Ontogenetic variability in external morphology of spirlin <i>Alburnoides bipunctatus</i> from the River Rudava (Danube catchment)

The external morphometry of spirlin &lt;i&gt;Alburnoides bipunctatus&lt;/i&gt;, a threatened species in parts of its native range, was re-examined in specimens from the River Rudava, Slovakia, using geometrical shape analysis, and the relationships between morphometry and habitat use were evaluated. Spirlin less than 40 mm standard length (L&lt;sub&gt;S&lt;/sub&gt;)(early and middle juveniles) were found to differ significantly from those more than 51 mm (adults), whereas individuals between 41 and 50 mm L&lt;sub&gt;S&lt;/sub&gt;(late juveniles) represented an intermediate interval during which major changes in shape appeared to occur. Adults had proportionally smaller eyes, deeper body, and longer preanal part of the body than the early and ‘middle juveniles. These differences tended to coincide with the differences observed in microhabitat use of spirlin, namely a generally increasing preference for high velocity areas with increasing age of the fish. Thus, changes in external morphometry occurring during the ontogeny of spirlin might reflect an increasing affinity for more complex, lotic microhabitats as well as developments associated with sexual maturation