Fish Behavior- Symbiosis and Ectoparasites

Expected Learning Outcomes

By the end of this lesson students should be able to:

            1) discuss whether or not the cleaner symbiosis represents a mutualistic or parasitic interaction

            2) discuss the difficulties that parasitic isopods face carrying out their life history attached to fish hosts

Cleaning Symbiosis

            Many species of fishes are attacked by a variety of small external parasites, including isopods and copepods, which burrow into the tissues around the eyes and nostrils, under scales, and in the gills and mouths.  Fishes are commonly observed to spend time at sites known as “cleaning stations” where they assume a stationary, trancelike pose while parasite-eating fish and shrimp feed on the parasites.  Thus, the cleaning symbiosis has been identified as a prime example of a mutualistic interaction where both species benefit.  In this relationship, the client fishes are thought to benefit from the removal of parasites and the cleaners are thought to benefit from eating the parasites.  

            In the early 60s, Limbaugh  conducted experiments in the Bahamas in which he removed cleaning species from some isolated reefs.  When he observed that the number of fish decreased dramatically on reefs where cleaners were removed he concluded that the cleaner symbiosis was indeed an example of mutualism. 

            Later on, other scientists in Hawaii conducted similar studies but observed very different results.  In Hawaii, there was no change in fish abundance and no change in parasite load.  These results suggested that in Hawaii cleaners have a limited ability to control the level of parasites.  Moreover, the cleaners may be acting as parasites themselves by removing bits of flesh and mucous from the client fishes.

            In 1987, George Losey, proposed that cleaners were nothing more than clever behavioral parasites who take advantage of the rewarding aspects of tactile stimulation.  Studies conducted in aquaria had shown that whether they were infested by parasites or not that fish responded to objects brushing against their bodies in the same manner that they respond to cleaners.  In these cases, fish appear to pose for cleaning even when free of parasites.  Moreover, Losey pointed to a weird behavior observed in some species of parrot fish known as “pseudo-cleaning” in which parrotfish assume a trance-like state inside of soft corrals.  These trances, which may take place for minutes at a time several times per day, may be stimulated by the caressing of branches of the soft corals.  Thus, have cleaners evolved to take advantage of being able to get an easy meal by being able to induce client fishes into a trance?  Obviously, there is room for a lot more research on this topic.

            In the Caribbean the four most common cleaners are gobies, the initial phase of the Bluehead wrasse, Pederson’s cleaner shrimp, and jeuvenile Spanish Hogfish.  You can see lots of cleaner behavior is you pay attention during your dives.

Ectoparasites.

            Large isopods from the family Cymothoidea attach to the heads or internal gill structure of common reef fishes.  Cymothoids attach to the skin of a fish using 7 pairs of hook-like legs.  They generally remain attached to their host for life.  Although they look menacing attached to their host, it appears that they do not attack their hosts, but that instead they scavenge pieces of food that float by.

            Living your life attached to a host presents particular challenges.  For example, like all other arthropods, isopods must molt in order to grow.  If they shed their skin all at once, like most other arthropods, then they would lose their grip on their hosts.  Thus, Cymothoid isopods molt in states; first the front half of the animal molts followed by the back half. 

            Obviously, finding mates is difficult for isopods living on swimming hosts.  Usually, a host contains only a single female, but you can occasionally observe a mated pair.  If a male settles on a host with a female he will mate with her.  The females brood their offspring and release them into the water column.  The juvenile isopods must find a new host and avoid being removed by cleaners.

            Eventually, when the female dies the male changes sex to become a female and waits for a new male to arrive so that reproduction can occur again.

            Isopods are not found on all species.  Interestingly, isopods appear to be choosy about on which species they settle and where on the fish they become attached.  The isopod Anilocra chromis attaches to both Blue Chromis and Brown Chromis.  But in Florida and the Dominican Republic it attaches to Blue Chromis and in the U.S. Virgin Islands it attaches only to Brown Chromis even though both species are common at both locations.    Scientists conducted transfer experiments in aquaria in which isopods from the US Virgin islands were attached to Brown Chromis from the same site and to previously uninfected Blue Chromis.  They observed that the Blue Chromis reacted violently against the host and that most had lost the isopod by the end of one day.  On the other hand, the Brown Chromis appeared undisturbed by the attachment of the isopods and most of them retained the isopod for a week.  Thus, the host species appear to be predisposed to accept isopods.  How interesting is this??