Fish Stories: The Elusive Swamp Eel

OSUM Study Monopterus alba

Picture of OSUM Study Specimen Monopterus alba (you’re not missing much, the cryptically camouflaged live specimens are normally not much more colorful!)

The Asian Swamp Eel is known to occur as a native fish species throughout much of southeastern Asia, including India, China, Japan, Malaysia and Indonesia.  There are reports of sightings of Asian Swamp Eels from several other countries in the vicinity including Australia.  The species is now established in several countries outside their native range in western Africa, South, Central and North America.  Populations are established in the United States; first in Hawaii, then in Georgia and most recently in Florida.  Possible modes of invasion include escape from aquaculture ponds where the eels are raised for food.

Swamp eels are known as one of several fish species named as a source of a nematode parasite that causes human gnathostomiasis.  The roundworm nematode must be ingested by the human host for successful infestation, but thoroughly cooking or freezing the intermediary host will kill the parasites.  Thus it is inadvisable to use the swamp eels for sushi!

Other than being a host for a human parasite it is currently thought that the Asian Swamp Eel is relatively benign as far as their impact on local ecosystems, unlike some other more deleterious invasives.  Although they are recognized as a pest neither the federal nor state regulatory agencies in the U.S. restrict the possession or culture of the Asian Swamp Eel.

According to the USGS the potential exists for establishment across the southern U.S. since they can survive relatively cold climates.  Asian Swamp Eels are primarily nocturnal, making their movements hard to detect as well as difficult to prevent.  They prey on small animals like fish, crayfish, worms, snails and aquatic insects but also feed on detritus.  One of several characteristics that enable this species to invade new territories is their ability to breath air, allowing them to occupy ephemeral water bodies, burrowing into wet earth during the dry seasons, and to move across land (it is likely that the swamp eels behave like lungfish in creating a cocoon of mucous that serves to retain moisture, since they are able to secrete copious amounts of of the slimy material).  Given they are air breathers application of chemicals like Rotenone that removes oxygen from the water would not be an effective control.

The skin of the larvae is highly vascularized so the young individuals use their large pectoral fins to pass water over their skin.  As the larvae mature they lose the pectoral fins as well as permeability of most of the external tissues.  Adults retain the vascularity of the skin of their mouths and throat linings and develop an air breathing apparatus called the suprabranchial, or labyrinth, organ.  The epithelial material of the mouth and pharynx develops from the first gill arch, while the labyrinth organ develops from the dorsal ends of the remaining gill arches; thus the organ sits atop the lower gill arches.


Gourami with gill cover raised, note untransformed lower gill structures below transformed labyrinth organ

The gill opening for Asian Swamp Eels consists of a single slit across the throat, so they must need to keep their head off the bottom in order to breath underwater through the gill slit.  But of course they are able to breathe through their mouths, so the only time the gill slit would be used is when they are eating or need to expel objects from their gills.

OSUM Study Monopterus alba gill slit

Picture of OSUM Study Specimen Monopterus alba, focused on gill slit

 Regarding reproductive behavior the males build a large “bubble nest” from their saliva and plant matter in submerged aquatic vegetation in shallow nearshore areas, and guard the eggs and young until they are ready to leave the nest.  They share this behavior with some other fish species such as gouramis, snakeheads and bettas (which, interestingly, also have labyrinth organs).  This is one of several fish species that naturally exhibits sequential protogynous hermaphroditism; starting out as a female, passing through an intersexual stage, and transforming to a functional male.

Other distinctive characters that typify the swamp eel Family Synbranchidae include the lack of all fins excepting the dorsal, caudal and anal fins that are reduced to skin folds, small, sometimes skin-covered eyes, and the lack of a swim bladder, scales and ribs.  These characters render the swamp eels well suited to their secretive lifestyle, slithering across the mud, through tunnels, canals and hiding in crevices and dense mats of aquatic plants.

 OSUM Study Monopterus alba head

Picture of OSUM Study Specimen Monopterus alba, note small, skin covered eyes

Big Minnows?

Not an Oxymoron

There are some concepts that fit the definition of an oxymoron, but unbeknownst to the majority of the non-ichthyologically educated public, the phrase “Big Minnow” does not qualify for that concept.  When most of us think of minnows we think of very small fish, but often the fish species that come to mind don’t fit the proper definition either.  Fishermen  frequently speak of “minnies” as any small fish that is used for bait, but let me take an opportunity to tell you some of what I’ve learned about minnows.  First of all, the scientific definition of minnows limits the group to the fish family Cyprinidae, so all those little bass, darters and sticklebacks may be used for bait, but they are not minnows.

