Small Life in the Sand of Rapa Nui
In just a handful of sand from the sea can be hundreds of thousands of small meiofauna animals.
This worm with many appendages is a nerillid, a member of polychaetes, which include meiofaunal as well as many larger forms.
Everyone knows the animals you can find on the beach or by snorkeling: mussels, snails, corals, sea stars, sea cucumbers and others. What few people know is that within the sediment, there are animals so small that they can hardly be seen with the bare eye. You need a microscope to see them. These animals live in the small spaces between sand grains. Typically, their size is around 100 micrometer, that is a tenth of a millimeter.
The community of animals between sand grains, generally called meiofauna, can be very rich, both in numbers and in diversity. Thousands of meiofauna species from almost all main animal groups (often called phyla) have been described from around the world. Some phyla are exclusively known from the meiofauna, others have only some representatives in the meiofauna. Often the meiofauna representatives look somewhat different from the larger relatives of the phylum, because they are adapted to the life between sand grains. To be able to move in the spaces between sand grains, a slender body form and a lack of long appendages are common features. There are, for example, snails in the meiofauna that, at first sight, look like a worm. They have no shell and are long and slender.
Flatworms sometimes have eyes. This one has a drop-shaped body form, other flatworms are longer and thinner.
Besides being rich in diversity, meiofauna animals can also be very abundant. Just a handful of sediment can have hundreds, even thousands, of meiofauna animals inside, but the numbers greatly differs much with the type of sediment. Meiofauna animals feed on bacteria, small algae, detritus (that are smallest pieces of organic substances) or other meiofaunal animals. Meiofaunal animals are found wherever you look into marine sediments, but there are many places in the world where nobody has looked before. One such place is Rapa Nui. Very few people have observed meiofauna and nobody has ever published anything about it. In November 2019, I undertook a small project to take a closer look at the Rapa Nui meiofauna. The project is a cooperation with ESMOI (Ecology and Sustainable Management of Oceanic Islands). To my delight, I found a wonderful diversity of exciting animals.
Roundworms or nematodes are also very abundant, although the figured roundworm, an epsolonematid with its characteristic body form, is not as often seen as other nematodes.
The sandy beaches of Anakena and Ovahe would seem to be the first place to look for meiofauna, but these sands are made up of small fragments of corals and shells. They contain very little organic substance on which meiofauna could feed and the sand is exposed to strong wave power. Therefore, the meiofauna is actually quite poor there. A rich meiofauna was found in several samples of smaller accumulations of sediment between rocks. Many different species were found in the sand from the public swimming pool right in front of the Tongariki Cultural Center. These species include nematodes (roundworms), flatworms and small crustaceans called harpacticoids, which are present in the meiofauna all over the world. But the sediments also include beautiful worms, for which there are no common names, such as polychaetes and gastrotrichs. In some sediments, marine mites and small marine isopods (crustaceans related to woodlice) were found. The full determination to which species the animals belong and whether they are new species or species known from other parts of the world requires a careful comparison with existing species descriptions and in some cases the collaboration with other experts. For these reasons, the animals were documented with a camera attached to a microscope, so that all their characteristics can be checked later.
Los ostracods son animales asombrosos. Son crustáceos y tienen una cáscara como un mejillón. Ostracods fueron encontrados a menudo en los sedimentos de Rapa Nui.
The presence of a rich and diverse meiofauna on Rapa Nui opens up the question as to how the animals got here. Many larger marine animals, such as large worms, echinoderms, snails and mussels have small larvae that live in the plankton of the open water for a while before they settle to the ground. In this way, they can be transported by currents over long distances. In contrast, the small meiofauna animals do not distribute larvae among the plankton, but their younger stages remain in the sediment. This means that they cannot spread over long distances. Nevertheless, they reached Rapa Nui after the island was born as a marine volcano. They could have come either on or in ships, or some of them could have reached the island on floating seaweeds, wood, volcanic pumice or, nowadays, on floating plastic litter. It is an interesting research question to find out how meiofauna animals came to Rapa Nui.
Harpacticoids are small crustaceans that are very abundant and can be found in almost every sample of sediment.
The head of a tiny snail without a shell, found near Ovahe beach. The two round structures are statocysts (gravity sensors) and the teeth right of it are typical for snails and most other molluscs.