Tuatara tuesday – an iconic parasite for an iconic species

By Hilary Miller 26/10/2010

As you might expect from an animal that is so evolutionarily distant from its nearest relatives, the tuatara also has some unique parasites to call its own.  One of these is the tick Amblyomma sphenodonti (sometimes also called Aponomma sphenodonti), pictured here.

Tuatara ticks Amblyomma sphenodonti

Like many ticks, A. sphenodonti are host-specific, spending all three of their life stages feeding on tuatara but dropping off into the soil in between stages.

So why should you care about tuatara ticks?  Well, these ticks are evolutionarily distinct in their own right, and are actually quite rare – far rarer than the tuatara themselves.

The taxonomic history of the tuatara tick is a little complicated, so bear with me for a minute.  The tuatara tick is “hard” tick in the family Ixodidae, and was originally named in the genus Aponomma, a group of ticks that predominately parasitise reptiles.  However, a revision of the Aponomma genus placed some of the these species into a new genus Bothriocroton, and moved the rest, including the tuatara tick, into the existing genus Amblyomma. A few years ago, with the help of a keen undergraduate student, I did a small genetics study comparing the tuatara tick with both Bothriocroton and Amblyomma ticks and found that it’s actually not particularly closely related to either group (see the tree below – click on it to enlarge).   The tuatara tick should probably actually be in its own genus, highlighting the fact that it has likely had a long evolutionary relationship with its evolutionarily distinctive host.

Phylogeny of hard ticks, based on 18S rRNA sequences. A. sphenodonti is shown in bold. Other members of the Amblyomma genus group in the top part of the tree, while Bothriocroton ticks form a group in the lower half of the tree.

Surveys carried out in the late 80s-early 90s found A. sphenodonti on only 8 out of 28 natural tuatara populations, and the Department of Conservation lists it as “Range Restricted” in its Threat Classification System. They are also virtually absent from populations established by translocations.  This is partly because up until recently, the ticks were removed when animals were translocated.  However for recent translocations, such as into the  Zealandia wildlife sanctuary in Wellington, the ticks have been left on, but disappeared naturally within the months after the translocation.  The most likely cause of the disappearance is the low density of tuatara in the new location, meaning that when a tick drops off a tuatara at the end of one of its life stages, finding a new host for the next life stage is difficult.  This inability to find new hosts when they are at low densities may have also contributed to the demise of tick populations in the wild.

The case of the tuatara tick highlights how, when populations become endangered, their natural “flora and fauna” are also at risk.  Parasites are the most diverse and species-rich metazoan group on earth, and form a highly important part of host ecosystems, so they deserve our conservation efforts just as much as their hosts.

Further reading:   Miller HC, Conrad AM, Barker SC, and Daugherty CH (2007) Distribution and phylogenetic analyses of an endangered tick, Amblyomma sphenodonti. New Zealand Journal of Zoology, 34: 97-105.