Next, allow me to elaborate on the idea that not all minnows are small: Did you know that the minnow family includes common carp?  Oh, and there’s a subject that needs some correction as well!  You see, carp are not only the bottom feeding scavengers that the term conjures up in our minds, but instead include a diverse assemblage of fish species such as Koi, Goldfish and “Asian Carp”, a term that begs for explanation in of itself.  We’ll return to that area of controversy later in this post.

Pikeminnows are the largest native North American minnow species, reaching six feet in length with a maximum recorded weight of about 40 pounds.  The biggest minnow species at up to 10 feet long and almost 140 pounds is the Giant Barb Catlocarpio siamensis, found in southeast Asia where it is sometimes kept in ponds for food.

Ptychocheilus umpquae 74564


OSUM 74564 Ptychocheilus umpquae Umpqua pikeminnow



Catlocarpio siamensis Giant barb (photo from National Geographic)

Speaking of Asia, let me take a few sentences to flesh out the term “Asian Carp”.  Recently, sensational news reports revealed invasive fish species now found in several tributaries to the Mississippi River that are included under the umbrella term “Asian Carp”.  It is a fact that all carp originally came from Asia and parts of Europe, but the carp that are currently most notorious are actually two species, the Bighead Carp and the Silver Carp.  Two other species, Grass Carp and Black Carp, are also relatively recent invaders, but the Common Carp has been here since the mid-1800’s when they were imported by entrepreneurial European farmers in an effort to continue their husbandry of the fish that they had raised in ponds back in their homelands.  Unfortunately the U.S. Department of Conservation decided that a little bit of a good thing would only be better if it were increased, so they transported Common Carp in rail cars across the United States.  Common Carp in the wild are bottom feeders and require removal of the “mud vein” to make them palatable, so the invasive fish became a nuisance since they actually muck up the waters and decrease aquatic habitat quality.  Fast forward to the past 50 years when another well-intentioned effort resulted in disaster:  Overflowing Bighead and Silver Carp hatcheries in Missouri introduced those species to the Missouri River from where they proliferated and now represent a substantial portion of the fish fauna in much of the Mississippi River mainstem and watershed.

Silver Carp2 from the Spoon River IL summer 2007 by NT

Hypophthalmichthys molitrix, Silver carp (photo by Brian Zimmerman)

Now, most minnows are indeed small, and the smallest minnow, by the bye, is also the smallest known fish species in the world (Paeodocypris progenetica) for which there is no common name that I could find.  Found only in Sumatra, the species matures at about 10mm long.  For comparison the smallest minnow in North America is relatively a whopper at 38mm.  Minnows are a very well researched group of animals and there is a lot more to talk about;  for example going into more depth I could tell you that the only true minnows are the fishes placed in the subfamily Leuciscinae, or that true minnows only have teeth in their throats…but we’ll leave those tidbits for another blog.

Stinky Cheese Log

What (I’m certain you are asking yourself), does a “stinky cheese log”  have to do with a blog on fish?  And for that matter, what in the world is a stinky cheese log?  Rest assured, Dear Reader, that I had no idea that such a thing existed until very recently.  Well, it is a much better known fact that catfish are attracted to malodorous items such as dead animals that have lain on the bottom of a river for several days, so it may not come as a surprise that catfish would be attracted to a stinky cheese log with their highly developed olfactory sense.    These fetid amalgamations of food stuffs consist of cheese, corn, soybean and other grains, and well, I’m not sure what else but feel free to use your imagination.   There are many recipes for smelly catfish baits, and the makers of the stinky cheese logs we used seem to have a recipe that would vie for the prize in a contest.

The operation in which we needed to deploy the baits was a test run to try out equipment for a project backed by the ODNR to comprehensively assess the current fish fauna of the Muskingum River,  pre-Asian Carp invasion.  The rationale for the project is based on recent e-DNA results taken from the Muskingum River that showed several positive samples for Silver Carp (e-DNA, or Environmental DNA, is a recent innovation where genetic material in an animal’s  waste products is taken from water samples).  To date no live specimens have been captured in the Muskingum, but the e-DNA may have come from adult Silver Carp cruising up the Muskingum from the Ohio River where several adults have been captured.

