The Tongue Biter Project

The study of co-evolution between parasites and their hosts is a powerful approach for investigating a diverse range of fundamental evolutionary patterns and processes, including diversification, evolutionary constraints and trade-offs, convergence, and whether particular evolutionary changes are irreversible. One of the single most remarkable examples of host-parasite co-evolution is shown by cymothoid Isopods, which are obligate parasites of fish, including many commercially important species. Cymothoids are highly diverse with ~390 species across 43 genera and exhibit striking host utilisation strategies. Some species, for example, grotesquely supplant their host’s tongues; the only natural example of anatomical replacement by another organism. Due to their bizarre life-histories, their large size relative to their hosts, and their economic impacts in fisheries and aquaculture, cymothoids have been studied over many years. However, the majority of this work is related to traditional taxonomy, and most ecological and evolutionary studies have been based on small sample sizes and few species. As a result, almost nothing is known about the evolutionary relationships within the family or the processes that have shaped the remarkable diversity of extant speciess, and never have these parasites been studied in relation to their hosts phylogeny. Here, we will use a phylogenomic approach (using differences in thousands of DNA sequences)  to unravel this complex evolutionary history.




















The Tongue Biter Project has several aims. The first, is to get people interested and involved in systematic research; but this is a Citizen Science with a difference. We are asking people to actively collect specimens for our analyses!  Cymothoids are a truly global family, found in waters all over the world, except the arctic. Therefore, for any one person to collect a representaive sample of the family would take decades of travel, fishing and scouring fish markets, which would be fun, but highly inefficient. Broad sampling is important in phylogenetic reconstruction to ensure that the hypothesis produced is as accurate as possible. Therefore, despite the name of the project we are actually interested in all cymothoids, not just the 'tongue biters' (they are also found in the gills, burrowed under the skin and attached externally on the scales). Since there are no restrictions on capture, storage and the transport of these animals it is the perfect opportunity to get people involved in systematic research, find out more about these weird creatures, learn about taxonomy, evolution, parasitism, get some cool stuff, and pick up some identification skills! To find out how to get involved head over to 


Another approach for tackling the issue of broad sampling is to utilise museum specimens. Over many decades Isopod collectors and taxonomists, though few in number, have collected thousands of cymothoid specimens deposited in collections all over the world. However, many of these specimens are very old (causing DNA to degrade) or were inappropriately preserved for DNA extraction. This project will develop new protocols for successful DNA recovery from museum specimens that may also be applied to other marine crustaceans. 


Finally, as a basis for understanding such comparative questions it is essential to first develop, and test, sound phylogenetic hypotheses. This is the primary focus of my research which I will use to address these intriguing questions:


1. What are the subfamilial and generic relationships among Cymothoidae? Is the early hypothesis of three putative lineages corresponding to parasitic mode of attachment (i.e. buccal, gill, scale) supported and can we infer an ancestral state?


2. Have these different strategies evolved progressively and are they evolutionary 'dead-ends', such that transitions from the most specialised strategies are no longer possible?


3. Is there evidence of convergence in parasitic strategies?


4. Is there evidence of co-speciation between hosts and parasites?


5. Do divergence time estimates correspond to geological or host speciation events?


6. How host-specific are cymothoid lineages from a phylogenetic perspective and how is specificity partitioned across clades?