You’ve probably heard of relationships between animals before: fish hitching a ride with a shark, clown fish hanging out in their anemones, or barnacles clinging to the fins of whales as they go for a swim; These are all unique in their own way. But have you ever heard of a fish living in the butt of another animal?
|A Thelenota ananas sea cucumber |
(one of the species used in this study).
Photo by Leonard Low at Wikimedia.
The sea cucumber, an echinoderm (along with sea urchins and sea stars), is found in shallow, sandy areas in all the world’s oceans. They eat by gulping in sand through their mouth, extracting decaying organic matter through their digestive tract and then excreting all of the unused matter from their anus. As if the way they eat isn’t strange enough, even more strange is the fact that sea cucumbers breathe using their anus. The sea cucumber gulps in water through its anus taking the oxygen out of the water… and that’s where the pearlfish come in, literally!
Several undergraduate scientists, Brooke Luciano, Ashleigh Lyman, Selena McMillian, and Abby Nickels, from the University of California in Santa Cruz, were on a Marine Ecology field course in French Polynesia. As part of their semester abroad, they wanted to study the interactions of pearlfish and their host, the sea cucumber. They focused their study on four main questions: 1) Is there competition to find a host?, 2) Is the pearlfish host specific and do they return to their original host after leaving?, 3) When the pearlfish finds a host are association cues present between the two?, and 4) Is the pearlfish nocturnally active?
To find results to their five hypotheses, studies were conducted. Two species of sea cucumbers were collected from sites outside of Opunohu Bay on the island of Moorea in French Polynesia. To remove the pearlfish from the sea cucumbers, the cucumbers were placed in a shallow, oxygen depleted container of water.
To answer the first question (Is there competition to find a host?), the pearlfish and the sea cucumber it inhabited were both tagged and observed to see if any fighting occurred between pearlfish. They also recorded if and when a pearlfish chose a different host to inhabit. Once they observed an incident of two male pearlfish fighting to the death inside a sea cucumber and resorting to cannibalism inside the cucumber. They also saw a pearlfish eating its way out of the sea cucumber it was inhabiting.
It's reported that most pearlfish enter tail-first, like in this video.
But cases of pearlfish entering head-first have been reported.
To answer the second question (Is the pearlfish host specific and do they return to their original host after leaving?), the pearlfish were tagged and then placed into a tank with multiple sea cucumbers including their present host. They then observed which cucumber they chose to inhabit, if they returned to it, and for how long. Conducting these studies concluded that no selectivity was found while observing the fish; most fish inhabited the first cucumber they came across even if it wasn’t theirs.
To answer the third question (When the pearlfish finds a host are association cues present between the two?), observations of the fish interacting with a potential host were recorded. Pearlfish smelling the length of their potential host was observed before actually entering the anus of the cucumber were recorded. The pearlfish were also observed listening along the sides of the cucumber, checking for another pearlfish already inside, and after checking, the pearlfish performed a type of knocking around the anus, encouraging its entrance into the body cavity of the cucumber. The sea cumber needs to open its anus to allow entrance for the pearlfish.
To answer the fourth question (Is the pearlfish nocturnally active?), night observations were done. But observations done at night showed no nocturnal behavior. This is strange because in the wild it has been observed that pearlfish live in the cucumber during the day, using them for protection, and then emerge at night to feed and scavenge. The reason no nocturnal behavior was observed in this study is thought to be because the pearlfish were under stress.
The relationship between pearlfishes and their sea cucumber hosts is one of the more intriguing cases of parasitism in the fish world. So if you happen to be a sea cucumber, make sure to hold your breath the next time you see a pearlfish swimming your way!
Luciano, B., Lyman, A., McMillian, S., Nickels., A. 2002. The symbiotic relationship between Sea cucumbers (Holothuriidae) and Pearlfish (Carapidae). A project of the Marine Ecology Field Quarter at the University of California, Santa Cruz, pgs 1-8. Available online: http://bio.classes.ucsc.edu/bio162/Previous%20Class%20Material/Moorea%202002/Readings/cucumbers.pdf