"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

January 20, 2014

Phronima sp.

Today's guest post is by Katie O'Dwyer, a PhD student currently at University of Otago in the Evolutionary and Ecological Parasitology research group. In one of my conference reports last year, I mentioned some of the research that she is currently conducting on parasitic flukes that live in periwinkles. She has provided us with a post about a parasite that she came across while walking along a beach in New Zealand.

Phronima and its salp barrel.
Photo by Katie O'Dwyer, used here with permission
After recently finding some salps containing the amphipod Phronima, washed up on a beach in New Zealand, I decided this was a worthy group to compose a blog about. It helped too that I was already interested in this group of crustaceans, having assisted with some work on them in Ireland. Read on for some interesting information on this little studied group of parasitic organisms…

Imagine a parasite which can create its own mobile nursery for its young, a parasite which is thought to be the inspiration behind the chestbusting xenomorph in the popular movie Alien. Well imagine no more! Introducing Phronima, the pram bug. These amphipods are members of the Phronimidae, a group of ten species of hyperiid amphipod, which occur in the water column throughout the open ocean. This sets them apart from their close relatives, which typically inhabit the benthic environment of the seafloor. So what has allowed this particular family to adapt to the pelagic or open water environment?

Those adorable little babies!
Photo by Katie O'Dwyer, used here with permission
Enter salps. What is a salp? Salps are gelatinous zooplankton which drift throughout our oceans. They may occur singly or in huge chains composed of individual salps linked together. Phronima is equipped with impressive front claws and with these they attach to an individual salp and carve away its insides until it forms a barrel. Phronima then climbs inside and sails the sea from inside a gelatinous barrel, collecting food from the water column. A number of questions may now come to mind regarding this symbiosis; has Phronima killed its host, which suggests that it is a parasitoid rather than a parasite, and why does it carry this barrel around as it must be pretty energetically expensive, right?

Well, as mentioned, these organisms live in the open ocean which presents several challenges to collecting samples for answering these questions. However, some dedicated researchers have indeed managed to study these fascinating creatures on the rare occasion that such an opportunity arises. From their research they have found that the salp in fact still contains live cells, although it hardly resembles a salp anymore with just a barrel of tissue remaining. The presence of live cells means that the barrel maintains its structure and that is important for Phronima to have a sturdy home. As the barrel barely resembles a live salp any longer, Phronima should really be considered as parasitoids rather than parasites.
Do a barrel roll!
Photo by Katie O'Dwyer, used here with permission

As for the energy involved in carrying around this barrel, the barrel provides a larger structure than the amphipod itself and this enables the Phronima to be more buoyant in the water column. However, some energy is still required to carry around this jelly barrel. Overall energy usage by Phronima is higher than that of benthic amphipods but on the lower spectrum compared with other pelagic or open water amphipods. This suggests that Phronima have indeed adapted to a unique niche which enables them to travel in the water column with their young and access new food resources without this behaviour being too energetically costly.

One unusual finding in the research thus far is that male Phronima are also found in barrels. If Phronima is known as the pram bug, which suggests the barrel is important for carrying offspring, then why should males carry a barrel too? Could they use it as part of some mating strategy, where they pass the barrel on to the female they mate with? Due to the difficulties associated with studying organisms that dwell in the open ocean many questions remain unanswered and this leaves us ever more curious and fascinated by creatures such as Phronima.

Hirose, E., Aoki, M. N., & Nishikawa, J. (2005). Still alive? Fine structure of the barrels made by Phronima (Crustacea: Amphipoda). Journal of the Marine Biological Association of the United Kingdom 85: 1435-1439.

Bishop, R. E., & Geiger, S. P. (2006). Phronima energetics: is there a bonus to the barrel? Crustaceana 79: 1059-1070.

This post was written by Katie O'Dwyer.


  1. So these are real-life barrel riders? You could have had a Hobbit tie-in as well ;)

  2. The males carry the barrels around like a hotel room. Put up a disco ball, some rocking music, a lot of X and babies are born. Then he locks em in the room and bails. Typical huh.

  3. When was the phronima discovered?

  4. The first species of Phronima that was described was Phronima sedentaria in 1775 - see: http://www.marinespecies.org/aphia.php?p=taxdetails&id=103272