With a Fish in Your Pooper, Things Are Never Super (A Guest Post)

By Brittanie Delorit


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!


Article:
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

What Has No Legs And The Most Amazing Feet Ever?

 

This starfish photo is by Mike Murphy at Wikimedia.

We often think of echinoderms, like starfish, sand dollars, and sea urchins, as static ocean decorations. But if you watch them for long enough (or on fast-forward if you lack the patience) you will find that they have exciting motile lives. They hunt, they flee predators, and they mate. But how do they get around without any legs to stand on? Their secret is tube feet.

If you look at the underbelly of these critters, you will see lots and lots of little tubes with suction cups on the ends. These are the tube feet. Tube feet work through hydraulic pressure, the pressure created when incompressible fluids are pushed around. Tube feet extend when a muscular bulb at the top of the foot (called an ampulla) contracts, forcing water down the length of the tube. As the tube foot extends, it swings like a pendulum and then lands and plants itself on the surface. If the surface is smooth, muscles can contract causing the cup-shaped tip to form a vacuum, sticking the foot to the surface. When the ampulla relaxes, the tube foot retracts. To get around, the animal contracts and releases these ampullae in waves, causing the tube feet to extend and retract in a coordinated way that moves the animal in a particular direction (albeit very slowly). They can also use their tube feet in a coordinated way to manipulate objects, like food items.


If you take a close look at this Pycnopodia helianthoides, you can
see the structure of its tube feet. Photo by Stickpen at Wikimedia.

But tube feet aren’t just for movement! They can also be used for breathing, smelling, tasting, and even seeing! These abilities relate to the structure of the membrane in the tube feet. Echinoderms are slow moving and have a low metabolism, so they can get away with taking in oxygen and expelling carbon dioxide at low rates. The membranes in the tube feet are permeable to both of these gasses, and thus play an important role in respiration in these species. Additionally, tube feet often have chemoreceptors (receptors sensitive to smell and taste chemicals) and photoreceptors (receptors sensitive to light). It is largely through their tube feet that echinoderms perceive their world.

Echinoderm tube feet are far simpler than our own feet, with fewer muscles, no bones, and no toenails to trim. Yet their feet can look out for predator shadows, grab and taste prey and walk up walls. Sometimes, simplicity is just cooler than complexity.

Want to know more? Check these out:

1. Lesser, M., Carleton, K., Bottger, S., Barry, T., & Walker, C. (2011). Sea urchin tube feet are photosensory organs that express a rhabdomeric-like opsin and PAX6 Proceedings of the Royal Society B: Biological Sciences, 278 (1723), 3371-3379 DOI: 10.1098/rspb.2011.0336

2. Santos, R. (2005). Adhesion of echinoderm tube feet to rough surfaces Journal of Experimental Biology, 208 (13), 2555-2567 DOI: 10.1242/jeb.01683