Primary data used to calculate predator-induced morphological changes in two Daphnia species-associated clones

<p>The expression of inducible defences in reaction to an inconsistent predation pressure is especially well investigated in the freshwater keystone filter-feeder <i>Daphnia</i>. Out of their many inducible defences, which increase their fitness, the highest diversity of induced traits is found within their morphology. In recent time the focus of studies on these has switched, from the previously thoroughly covered large-scale defences, e.g., elongated/enlarged tail-spines, helmets and crests, to rather small-scale predator-induced changes. Inconspicuous spinules that cover the dorsal and ventral carapace margins have only rarely been featured in studies. We, therefore, tested two well-studied <i>Daphnia</i> species (<i>D. magna</i> and <i>D. longicephala</i>) for predator-induced changes concerning the spinules, i.e., their area of distribution relative to the body length (relative dorsal/ventral spinules-bearing area, relSBA) and their change in length (mean spinule length of the five central spinules along the dorsal/ventral carapace margin, mean SL). As invertebrate and vertebrate predators display differently built catching structures, those of invertebrate predators being more delicate, the defensive structures against these predator groups might vary accordingly. To test this, we exposed the daphnids to the vertebrate <i>Leucaspius delineatus</i> and a well co-studied predator of the respective <i>Daphnia</i> species, i.e., <i>Triops cancriformis</i> or <i>Notontecta maculata</i>. Our study discovered induced changes of the spinules' lengths and an area they cover. These changes of inconspicuous traits appear not only to be predator-dependent, but also <i>Daphnia</i>-specific. The inconspicuous morphological changes of <i>Daphnia</i>, in combination with the large scale defences, here measured pointers for the expression of inducible defences were the tail spines (relative tail spine length, relTSL) in both species and the crest in <i>D. longicephala</i>, and some ultrastructural defences<i> </i>are suspected to be rather specifically customised collective defences.</p>