Pearlfish: A LIttle Known Fish Family Gem, by Jordan WIlliams

Did you know that there are approximately 33,788 discovered species of fishes living on this planet as of 2016? It is actually really incredible when you stop and think about it. To put that number in perspective for you, that is over six times more species of fish than there are of mammals. Both mammals and fish fall under the phylum Chordata, which means every member of has at least a notochord and a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail for at least some period of their life cycles. But which came first? Fish of course! They were swimming around at least 300 million years before the first mammal even came into existence! Fish have had about 510 million years to fine tune their evolutionary adaptations for living in their aquatic environments. That isn’t to say that time is everything; just look at the 950,000 species of insects we can observe! And they are 200 million years younger than the fish (Ramel)! But as far as chordate evolution is concerned, fish have had the advantage of time over us mammals. Which finally brings me to my point: fishes have used all this extra evolutionary time to adapt and diversify in order to best suit their numerous aquatic environments. Fish showed up only 90 million years after the earliest animals, so they are some of the most well adapted and diversified animals on the planet.

Atlantic pearlfish  Carapus bermudensis   Source 

            But getting more to the point, fish are taxonomically separated into hundreds of families, 515 if you were curious. Taxonomy simply classifies animals into groupings that share common, “homogeneous,” traits. Pearlfish make up just one of these families, which makes them distinct from any other. The same can be said for the 514 other families, but I have a suspicion that you may find pearlfish just a little more distinct from the rest. Now without further ado, I hope to show you that the morphological and behavioral characteristics of the pearlfish family, Carapidae, makes its members fascinating and distinct from any other fishes.

            Morphologically, you would think that all fish share the same basic layout, but this is not the case. The minute you start trying to describe fish is the minute you’ll think of an exception to the characteristics you’ve just laid out! Just take a look at pearlfish for example. They are shaped like an eel and have no scales. Along with that, their anal and dorsal fins fuse together at an acute tip. Speaking of fins, the caudal and pelvic fins are completely absent in this family. They also have translucent bodies. Put all this together and we are a long way from what we first thought of when we heard the word ‘fish.’ But even with that description, there are still 36 accepted species of pearlfish swimming around. They are taxonomically grouped in three main genera: Echinodon, Carapus, and Encheliophis. The distinction between the three comes by how dependent they are to invertebrate hosts. Echinodon species are free-living. So to fend for themselves out in the tropical waters of the Pacific, Atlantic, and Indian oceans, they must be the biggest and strongest of the bunch.

Star Pearlfish, Carapus mourlani (Petit 1934)  Photo by John E. Randall. Source

            Alternatively, Carapus species tend to be a little smaller because they commensally hide inside their hosts during the day and come out to feed only at night. This leaves the parasitic Encheliophis genus left. Its members are usually the smallest based because it has no real reason to leave its host. Being a small parasite makes it more likely that the host will survive, in turn, raising your own odds of survival. (FAMILY Details for Carapidae – Pearlfishes).

But since taxonomists break the family up into these three main genera only by their level of dependency, there has been a lot of back and forth between which species belongs to which genus. For example, the Silver Pearlfish was thought to be of the parasitic family Encheliophis up until very recently when scientists discovered crustaceans in its stomach contents (Luna). This indicated that it had been out feeding at night, as Carapus species are known to do. However, it is still debated whether or not these fish obtain the majority of their nutrition from their hosts or from nocturnal feeding.

Regardless, we now understand what pearlfish look like and that they are fall into their genus based on dependency on hosts. But it’s time we combine the two and find out just what is really going on with these fish, behaviorally. Pearlfish hosts include holothurians, bivalves, and starfishes (Simon 2014). Most likely the name pearlfish came from when the species were first discovered as people were harvesting oysters or another kind bivalve. Pearlfish that do utilize hosts are quite clever about it. My favorite example of this is how they get into sea cucumbers (holothurians). First, pearlfish will sniff out which end they want to enter from… which happens to be the rear, as sea cucumbers breath through their anus while ingesting sediment on the ocean floor. If the sea cucumber continues to breathe normally, which is often not the case when they sense pearlfish are nearby, the pearlfish can just swim on into its body cavity. But if the sea cucumber puckers up, then the pearlfish truly shows us why it is such a fascinating fish. Remember how the dorsal and anal fin come to a point and how pearlfish lack scales? Good, because you’re about to find out why. The pearlfish will stab the tip of these fins into the sea cucumber’s anus and wait for it to start suffocating. When this happens, the sea cucumber will gasp for breath. But, this allows just enough time for the pearlfish to wriggle backwards into the orifice (Miyazaki 2014). From there, it can hide, eat, or even reproduce all while inside the coelom. Parasitic pearlfish eat this coelom and the gonads, as if the sea cucumber wasn’t violated enough already (Nielsen 1999). Click here, to  see a pearlfish swimming into a host.

            As you can see, Carapidae is a fascinating fish family because of their morphology and behavior. They are one of the most striking examples of fish adaptation and live in a strange and intriguing manner.

References

"FAMILY Details for Carapidae - Pearlfishes." FAMILY Details for Carapidae - Pearlfishes. Accessed April 4, 2016. http://www.fishbase.org/summary/FamilySummary.php?ID=187.

Luna, Susan M. "Encheliophis Homei Summary Page." FishBase. Accessed April 4, 2016. http://www.fishbase.org/summary/SpeciesSummary.php?ID=4832.

Miyazaki, S., T. Ichiba, J. D. Reimer, and J. Tanaka. 2014. Chemoattraction of the pearlfish Encheliophis vermicularis to the sea cucumber Holothuria leucospilota. Chemoecology 24(3):121–126.

Nielsen, J. G., D. M. Cohen, D. F. Markle, and C. R. Robins. 1999. Ophidiiform fishes of the world (order Ophidiiformes): an annotated and illustrated catalogue of pearlfishes, cusk-eels, brotulas and other ophidiiform fishes known to date. FAO Rome.

Ramel, Gordan. "The Evolution of Mammals." The Evolution of Mammals. Accessed April 4, 2016. http://www.earthlife.net/mammals/evolution.html.

Simon, Matt. "Absurd Creature of the Week: This Fish Swims Up a Sea Cucumber’s Butt and Eats Its Gonads." Wired.com. February 21, 2014. Accessed April 4, 2016. http://www.wired.com/2014/02/absurd-creature-of-the-week-pearlfish/.