The “Pinocchio-shrimp effect”: first evidence of variation in rostrum length with the environment in Caridina H. Milne-Edwards, 1837 (Decapoda: Caridea: Atyidae)

<p>External morphology has always been the first criterion used to separate species of shrimps,</p>

<p>especially in the freshwater genus Caridina H. Milne-Edwards, 1837, but more doubts have been</p>

<p>expressed regarding the relevance of some of the morphological characters. We collected 27</p>

<p>specimens of Caridina from seven different localities during field work conducted on the island</p>

<p>of Pohnpei (Federated States of Micronesia). After genetic verification that they all belonged to</p>

<p>the same species, 19 morphological variables were measured and correlated with the elevation</p>

<p>of the collecting stations using correlation analyses. We provide evidence that the length of the</p>

<p>rostrum showed strong negative correlation with the elevation. This could be explained either</p>

<p>by the physical stress exerted on the rostrum by the stronger water currents in the stations at a</p>

<p>higher elevation, as a defence against predators in the lower stations, or a combination of both</p>

<p>possibilities. The taxonomy of these shrimps is thus challenging and should not rely only on</p>

<p>rostrum length, but on other characters such as the number of teeth on the dorsal margin of</p>

<p>the carapace, which is not correlated with rostrum length and therefore, with the environment.</p>