PURE

The Endangered Sea Snail

by Ernesto Díaz Cabrera
Estudiante Doctorado en Ciencias, mención Ecología y Biología Evolutiva de la Universidad de Chile
Doctoral candidate in Science of Ecology and Evolutionary Biology, University of Chile

Pure is a sea snail which has historically been used by the Easter Island community as a food source (in the past it was a regular part of the diet) as well as an item in decorative crafts, especially for necklaces and native dress. The gradual increase in tourism on the Island has heavily increased the pressure for extraction of Pure.

Pure (the scientific name is Monetaria caputdraconis which means “dragon’s head”) is endemic to Rapa Nui and the Salas y Gómez Island, which means that it’s not found on any other part of the planet. It belongs to the family of snails called Cypraeidae, a type that is particularly appreciated by shell collectors throughout the world for its variety of forms and bright colors. This family has more than 200 species and is widely found in shallow waters throughout the tropical Pacific and Indian Oceans from Hawaii to East Africa.

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Pure, Monetaria caputdraconis, in its natural environment. An outstanding anatomical characteristic of the family to which it belongs is the presence of a retractable covering which can completely cover the shell and which presents protrusions (called papillae) which help camouflage the animal as well as assist in breathing. In addition, the fact that the covering covers the shell makes it more brilliant (as can been seen below) and, therefore, becomes more desirable for collectors.
This sea snail lives mainly under the rocks in the inter-tidal reef zone, the area bounded by high and low tides and in the rough zone where the waves break on the edge of the reef. It feeds on mirco-algae and other marine organisms within a complex life cycle that passes through several stages where the individuals exhibit amazingly different characteristics of size, shape, behavior and habitat. The Pure incubates its eggs for 7 days, then deposits them on the rocky seafloor. A small larva will emerge (a snail’s larva is called a ‘veliger’), which will remain in the water, forming part of the plankton, until it settles again and transforms into an adult.

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Pure is found on both Rapa Nui and Salas y Gómez Island, which indicates that at some time there was transport of individuals from one island to the other or between them. This process of interchange between physically separated populations is known as “population connectivity”, and plays a major role in the distribution of these snails, in maintenance of genetic diversity and, eventually, in the survival of the species. For this type of organism, with the adult phase being fairly immobile, the planktonic larvae are ones with the ability to disperse over long distances, connecting separated populations.

The Ecology and Sustainable Management of Oceanic Islands (ESMOI) Institute is working on a study on the connectivity of Pure between the two islands. Data extracted from genetic analysis indicates that most of the individuals present on either island come from the same original population, which is called self-recruitment.

The first available results have shown that individuals on Rapa Nui are genetically identical to those on Salas y Gómez, which means that we can assume that they are a part of the same population. However, we have registered very low levels of connectivity and a high level of self-recruitment on each island. This would lead us to believe that migration of Pure between the islands is not so common, but that it is sufficient to keep them genetically connected. The movement seems to be slightly higher from Salas y Gómez to Rapa Nui (0.24%) than the other way around (0.18%).

Studies such as these have important implications for the conservation of biodiversity and allow us to optimize our efforts for a sustainable management of them and their conservation over time.

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