Plankton make up about 95% of life in the ocean. It is made up of phytoplankton, the base of the food web. As well as zooplankton, the essential link between microscopic plants and the rest of the food web (Sardet, 2013).

Amphipods are crustaceans that are found in marine and freshwater habitats. They have segmented bodies, separated into three distinct sections: head, thorax and abdomen. Heads bear up to six pairs of appendages. There are over 5,000 species of Amphipod. Most live on or in the sea bed but those belonging to the suborder Hyperdii are exclusively marine, pelagic and are a part of the plankton community (Hayward and Ryland, 1995)

Hyperiid Amphipods are characterised by large heads with very large compound eyes, which gives them a panoramic view of their surroundings. Aiding detection of predators and potential food. They have fat bodies with long slender tails. They specialise on feeding on particular gelatinous organisms and make homes out of the transparent envelope bodies. Example prey items are: Salps, Siphonophore, jellyfish or Pyrosomes (Hayward and Ryland, 1995; Sardet, 2013).

Lifestyle of Phronima

Phronimidae is a family of Amiphipod, known collectively as the Pram Bug. This parasitic family contain eleven species, ten of which have the genus Phronima. Species of Phronima can be distinguished by their ownership of two claws used for feeding and defence. Phronima measure to about 25mm and predate largely on Salps. They eat and live inside the Salp. Attaching themselves to the remains using hind and fore legs. They swim with the current using their tail, carrying their ‘barrel’ with them. Bristles on their hind legs are used to collect food particles from the water. Phronima will leave the barrel to catch bigger prey. When they do the food is eaten outside the home until it is small enough to fit inside (Aoki et al. 2013; Sardet, 2013).

Living inside the body of their victim is a behaviour of both males and females. Males will abandon their barrel at the slightest disturbance however, while females need more persuasion to leave (Sardet, 2013).

video directed by S. Mirshak. original idea C. sardet


Female Phronima use the barrel as a nursery as well as a food parlour and hunting platform. Females take good care of their young, which is rather unusual for crustaceans. Not only do the females lay and incubates their eggs in a marsupial like pouch, they feed and protect the young too. Which its where it gets its name pram bug. The female feeds the young for two to three weeks and make sure they stay together. If the young fall out of the barrel she will catch them and retrieve them. When the offspring are mini versions of mum they are ready to leave home. Different species on Phronima have different numbers larvae phases. Ranging from one to five (Aoki et al. 2013; Sardet, 2013).

Hiding Phronima

Two avoid being eaten, Phronima sedentaria have chromophores all over their cuticle and claws. When expanded the pigment turns the animal red. When contracted, however, the animal is transparent like its host to confuse would be predators (Sardet, 2013).

Phronima sedentaria also avoids visual predators by diel vertical migration. This is a behaviour that describes vertical migration in the water current. Phronima sedentaria spend night where there is better feeding opportunity, at the surface and retreats during the day. The surface water at night are also the upper physiological tolerance of the amphipod. There was a 50% mortality rate when exposed to the typical water temperatures of night for a full day. Surface waters would only increase during the day so the daily migration may be a thermotaxis response as well as a phototaxis (Elder and Seibel, 2015)

Ecological Importance

Amphipods are very desirable prey items for animals like fish and are essential links in the food chain. Particularly in Antarctic benthic communities (Sardet, 2013).

In Antarctica, Krill is the backbone of the food web. They feed on plankton trapped in sea ice and in turn feed everything else or everything else’s food. Salps also feed on the same prey as krill. There are two important differences in their niche roles though. Salps are more efficient at extracting their food in the open ocean. Krill are nutritionally much more valuable (Leob et al. 1997; Nicole et al. 2008)

Being gelatinous makes Salps, essentially, nutritionally invalid. They are not prey to much because their bodies do not offer enough for a polar animal to make a decent meal out of. If krill were to be out competed by Salps due to loss of sea ice, Amphipods could potentially fulfil the role of krill from benthic communities (Pardo, 2017).

In summary

Just because something is small or not very well understood does not mean it is unimportant or boring. Amphipods including Phronima are ecologically valuable in supporting food web we depend on for fish.


Aoki M.N., Matsumoto-ohshima C., Hirose E,. Nishikawa J (2013) Mother–young cohabitation in Phronimella elongata and Phronima spp. (Amphipoda, Hyperiidea, Phronimidae). Journal of the Marine Biological Association of the United Kingdom, 93(6), 1553–1556. doi:10.1017/S0025315413000143

Elder LE and Seibel BA. (2015) The thermal stress response to diel vertical migration in the hyperiid amphipod Phronima sedentaria. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology. Vol 187. Pp 20-26

Hayward PJ and Ryland JS (1995) handbook of the marine fauna of northwest Europe. Oxford university press. First edition. Pp 361

Leob V., Siegel V., Holm-Hansen O., Hewitt R., Fraser W., Trivelpiece W., Trivelpiece S (1997) Effects of sea-ice extent and krill or salp dominance on the Antarctic food web. Nature 387, pp897-900. Doi:10.1038/43174

Nicol S,. Worby A., Leaper R (2008) Changes in the Antarctic sea ice ecosystem: potential effects on krill and baleen whales. Marine and Freshwater Research 59(5) 361-382

Pardo LM. (2017) Amphipods could play role similar to Krill in Antarctic benthos. Ideal, Research centre dynamics of high latitude Marine environments. {online} Amphipods-could-play-role-similar-to-Krill-in-Antarctic-benthos/#

Sardet C. (2013) Plankton. Wonders of the drifting world. The university of Chicago press. Pp 154-164

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