Fish as parasites: an insight into evolutionary convergence in adaptations for parasitism

Fish are the most diverse group of living vertebrates on the planet with 32 000 living species. They have diversified to fill a wide variety of ecological niches. Some species have formed close ecological interactions with other aquatic species that can be best described as symbiotic or even parasitic. Some fish species have evolved different ways to exploit invertebrates, ranging from using their body as a site for depositing their eggs and larvae to actually sheltering inside the invertebrate themselves and feeding on the organs of their host. Other fish species are frequently associated with larger aquatic vertebrates, attaching to them for either phoretic or feeding purposes or both. The aim of this review is to provide an overview of some general patterns in these symbiotic or parasitic relationships, comparing them with more 'traditional' parasites and symbionts, and discuss the insight they can offer on both the evolutionary process that leads to parasitism, as well as the evolutionary pathways of fishes as a whole.