Zombies, Ghosts, and Multi-legged Hosts

Here is an article I wrote for the 2016 fall issue of Fly Fusion magazine. Fly Fusion is probably the most informative and artistic fly fishing magazine in print. I have not written for them lately but I do still have a subscription - something I would suggest to anyone with an interest in fly fishing. This article was written with Halloween in mind. I hope you enjoy it.

   It was clear from the get-go that something strange was afoot. Was this really happening? Could I be dreaming? This just didn’t make any sense...

  Strange indeed; knowing what was normal, what was expected, I was having a difficult time reconciling what I was bearing witness to. The funny thing was that I was not the least bit afraid. The fact that it was already mid-morning may have had something to do with my mindset leaning more to simple curiosity than to morbid misgivings, but with the seemingly sinister events unfolding before me, I simply had to know more.

   I cautiously waded a little closer to a small patch of weed growth in shallow water. Three brook trout circled above it like sharks around a wounded mackerel, their dark olive backs and white fin edges standing out in stark relief against the verdant background. Every so often a pale colored scud could be seen spiraling lazily towards the surface, only to be quickly picked off by the nearest fish. Suicide, I thought. What would possess these scuds to leave the safety of the weeds like that, in broad daylight no less? I knew they didn’t hatch into a winged terrestrial form like aquatic insects do. They really had no reason to head for the surface. It would be several more years before I would learn that those scuds were in fact possessed – infected by an insidious parasite and no longer in control of their own bodies. Yes my friend, zombies really do exist.

    Scuds normally lead peaceful and unassuming lives. Much of their time is spent simply scavenging among aquatic vegetation and along the bottom for bits of dead plant and animal material to eat. As scuds grow they periodically shed their plate-like exoskeleton through a process called molting. Once maturity is reached they will form mating pairs, swimming in tandem as they propagate the next generation. The eggs develop in a pouch on the female called a marsupium and the young are born looking just like their parents, only smaller. There is no larval stage or metamorphosis. The average lifespan for a scud is about one year and depending on conditions they may produce from one to several generations in a summer. That effectively sums up the serene cycle of life for a scud.

   Of course being a perfectly packaged piece of protein means that scuds are a sought after food source. Many birds and fish will go out of their way to chase down any scuds that have been careless enough to get caught out in the open. Unfortunately for the scuds they have little to offer in the way of escape strategies. Even with all of those legs scuds are relatively poor swimmers – often swimming in slow spirals, sometimes on their side, and sometimes even upside down. And with their Mr. Magoo-like eyesight they are constantly bumping into things. With antics like that it’s no wonder that scuds make up a significant part of a trout’s diet wherever the two coexist.

   For the slow moving scud staying hidden is really the only available defense. Cryptic coloration that matches their habitat is a good start. Being found in the weeds, or prowling in the muck, scuds tend to be shades of green, olive, or olive-brown. They can’t quite change their color with chameleon-like adeptness but scuds can adapt over time to changes in the background color of their environment using subtle shifts in shading. Staying out of sight is another strategy that keeps scuds safe. Scuds are normally photo-phobic – meaning they tend to avoid the light, preferring instead to frequent dark and shadowy places. During the day they can be found tucked deep in the vegetation or hiding under logs and debris. It is during periods of low light that scuds tend to become more active, sometimes venturing out into the open (even cloudy days can be enough to coax them out of hiding). Regardless of the lighting conditions, a scud’s instinct is to always dive for cover when threatened.

   Knowing all of this it’s easy to understand why I was so intrigued upon witnessing the exact opposite behavior on my earlier fishing expedition. And here is where our story takes an unsettling twist, one that fly anglers can use to their advantage.

   Parasitism is not uncommon in the invertebrate world. Small bugs often serve as intermediate hosts to even smaller bugs looking for a free lunch. One of the most common scud parasites is a tiny creature known as a spiny-headed worm (Acanthocephala). Scuds become infected with spiny-headed worms when the scud ingests eggs that are mixed in with the organic debris they are feeding on. The eggs hatch in the gut and the “larvae” (acanthella) burrow through the scud’s intestinal wall and into the body cavity where they continue their development. The acanthella develop into sub-adults known as cystacanths – at this stage they are fully formed worms but lack reproductive structures. These tiny worms migrate to a central region, usually along the back of the scud, where they encyst and wait for a definitive host to eat the scud. Depending on the parasite species the definitive host is usually a fish or bird. Final development and reproduction takes place in the gut of the definitive host and the parasite eggs are released when the host animal defecates. Scuds feed on the feces and the cycle continues.

   Once the first phase of development has been completed within the intermediate host, the life of the spiny-headed worm (or continuation thereof) hangs in the balance. If the scud is not eaten by a definitive host the scud will eventually die of old age, and all will be lost for the progeny of the spiny-headed worm. Not leaving things up to chance, the ancient acanthocephalan has evolved a few clever ways to beat the odds.  By manipulating the hormonal system of the scud, acanthocephalans are able to control the scud’s behavior. Instead of avoiding the light, infected scuds in the cystacanth stage become attracted to light and are more active during the day. Infected scuds will also develop a tendency to swim towards any disturbance in the water (such as feeding fish or birds) rather than head for cover. Acanthocephalans also make infected scuds stand out more by altering their coloration. Cystacanths are typically bright orange or yellow – they can easily be seen in infected scuds as a small spot along the back. But more importantly, the cryptic coloration of a normal scud often becomes muted and takes on a pale, bluish-grey, ghost-like hue as the infection progresses (freshly molted scuds will also exhibit the same bluish-grey color but they will lack the orange spot and will have an even greater tendency to stay hidden). These two factors combine to make an infected scud stand out like a neon sign. No guarantee that the scud will be consumed by the correct definitive host, but quite likely that some sort of fish or fowl will gobble it up.

A scud (Gammarus lacustris) in the early stages of infection, before the natural color has faded.

   In fact, it has been shown experimentally that fish will preferentially feed on infected scuds even when both infected and uninfected scuds are readily available in equal numbers. This should come as no surprise. Most fish rely heavily on search images to direct their feeding effort. If they are seeing and successfully capturing more pale scuds than normal ones, the search image for the pale scuds becomes a stronger feeding trigger.

A scud (Gammarus lacustris) with its natural color faded to a bluish-grey due to the infection.

   If infected scuds stand out more, and are eaten more frequently than their uninfected counterparts, doesn’t it make sense to use a fly pattern that specifically mimics an infected scud? Once I had all of the facts, it didn’t take me long to answer that question for myself. Several years ago I tied a bunch of simple baggie shrimp on #10-12 hooks, some using pale blue dun rabbit dubbing and some with the traditional olive dubbing. I added a small orange dot to the back of some using a marker pen but left a few of each color unmarked. After testing these patterns on several local trout lakes the olive scud with the orange spot turned out to be slightly more effective than the plain olive pattern, while the blue dun scud out fished both olive patterns by a wide margin. The presence or absence of the orange spot on the blue dun scuds didn’t seem to matter. This experiment was all very unscientific of course, but the results were enough to convince me to permanently add the Blue Dun Scud to my repertoire.

   In the intervening years I have come across fly tying recipes for other scud patterns in blue, light grey, and pearl – all good candidates for an infected scud imitation. I prefer the simplicity of the old-school blue dun baggie shrimp but some of the newer synthetic fly tying materials are certainly worth experimenting with. The goal is to get that translucent bluish-grey look of a zombie scud. Of course I still carry olive scuds for those times when olive is what the fish seem to prefer, but more often than not it is the blue dun scud that is in tatters on the end of my line.