From land to sea: evolutionary domain transitions in marine

Oribatid mites are primarily terrestrial species and the majority inhabits soil, litter, trees and canopy. The superfamily of Ameronothroidea, however, has managed to conquer coastal areas worldwide, whereas single families of this taxon are restricted to shores of specific climate zones. Although these animals dwell in this extreme environment and are exposed to daily tidal inundation, they are still air-breathing organisms and look at first sight like typical terrestrial mites. Why, how and when these mites have invaded the marine intertidal is unclear but based on their shared marine associated lifestyle, the families of this group were supposed to be derived from a common ancestor and hence unified under the superfamily of Ameronothroidea. Some authors believe that glaciation pushed these mites closer to the coast and others think that strong competition was the reason. However, in vertebrate taxa it was shown that mass extinction events or biotic turmoil basically may have triggered domain transitions and the same may apply to the mites. Based on morphological and biogeographic indications, the following hypotheses are suggested: (I) the land-to-sea transition took place independently in different climate zones and (II) polar intertidal mites are derived from a terrestrial ancestor that has subsequently invaded shorelines, whereas tropical taxa originate from an aquatic ancestor that has occupied brackish waters like estuaries and then marine waters. Aim of the present joint project is to reconstruct the evolution of these mites and to show how they colonized the intertidal area, when did this happen and what caused the shift in their lifestyle. Additionally, morphological and ecological adaptations and preadaptations necessary for the invasion of this habitat, as for example plastron structures allowing to breathe underwater, will be studied in detail. Austrian researchers will reconstruct the evolutionary history of the Ameronothroidea based on comprehensive morphological and molecular genetic data and Japanese scientists will perform extensive ecological and biogeographic investigations as well as laboratory experiments to reveal ancestral traits and preadaptations to the marine littoral. Tobias Pfingstl, a specialist for the biology and systematics of intertidal oribatid mites, will supervise the project in Austria while Satoshi Shimano, an expert for microbial ecology and applied soil zoology, will manage the studies in Japan. The combination of results of both approaches is supposed to clearly answer the above mentioned questions and to give a detailed insight into the evolution of marine associated oribatid mites.

In this Joint Project, Austrian and Japanese researchers investigated the biogeography and evolutionary history of mites living on the coasts of Japan. A total of five new species could be detected, whereas one species could even be discovered via the social media platform Twitter. Due to this unusual discovery, the 'twitter mite' was selected as one of the top-ten most interesting discoveries of new marine species in 2021. All these findings considerably contribute to biodiversity research and show how much species may still exist undetected in these modern times. Furthermore, the studied mite groups show a clear climate related distribution. One group, also called Ameronothridae, prefers coastlines of the colder northern Japanese regions where an annual mean of 17C air temperature is not exceeded. Two other groups on the other hand, the Fortuyniidae and Selenoribatidae, only occur on the southern and subtropical shorelines of the Japanese landmasses where the annual mean of 17C is clearly exceeded. Genetic investigations also revealed that recent mite populations living now on different islands without any connection, had been able to expand their distributions considerably and exchange genes between populations during the latest glacial maximum (approx. 22000 years ago) when the sea-level dropped significantly resulting in land-bridges between formerly separated islands. Changes in climate thus have a direct and observable influence on the distribution of these animals, therefore they can be used as indicators to observe and even predict the effects of the present global warming. In the course of this project, the claws of these animals were also studied and it could be shown that the shape of the claw is strongly correlated with the habitat the animals live in. These tiny mites are subject to daily tidal flooding and the strong surf of the ocean, and to prevent being washed away they have evolved specific claw shapes. Species living on rocky coasts show robust and strongly curved claws, while species living on softer substrates, like mangrove forests, possess weaker and less curved claws. This simple correlation between claw shape and environment could have implications for technical inventions, for example the construction of legged robots which are supposed to walk in hostile terrain. The diversity of these tiny mites living on the coasts is still largely unknown and there is danger that many species may go extinct before we even knew them. However, these organisms could be of great value for human beings as they could be used as indicators for climatic changes and some of their anatomical features may inspire bionic research.