Biodiversity and biogeography of intertidal Oribatida from Bermuda and the Caribbean

Bermuda is a small remote subtropical archipelago in the Western Atlantic Ocean, which was elevated above sea level approximately 900.000 years ago. A recent study discovered an unexpected high number of small mites living in the intertidal zone and revealed interesting biological aspects of these animals. These mites dwell in the area between low and high tide which means they are completely flooded during high tide. Special body surface structures allow these animals to retain a layer of air to their bodies during submersion and hence they are able to breathe underwater. A genetic investigation demonstrated that the found species are geologically older than the islands of Bermuda and therefore they could not have evolved there. So where is the real origin of these species? The closest coast is the North American East Coast (North Carolina) with a distance of 960 kilometres but there are no records of these mites from this area. In the more distant regions of Central America and the Caribbean Islands occurrences of close relatives have been reported and consequently Bermudian mites may derive from this area. However, another question is how these small flightless animals were able to travel such enormous distances across the open ocean. There are three possible dispersal mechanisms: (1) dispersal by wind, (2) transport in the plumage of a bird and (3) drifting along ocean currents. Presently, there is no clear evidence for any of these possibilities but it is supposed that drifting along ocean currents is the most likely mode of transport. The proposed project intends to answer these questions. The geological and the oceanographic history of the Caribbean are well documented and we know the approximate geological age of each island and we know the direction and speed of the Caribbean ocean currents. This knowledge represents an excellent framework to investigate when, how and in which direction these mites have colonised certain Caribbean islands and regions. A comprehensive anatomical and genetic investigation of species from selected areas will reveal the geographic origin of Bermudian intertidal mites and a comparison of detected dispersal patterns with ocean currents and migration routes of shore birds will reveal how these mites are able to travel long distances over the open sea. Tobias Pfingstl will be the project leader; he investigated the Bermudian intertidal mites in detail and is an expert for these animals. This project will be performed in cooperation with the Association of Marine Laboratories of the Caribbean and with experienced international researchers working on marine associated organisms, bird migration and dispersal mechanisms.

Littoral oribatid mites are tiny arthropods that inhabit coastal areas where they feed on algae. The biodiversity and biogeography of these animals in the Caribbean was investigated in this project. Prior to this project, there was only a record of a single species from the Lesser Antilles and there were two reports of undetermined species from Costa Rica and Saint Lucia. In the course of this project, we discovered a total of 12 species belonging to 8 genera and 2 families, whereas 2 genera and 5 species were completely new to science. The distribution of the species strechtes partly over the whole Caribbean area (Central America, Florida, Lesser and Greater Antilles) and hence suggests these mites to represent an important component of the littoral fauna. Based on morphological and molecular genetic analyses, the phenomenon of 'cryptic diversity' could be demonstrated in these intertidal organisms, i.e. morphological appearance does not show apparent divergences though there are distinct genetic differences between the populations from different islands. The reason for this similarity could be morphological stasis, which means that a strong selective constraint limits morphological changes. The littoral zone represents an extreme habitat, hence slight morphological changes could have negative effects. Therefore the mites show a constant anatomy, regardless of their relationship. These mites are small flightless arthropods which are not able to swim actively, in this sense it was unclear how they are transported from one island to another. Our investigations indicate that they are transported along ocean currents but such a dispersal event happens only in extremly rare cases. Based on genetic data, we could demonstrate that different species also show different dispersal abilities. For example, the ocean represents an insuperable barrier for the species Thalassozetes barbara but Carinozetes mangrovi, on the other hand, is able to cross this barrier in some cases. The reason for this difference may be caused by their habitats, Thalassozetes barbara only occurs in algae growing on rocks whereas Carinozetes mangrovi predominantly dwells in algae growing on mangrove roots. If strong surge breaks off a piece of this habitat, then the wooden mangrove root, in contrast to the stone, will be drifting for a long time on the water surface and hence will transport the mites for long distances. Accordingly, ocean currents are important pathways and our results indicate that dispersal of these mites basically occurs along these currents. In the Caribbean the Gulf stream represents an important role in this respect. This project generally shows that the diversity of these littoral mites is unexpected high and therefore it is assumable that other yet incompletely investigated coastal areas may harbour further numerous yet unknown species.