|C. plocamia photo by |
Rubén Arturo Guzmán Pittman
The study we are looking at today focuses on a little parasitic crustacean that belongs to a group known as the Hyperiidea. They are amphipods that have evolved to live inside gelatinous animals of the open ocean. In the case of Hyperia curticephala, it dwells within the bell of the medusa Chrysaora plocamia - a rather large jellyfish that can grow up to a metre (a bit over 3 feet) in diameter.
|H. curticephala image from here|
When scientists examined the stomach contents of small (about 6-10 cm / 2-4 inches long) palm ruffs, they found them to be packed full of H. curticephala and nothing else. As they grew larger, the fish started having a more varied diet, but hyperiids still make up for over 97% of their prey. The amount of H. curticephala in the stomach of palm ruffs reaches a peak in February, just as the parasite also reaches very high abundances in the medusa when some individual C. plocamia can be infected with over a thousand amphipods (which in turn provides a floating banquet for any hungry palm ruff). The abundance of H. curticephala also reaches a high during November, but this was not reflected in the stomach content of the fish - so why is that? The scientists suggested that during this season, most of the medusae available are still quite small and while collectively they might be harbouring a high abundance of H. curticephala, because of their smaller bell size they are inaccessible to the palm ruff (which needs to get in or under the medusa's bell to reach the hyperiids). But by February, the medusae have grown to sufficient size that the fish are able to swim inside the jellyfish's bell to peck at the hyperiids.
Smaller fish can easily swim inside the jellyfish to feed on the parasites and are often found loitering within the host medusa (which also provides them with protection). Larger juveniles cannot enter the bell and have to settle for pecking off parasites, which happens to be in more accessible positions. In this manner, the palm ruffs act as cleaners for C. plocamia, protecting the jellyfish from the parasitic H. curticephala rather like cleaner wrasses that eat ectoparasites off coral reef fishes.
Riascos, J.M., Vergara, M., Fajardo, J., Villegas, V., Pacheco, A.S. (2012) The role of hyperiid parasites as a trophic link between jellyfish and fishes. Journal of Fish Biology 81:1686–1695