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Sex and parasitism on the open sea and in a fish’s mouth.

Paper: Cook, C. and Munguia, P. (2015), Sex change and morphological transitions in a marine ectoparasite. Marine Ecology, 36: 337–346. doi: 10.1111/maec.12144



Parasites make a living off their selected host, using it as a food source or sometimes even as shelter. Parasitic organisms do not provide any benefit to their host and thus are required to adapt to environments where they are unwanted. Parasites must be able to spread to other hosts in order to survive or reproduce successfully. This complicated lifestyle can lead to creative strategies to get through life. As on land, parasites are common in the ocean. All types of animals suffer from parasites such as barnacles, snails, worms, leeches or even crabs. These parasites may harm or sometimes kill their hosts.

The parasitic isopod, Cymothoa excise, infects Atlantic croaker, Micropogonias undulates.  Isopods are a type of crustacean related to pillbugs (or roly polys) you may find in your garden. Some isopods are free living reaching over a foot in length in the deep sea. Some parasitic isopods infect the skin of fish. The isopods in this study live in the mouth of fish, attaching to the tongue or gills.

Our isopods must swim through the water to encounter a host fish to live inside, which can be quite a challenge. After securely attaching to the tongue, the isopod can focus on the next phases in its life – growth and reproduction. Male and female isopod are needed for sexual reproduction, but relying on luck to have both sexes float by at the same time is too risky. These isopods solve this problem by changing sex as needed to ensure there are males and females available. An isopod may be a juvenile, then a male, and then a female if one does not already live on their fish. This sex change over an individual’s life is known as sequential hermaphroditism.

The sex changes of the isopods are known, but not fully explored or understood. Researchers have observed the sex changes but what triggers them, or how prevalent it is was unknown. This study examined the morphology of the different stages, along with the reproductive output and explores the best timing for transitioning from male to female.




Seen at the National Museum of Natural History in Sofia in Bulgaria.

Isopods seen at the National Museum of Natural History in Sofia in Bulgaria. Credit: Orin Zebest.

The Study

The investigators collected Atlantic croaker over 22 months and examined the fish for parasites. Isopods were removed by hand, measured and identified based on stage and sex according to the following categories: juvenile, male, female or transitional (between sexes).

Over the length of the survey almost 100 croaker were caught through trawls (3 m depth) in the bay of Port Aransas, Texas. Overall 1 in 5 fish were infected by the isopod. Infection rate did not change with the season, but the intensity of infection (i.e. number of isopods per fish) changed throughout the year with spring having a low intensity increasing as summer went on and lowering in winter. Infected fish were home to a range of one up to six isopods.

The first isopod on a fish infected the tongue and was female or transitioning (except in one case when a small male was found alone in the gills), whereas all additional isopods were male. When two isopods are present they inhabited different areas of the tongue. Any more than two isopods were found in the gill arches.



Fig. 1. (A) Seasonal frequency of isopods infecting fish as a function of fish size. Arrows indicate the median size of infected fish. (B) Seasonal frequency of infected fish as a function of the number of isopods per infection.


Fig. 2. Location and frequency of isopod parasites occurring on Atlantic croaker (n = 70). Locations were divided into three areas – the gills, the tongue and a cavity behind the tongue next to the gills (T-G). Infected fish had one to four parasites.




As males transition to females they become larger. This study found larger females have higher reproductive success than smaller females. Larger fish can also support larger isopods. The researchers also determined six body traits that differ between males and females, showing morphological identification of sexes is possible.



This study helps us understand the fascinating life history of a parasitic isopod.  It raises questions about how the isopod undergoes sex changes and what controls these changes in the fish’s mouth. We still do not know what prevents more males from transitioning in this environment. Researchers can continue to study this relationship to determine what pheromones may be directing the transitions or to explore the competition occurring among males to determine which isopod has access to the female. Also, this study does not investigate the effect of the parasite on the host. Evolution of parasites is tied to evolution of the hosts and this paper helps to fill in part of the fascinating story while piquing our interest in learning more about the parasite and its relationship with its host.



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