We set out on the morning of a fine September day in the Fish Division flat bottom boat.  Our cargo consisted of five sets of hoop nets, the 8 foot otter trawl net, and various accouterments necessary for deployment and maintenance of the equipment.



At each hoop net site we dropped a concrete weight, buoy and flag tied to the hoop nets and another weight at the back end.



The hoops are rings of iron, seven in total, sized from a three foot hoop to a one foot diameter.  The netting is fitted around the hoops with enough material to drape loosely between the hoops, and includes a net wall with a slit that lets the fish swim in but prevents them from swimming out.  On the way back from setting the hoops (five sites in all) we stopped at each site and made three passes with the bottom trawl.

There were no big surprises for the bottom traul hauls for this stretch of the Muskingum River, notwithstanding that the gear has provided some remarkable discoveries with its ability to sample where no sampling equipment has gone before over the past few years.  But two days later we headed back out to the hoop net sites to pull in our catch.  Not knowing what to expect from the hoop nets I was in for a rewarding experience!  Picking channel kitties out of a net is a ticklish task, you must be careful of their barbed dorsal and pectoral spines while doing your best to ensure their health and survival (try not to break their spines to extract them from the netting).  Thankfully it wasn’t a particularly warm day which helped ensure they didn’t suffer overly much; every one was observed swimming away from the boat vigorously as we threw them in the water one by one after taking weight and length measurements. Now the nets are put away until next spring, when I look forward to helping with more hoop netting, trawling and electroshocking on the Muskingum River!

Paul with Flathead Catfish on the Muskingum River


 OSU Grad Student Paul Larson “weighing” one of the several big Flathead Catfish from the hoop nets

All photos in this post were provided by Paul

An Easy Way to Make Skeletons (or, Impress Your Friends!)

This method commonly used by ichthyologists, forensic scientists and taxidermists uses a scavenger, the dermestid beetle, to remove flesh that would be impossible for the preparator to excise by hand.  There are other methods that would accomplish this goal, such as boiling the carcass or permitting it to rot for a while in water, but  those processes result in disarticulation of the skeleton.   Plus the rotting, or maceration, method is quite putrid.

Dermestid beetles (Family Dermestidae) make great pets! 


Okay, not really, but they are easy to take care of.  The beetle colony needs minimal attention if they are in the proper surroundings.  At most you’ll need to check on them a couple times per week to ensure they have enough food, spritz a little water in the container, and make certain they are not attacked by spiders.  Spiders can devastate a dermestid colony, if you find a spider web in the room kill the spider and its eggs immediately.

A proper habitat for dermestid beetles is dry, shut off from the outside, and has some air flow.  Thus the room we use is ideal with an entrance door that only opens to the outside, thus ensuring that if the beetles did somehow escape their container they cannot enter the rest of the building.  Heat and air flow are provided from a heat lamp and a space heater, they do best in temperatures within a 65-85F range so keep a thermometer nearby.  Escape prevention is easy, simply house them in an aquarium or other container with smooth sides as they are not proficient climbers.  Do make certain the joints of the container are likewise smooth and nothing reaches to the top from inside.


Also make a cover with a screen top that fits tightly to the container, because the adults will fly if it gets warms enough.

When skeletal materials are not at hand the beetles thrive on any sort of meat (chicken livers are cheap and readily available,



but this red hawk made for great fodder.)  A bedding of old newspaper provides a place for hiding and an alternate food source (dermestids also do a number on any cellulose material so don’t set them loose in your home).

Excess flesh is removed from the specimen using a sharp knife.  Care must be exercised to avoid cutting through bones.  The skin and scales, and the gill basket are removed and set aside to dry, thus avoiding destruction by the dermestid beetles.  Once you’ve removed as much flesh as you are able to place the carcass under a heat lamp to dry.  When the carcass has taken on the consistency of jerky, it is ready to feed to the beetles.  Do check on the skeleton at least once per week to note when they have removed almost all of the flesh or they will eat connective cartilage and tendons between the bones.  Manually pick the bugs from the skeleton over their container allowing them to drop back in.  You won’t be able to remove all the dermestids so put the skeleton in a deep freezer to kill the individuals that hide in the crevices.  One more reason the beetles don’t make good pets.  After a couple days your skeleton is ready for exhibit although you may want to clean it up a bit more.



Skin, gill basket (upper right) and skeleton from a recent acquisition.