Lamprey Dominance Hierarchies

Cross reference:  Human Dominance Hierarchies  

Breeding Behaviour of Brook Lampreys Lampetra planeri: Experiments on Mate Choice
Only abstract available online.  I got the PDF through the library.   
Aggression was observed mainly at the beginning of the experiment between nest-building males and intruding males. In spite of widespread promiscuity there was a significant positive correlation in size between the partners, indicating an assortative mating. Males competed for matings and it is suggested that the most alert males are the ones most likely to mate with the females.

My comment
This is quite clearly an example of territoriality. 
The Territorial Imperative   
excerpted in:  Dominance Hierarchies in General    
    "Above all, there was territory. ..."