"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

September 8, 2023

Rhizolepas sp.

Parasitism has evolved a few different times in barnacles. Most parasitic barnacles belong to a group called the rhizocephalans, which are body-snatchers of decapod crustaceans like crabs and shrimps. Aside from them, there are two other known genera of parasitic barnacles: Anelasma squalicola - which is the bane of deep sea Squaliform sharks, and then there's the barnacle being featured in today's post - Rhizolepas, a rare little crustacean that parasitises seafloor-dwelling aphroditid scale worms. Both of them belong to a group called Thoracicalcarea, which happens to be a sister group to the rhizocephalans.

Left: Rhizolepas in situ attached to its scale worm host. Right: Rhizolepas removed from the host, showing its entire anatomy.
Photos from Figure 1 of the paper.

Rhizolepas has a general shape that broadly resembles typical stalked barnacles that can be found attached to piers or drifting debris, but it lacks the feeding legs that those barnacles use to filter food particles out of the water. Instead, it has a dense network of roots at its base that extend deep into the host's body which it uses to suck up nutrients directly from the host.

This blog post covers a recent study on Rhizolepas, and it's about time too because the last time anyone managed to collected a specimen of this little barnacle was back in 1960. The Rhizolepas specimen in this study was collected during a trawl in the seas off Kagoshima, southern Japan. Out of the ten Laetmonice scale-worms that were collected by the trawl, only ONE of them was infected with Rhizolepas. This provided an amazing opportunity to find out more about this rare little barnacle, so the scientists carefully removed the barnacle from its scale worm host and preserved it in high-grade ethanol for further DNA analyses.

How did Rhizolepas get to be the way it is now? Looking at its morphology is of relatively limited value - evolving towards parasitism does weird things to an organism's body. It is a process that turns copepods into fleshy blobs, and transform snails into sausages. So trying to work out the evolutionary origin of something like Rhizolepas based on its anatomy is an exercise in futility. But while its anatomy may have been modified beyond recognition, its evolutionary history is recorded in its DNA.

DNA analysis revealed that Rhizolepas' closest relatives are Octolasmis - a genus of goose barnacles that spend their lives attached to all kinds of different animals, including the shells and gills of crabs and the skin of sea snakes. The study also found another barnacle called Rugilepas, is actually nested among the various species of Octolasmis, and it provides a perfect transitional model for how Rhizolepas might have evolved from a regular stalked barnacle into a fully committed parasite.

Rugilepas lives on sea urchins, but they don't simply attach to their host, their presence induces a gall on the sea urchin's body which snugly encases the barnacle. However, unlike other gall-inducing animals in sea urchins, Rugilepas is walled off from the urchin's internal anatomy, and doesn't draw any nutrients from its host. Furthermore, while its feeding limbs are significantly reduced, they are not completely useless like those in Rhizolepas and Anelasma. So between Octolasmis and Rugilepas, we can get an ideal of the evolutionary steps that Rhizolepas might have taken on its path to becoming a parasite of scale worms

Based on its level of DNA divergence from other barnacles, Rhizolepas is estimated to have originated about 19 million years ago, during the Miocene. Given the external part of this barnacle no longer performs its ancestral function of feeding, the potential next step in their evolution would be to get rid of any dangly parts altogether, and become completely internalised within the host like their rhizocephalan cousins.

Barnacles are particularly pre-adapted for flirting with or even becoming completely committed to a parasitic lifestyle. Even among non-parasitic barnacles, these crustaceans are remarkably versatile in attaching to different living substrates, from sponges and corals, to whales and turtles. Perhaps this versatility gives barnacles an advantage in taking the next step from a mere hitch-hiker into a full-blown parasite. Since the oldest known barnacles date back to the mid-Carboniferous period around 330 million years ago, who knows what other marine animals they might have attached to or even parasitised throughout Earth's history?


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