Brook Stickleback
The brook stickleback is is the only 
completely freshwater member of the stickleback family (Gasterosteidae) and it is found in cool, clear, heavily vegetated streams and rivers across North America. During the summer breeding season (May-August), males compete for territories and and construct nests from algae, grass, and twigs in aquatic plants. Courting males are easily recognizable by their beautiful velvety black nuptial colouration, and complex courtship repertoire. If a male successfully courts his intended, she will enter the nest and deposit a clutch of eggs before before leaving the territory. The male fertilizes the eggs and performs the entirely of parental care; he aerates and defends the nest from predators until the eggs hatch (about 8-9 days) and protects the young until they are able to swim out on their own.
Pretty cool videos of brook stickleback nest-building behaviour....
...and spawning behaviour.
Although currently classified as one widespread, highly variable species, recent phylogenetic research based on molecular characters has revealed a deep genetic divergence in the brook stickleback, suggesting that the species began to bifurcate approximately four million years ago. The two lineages were isolated in the Mississippi drainage (M group) and Atlantic drainage (A group), and have had minimal contact since that time, making this an excellent species in which to investigate the evolution of mate recognition systems and the extent of behavioural divergence in the absence of reinforcing secondary contact.
Phylogeographic analysis of the Brook Stickleback (Culaea inconstans) based on mitochondrial DNA, with the Mississippi lineage (green) and Atlantic lineage (blue) indicated. From Mattern (dissertation - Univ. Toronto 2005).