A few months ago I wrote a Dispatch for
Current Biology about a newly published study on Anelasma squalicola - a parasitic barnacle that infects velvet belly lantern sharks. Unfortunately for most people, the Dispatch is behind a paywall, therefore I have decided to write a blog post about that study, which in turn is based on the Dispatch I originally wrote for
Current Biology, so here it is.
|Drawing of Anelasma squalicola and its host by Tommy Leung|
The trouble with studying the evolution of parasites is that it is often hard to tell what evolutionary steps they took to get that way. Evolutionary selection pressures experienced by parasites can be quite different to those with a free-living life, thus parasites often bear very little resemblance to their non-parasitic relatives. For example, Enteroxenos oestergreni
is a parasitic snail that lives inside a sea cucumber, but the adult stage of this snail is nothing more than a long, wormy string of gonads. To make things even more difficult, parasites are usually small and soft-bodied - which means they are not usually preserved as fossils and unlike say, birds or whales, there is not a good fossil record of various transitional form.
Parasitism has evolved in many different groups of animals, including crustaceans. Various lineage of crustaceans have independently evolved to be parasitic, some of them are so well-adapted that most people would not recognise them as crustaceans if they were to encounter one
. Some barnacles have also jumped on the parasitism bandwagon, of which the most well-known is Sacculina
which infects and castrate crabs
. The body plan of Sacculina
and other rhizocephalans bear little resemblance to the filter-feeding species often found attached to rocks or the hull of ships. Superficially, it resembles some kind of exotic plant (perhaps Audrey II from the Little Shop of Horrors
)- there is the bulbous reproductive organ call the Externa
which protrudes from the host's abdomen, but the rest of the parasite is actually found inside the body of the crab in the form of an extensive network of roots called the Interna
Aside from the rhizocephalans, there are only two known genera of parasitic barnacles - one of which is the star of this post. Anelasma squalicola
is one of those rare parasites that has retain some remnants of its non-parasitic past. Its host is the velvet belly lantern shark
- a deep water fish also known as the shark that warn off predators by wielding a pair of "light sabers"
. But such armament offers no protection against A. squalicola
. This barnacle attaches to the shark's body and burrow into its flesh. Anelasma squalicola
digs into the shark using its peduncle - for non-parasitic stalked barnacle, that is the structure they use to stick themselves onto a fixed surface. In A. squalicola
, the peduncle embeds itself into the shark's muscles, then sprouts numerous branching filaments that sucks the life blood out of the host. As a shark can sometimes be infected with multiple A. squalicola
, this can really take a toll and this parasite has been known to cause host castration
There are of course, other barnacles that attached to marine animals like whales
, but they are not truly parasitic as they still feed strictly by filtering food from the water instead of feeding off the host like A. squalicola
. One group - the Coronuloidea
- are specialists at this particular life-style. In fact, some of them do not merely stick to their host, they are partially buried in the host tissue and have special structures to anchor them firmly in place
. So it seems likely that the coronuloids might be the predecessor to a full-blown parasite like A. squalicola
, right? Even though they have kept up their filter-feeding life-style, they are already embedded in the host's body, so one can imagine that it is only one step away from feeding directly from the host itself.
But as plausible as that story may sound, according to the new study by Rees and colleagues, their analysis shows that the closest living relative of A. squalicola
the coronuloids but is actually...[drumrolls]...a filter-feeding goose barnacle
! The ancestor of A. squalicola
seems to have taken up the parasitic life-style about 120 million years ago in the early Cretaceous, when the sea was filled with marine reptiles. It was also during this period that more "modern" sharks underwent a dramatic increase in their diversity
. Given the lack of any other known stalked barnacles with similar life-styles and its relatively ancient origin, could A. squalicola
be the remnant species from a group that was once far more diverse, rather like the coelacanth or the tuatara?
But what about the Coronuloidea? Why did they
not go "full parasite"? Considering the radical changes the ancestor of A. squalicola
underwent from a life of filter-feeding to one parasitising a shark, why have none of the coronuloids done the same? Especially seeing how they seem to be in such a prime position to do so.
The affinity of A. squalicola
to modern rock-clinging barnacles should remind us that evolution does not always go the way we imagine it to be. You can come up a plausible hypothesis (like A. squalicola
evolving from the coronuloid barnacles) that seem rather believable, but ultimately it has to face the data. The evolution history of any organism is a convoluted tale, and sometimes it can challenge our expectations.
Leung, T. L. (2014). Evolution: How a Barnacle Came to Parasitise a Shark. Current Biology 24
Rees, D. J., Noever, C., Høeg, J. T., Ommundsen, A., & Glenner, H. (2014). On the Origin of a Novel Parasitic-Feeding Mode within Suspension-Feeding Barnacles. Current Biology 24
For another take on this story, I also recommend Ed Yong's post about the paper here.