Marcel Vermeulen


Phenotypic divergence across a flow regime in the Blackspot barb Dawkinsia filamentosa (Valenciennes 1844) (Teleostei: Cyprinidae) from Sri Lanka.

MSc Student
Advisor: Harald Ahnelt

Unit for Theoretical Biology, Department of Evolutionary Biology
University of Vienna


Habitat-related morphological divergence is a common phenomenon in fish. According to Langerhans’ (2008) steady-unsteady swimming performance model, individuals in lentic environments are more likely to develop a deeper body shape, whereas individuals in lotic habitats develop a stream-lined body shape. Although physical environmental factors such as water velocity are widely considered to be the strongest predictor of morphological variation, biological factors such as behaviour and food availability are as well important. For example, environmental variation within lentic and lotic habitats (different food availability) causes differences in head and mouth morphology.
Morphometrics is a traditional and quantitative method, which is used to compare the shape of organisms. By measuring from point to point, linear measurements of length, width and depth could be collected.
The Blackspot barb Dawkinsia filamentosa is a small freshwater fish inhabiting a variety of habitats including rivers and lakes. They can be found in the lowlands of Sri Lanka (distribution limit at 600 m above sea level) and on the Indian peninsula, being one of the most common cyprinid fishes. With the origin in southern India, the blackspot barb began colonising the island in the late Pleistocene across the Palk Isthmus.
Adult specimens of Dawkinsia filamentosa are characterized by a standard length of 80 – 120 mm, a complete lateral line containing 18 – 22 scales and an eponymous black blotch in the centre of the caudal peduncle. This blotch is an artefact of the juvenile colouration, which consists of three black bars. The mouth is subterminal and the barbels, distinctly developed in most congeners, are strongly reduced in length.
Within the genus Dawkinsia, two major species groups could be morphologically distinguished. The “filamentosa group” and the “assimilis group”. Dawkinsia filamentosa belongs to the “filamentosa group”, with males having dorsal fin rays filamentous elongated.  Sexual dimorphism in this species is restricted to fin characters including pectoral-fin length, dorsal-fin base length and caudal-peduncle length. With regard to these three sex-based morphometric characteristics, males have slightly longer measurements than females.