Palaeo-Evo-Devo of Malacostraca – quantifying crabs and their relatives

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Crabs and their closer relatives, together representing the group Meiura, are today present in diverse ecosystems with a large number of species. Besides different marine habitats, representatives of Meiura can also be found in freshwater and terrestrial habitats. The group Meiura is of considerable economic importance, and its representatives fulfil diverse ecological roles. A large part of the ecological diversity of the group Meiura is represented by the larval stages, which comprise a significant part of the plankton. Also in the fossil record, representatives of Meiura are known in large numbers, the earliest record being from the Mesozoic. Presumably, these crustaceans have already played an important role in the ecosystems at that time. Especially in the Cretaceous representatives of the group Meiura appear to have diversified significantly, based on numerous fossils of adult crabs and close relatives. Unfortunately, almost nothing is known about the immature stages including the larvae of the fossil representatives of Meiura, even though this knowledge would be important for the understanding of fossil ecosystems. In this project, it is planned to follow a new approach to tackle the problem of the lack of fossil larvae of the group Meiura. We assume that this lack does not necessarily mean that those fossils do not exist, but instead that they are often not recognised as such. The reason for this is that especially the later larval stages, the so-called megalopa larvae, can be determined as such based on characters of the trunk and the appendages. In fossil meiurans, however, usually only the shield and/or the chelae are found. In extant meiurans, ontogenetic changes of these structures are known, yet these are rather subtle and cannot easily be detected as qualitative and discrete characters. Hence, our approach in this project is to gather quantitative data of the shape of shield and chelae and to compare these. In this way, a number of conclusions can be drawn: 1) Distinct megalopa larvae and early juvenile stages can be identified as such. 2) Ontogenetic patterns can be compared between modern and fossil representatives (palaeo-evo-devo). 3) Changes in body size of distinct stages can be detected. To quantify the shape, the outline will be investigated with Elliptic Fourier Analysis, which has already been successfully applied for similar studies. With these data, changes in different evolutionary lineages within the group Meiura should be compared. Similar changes in different lineages at the same time would imply reactions in the same direction on the same changes of the environment. In this way, such data also contribute to a general understanding of a changing climate.