Ontogenetic variability in external morphology of round goby Neogobius melanostomus from Middle Danube, Slovakia By M. L’avrincˇ ı´kova´ 1 , V. Kova´cˇ 1 and S. Katina 2 Departments of 1 Ecology and 2 Probability and Mathematical Statistics, Comenius University, Bratislava, Slovakia Summary In the previous decade, four species of non-native goby have invaded the middle section of the River Danube. To under- stand the problems associated with these invasions better, we examined the external morphology of 184 round goby Neogobius melanostomus specimens using both triple regres- sion (distance-based measurements) and geometrical (coordi- nates-based measurements) analysis within an ontogenetical and epigenetical aspect. Using geometrical analysis of dis- tance-based characters, we found that the external body shape in four size groups of round goby remained essentially unchanged. Such direct development represents a strongly precocial (i.e. specialized) life-history, whereas earlier matur- ation in this non-native population suggests a shift back towards more altricial (i.e. less specialized, more generalized) life history. This combination of altricial–precocial trajectories seems to be typical for invasive round gobies and may be one of key factors for their successful colonization of new environments. Introduction The proliferation of organisms in aquatic systems outside their native range has been caused by both direct and indirect by human activities. Direct actions include the release of organ- isms by anglers and ornamental fish enthusiasts (Copp et al., 2005b), whereas indirect actions include ballast water trans- port (Sko´ra and Rzeznik, 2001), the construction of canals, dams or boat lock structures that permit passage of organisms between previously separated hydrosystems and that result in modified ecosystem function (Lodge, 1993; Riccardi and MacIsaac, 2000). Round goby Neogobius melanostomus (Pallas 1814) is a demersal fish species that has recently greatly extended its native range from the Black Sea, Caspian Sea and sur- rounding waters to include the Middle and Upper Danube, the River Moscow, and ultimately the Baltic Sea (Copp et al., 2005a) as far as the German coast (Corkum et al., 2004). Invasions have been facilitated by ship transport to novel environments as far away as the Great Lakes of North America (Jude et al., 1992), but possibly also within and among European river systems (Wiesner, 2005). In the River Danube, Ba˘na˘ rescu (1964) reported the upriver expansion of round goby since the 1960s, but it had been known earlier as far upstream as Vidin (Berg, 1949; Pinchuk et al., 2003). In 1997, the species was found for the first time in the Serbian part of the Danube (Simonovic´ et al., 1998). By 2000, it was present in the main Danube near Vienna, Austria. Since then, it has been observed in modest abundance, mainly in industrial harbours and to a lesser extent along the banks of the main channel (Wiesner, 2005). In 2003, round goby was detected as the fourth new gobiid Fig. 1. Schematic illustration of the distance-based measurements and landmarks taken from the lateral view of round goby from the Slovak stretch of the River Danube (see also Table 1) J. Appl. Ichthyol. 21 (2005), 328–334 Ó 2005 Blackwell Verlag, Berlin ISSN 0175–8659 Received: January 23, 2005 Accepted: June 15, 2005 U.S. Copyright Clearance Centre Code Statement: 0175–8659/2005/2104–0328$15.00/0 www.blackwell-synergy.com