The following is not suitable for those of a delicate disposition!
Phronima is a genus of Amphipod Shrimps found in the open oceans. They engage in massive vertical diurnal migrations; inhabiting the deep ocean by day (below 1000m) and rising closer to the surface at night (still below 150m) (Quiggley and O’Dwyer 2015).
Phronima are known as the ‘Pram Shrimps’ as they propel their young around in the hollowed-out bodies of creatures known as Salps (Quiggley and O’Dwyer 2015).
Salps are small, barrel-shaped creatures, about the size of the last segment of your thumb. They belong to a group of primitive animals called Tunicates, which live in the pelagic zone of the oceans. They move by pumping water through their bodies, producing one of the most efficient examples of jet-propulsion in the animal kingdom (Bone and Truman 1983).
Although Phronima Shrimps are only a few centimetres long, they are armed with impressive claws and an array of Pereiopods; with these, they hollow out some hapless Salp (Diebel 1988). Given the extremely primitive nature of the Salp’s nervous system, it is not clear if the it would even notice. Once hollowed out, the shrimp climbs inside and is able to propel itself 3-4 times faster that when it is free-swimming (Davenport 1994). The barrel is open at both ends, with the entrance, which faces forward, having an area three times larger than the exit. The propulsion is provided by the beating of the shrimp’s pleopods, but it is tempting to hypothesise that its efforts might be rendered more efficient, by the shape of the Salp’s barrel, which is not unlike the shape of a jet engine.
Photographs by Michele Clements
As mentioned above, the young Phronima are transported about, clinging the inside of a Salp’s Barrel while their mother propels them along. Scientists refer to this chamber as a marsupium, like a kangaroo’s pouch (Diebel 1988). It has been found that cuticle cells, on the inner surface of the barrel, that are ‘grazed’ away by the Phronima Shrimp, and her young, are regenerated (Hirose et al 2005). So the shrimps have a renewable source of food as they travel on their merry way. This relatively high level of maternal care, providing food and protection from predation means that Phronima produce only 600 eggs in the marsupium; a relatively low number when compared with other species of shrimps of a comparable size (Laval 1978).
It has been theorised that, at the appropriate time, adult female Phronima Shrimps rise to the very surface of the ocean to release their brood (Chun 1889a, 1889b), presumably to facilitate dispersal. Incidentally, this might have had something to do with the mass stranding of thousands of parasitised Salps on Narin and Dooey Beaches on the 12th January 2019.
Now, I’m not one to talk but… Phronima Shrimps were not very blessed in the good-looks department and are thought to have been the inspiration for the gruesome, chest-bursting creature in Ridley Scott’s 1979 film ‘Alien’ (O’Dwyer 2014)… How very, very appropriate!
Video by Michele Clements
Chun, C. (1889a) Bericht über eine nach den Canarischen Inseln im Winter 1887-88 ausgefürte Reise, II. Königlich Preussische Akademie der Wissenschaften zu Berlin 30: 519- 552.
Chun, C. (1889b) Über die Amphipoden-Familie der Scinidae Stebb. Zoologischer Anzeiger 12: 286-290 and 308-312.
Davenport, J. (1994) Observations on the locomotion and buoyancy of Phronima sedentaria (Forskål, 1775) (Crustacea: Amphipoda: Hyperiidae). Journal of Natural History 28: 787-793.
Diebel, C. E. Observations on the Anatomy and Behaviour of Phronima Sedentaria (Forskal) (Amphipoda: Hyperiidea). Journal of Crustacean Biology, Volume 8, Issue 1, 1 January 1988, Pages 79–90,
Laval, P. (1978) The barrel of the pelagic Amphipod Phronzma sedentaria (Forsk.) (Crustacea: Hyperiidea). Journal of Experimental Marine Biology and Ecology 33: 187-211.
O’Dwyer, K. 2014. https://theconversation.com/meet-phronima-the-barrel-riding-parasite-that-inspired-the-movie-alien-22555Accessed 13.01.2019.
Quigley, D. and O’Dwyer, K. 2015. The pram shrimp Phronima sedentaria (Forskål, 1775) (Crustacea: Amphipoda: Hyperiidea: Phronimidae) in Irish waters and a review of its association with gelatinous zooplankton. Irish Naturalists Journal 34:1-7