Does sediment composition sort kinorhynch communities? An ecomorphological approach through geometric morphometrics.

Ecomorphology studies the relationship between organisms' morphology and environment features. To better understand whether the shape of the body and the appendages involved in the movement is correlated to sediment composition in meiofaunal organisms, we study the evolved morphological adaptations to environment in selected taxa of the phylum Kinorhyncha: the allomalorhagid families Dracoderidae and Pycnophyidae, and the cyclorhagid genus Echinoderes. The selected taxa include the most diverse groups of Kinorhyncha worldwide, representing the 75.5% of the total phylum diversity. Widened, plump bodies and lateral terminal spines may be adaptive for species living in coarse, more heterogeneous sediments, as they could maintain a more powerful musculature to actively displace the sediment grains applying a greater force. Conversely, slender, vermiform bodies and lateral terminal spines would represent an adaptation of species inhabiting fine, more homogeneous sediments where there would not be much need to exert a high force to displace the sediment particles, and a more vermiform shape would even favour the burrowing of the animal through the smaller interstices. The studied kinorhynch taxa would also be adapted to the higher velocity of the sea-water and the intense erosion and transportation of heterogeneous sediments by possessing more robust bodies, avoiding getting laid off substratum under these conditions. These findings provide evolutionary evidence that body shape in the studied kinorhynch groups is adapted to environment.