentacles called colloblast. The tentacles can be up to twenty-five to fifty times the length of the body, although they may not be seen as the sea gooseberry can retract them into pouches on both sides of the body.
Predator and prey
Sea gooseberries use their tentacles to catch prey and feed. They spread them out to form a net to catch food. Sea gooseberries are not fruit but carnivores. They feed on zooplankton, such as fish eggs, larvae of crabs and barnacles, and small crustaceans. These are caught up in the adherent cells on the net from the tentacles before being drawn into the mouth.
Although see-through when above the water, sea gooseberries exhibit neon colours while in the water. This is not bioluminescence, however, but is caused by refraction. The cilia spread in different ways, giving off unique colours.
Sea gooseberries exhibit dial vertical migration, where they migrate up and down through the water twice during the day. When it starts to get dark at dusk, the sea gooseberry heads to the surface, and when it starts to get light, they migrate back down to the darkness of the depths. This allows them to avoid predators and follow the zooplankton through 1000 meter depth migrations.
Sea gooseberries can do a lot of damage to commercial fish such as cod or herring due to their voracious appetites. As with most animals, they also have their predators. The herring gets its revenge as it is one of the deadliest predators to the sea gooseberry. Another predator they face is another comb jelly called the Beroe. This marrow-shaped comb jelly will feed on the sea gooseberry.
Although they look similar to jellyfish, sea gooseberries are in a different phylum and are not related. Ctenophores evolved about 500 million years ago. Unlike jellyfish, they do not have a stinging cell but instead use a sticky cell.
Sea gooseberries are hermaphrodites that contain male and female organs. Reproduction usually takes place in summer and autumn. Eggs and sperm are released into the sea through their mouth, where fertilization occurs in the water. They do not have any parental protection after the eggs have been fertilized.