Thursday, January 28, 2016

Parasite Selfie: The “Duckface”

The first installment of 2016’s parasite selfies is about the Class Monogenea, which includes thousands of species of mostly microscopic critters (typically under 2 mm) that are found on the external surface of a fish, and are usually very host specific. They have unique attachment organs, called haptors, which come in a wide variety of shapes and sizes and are one of the best aids for identification. Together, the arrangement, shape and size of hooks, anchors, suckers and/or clamps are unique to each species. The haptor is located at the posterior end of the body while the adhesive head organs, including the pharynx and mouth, are located at the anterior end. One of the reasons I chose Class Monogenea for this blog post is because the shape of the muscular pharynx (Figure 1) resembles the all too common “duckface” selfie that seems to have infiltrated the social media accounts of celebrities and young college girls alike. Regular perpetrators of this stereotypical expression include celebrities like Kim Kardashian, Nicki Minaj, and the man who may be regarded as the father of the duckface phenomena, the one and only Derek Zoolander (#BlueSteele). I myself succumbed to the duckface craze during my undergrad days, and while embarrassing, the similarity between myself and a tiny, hungry parasite is too striking not to share. Do you see it?!

Figure 1. Duckface selfie. The monogenean shown is a species of Rhabdosynochus, which infects common snook, Centropomus undecimalis, in Florida waters. The arrow points to the muscular pharynx, which has been edited in color for your viewing pleasure, and to match the shade of my lipstick. Photo credit: FWC, Fish and Wildlife Health group for the image of Rhabdosynochus.

Ah, the little things in life that make working with tiny organisms fun! How ironic that the “unique” face my 19-year old self posed in a taxi ride to South Beach, Miami turns out to be a common trait shared by these microscopic parasites of fish.  Alas, not the type of “cool” I was trying to portray.

While an unflattering, contorted and unnatural pose for humans, the “duckface”, aka the muscular pharynx, is vital for the parasite to survive and facilitates the ingestion of food. The parasite illustrated in Figure 1 is a species of Rhabdosynochus, which is in the subclass, Monopisthocotylea and the family Diplectanidae. This particular genus includes parasites which infect the gills of snook, species of Centropomus, in Florida waters. Commonly found in moderate to heavy densities in wild healthy snook, it may or may not cause problems in aquaculture environments.

Species within the Monopisthocotylea (Figure 2) are primarily ectoparasitic grazers, thriving on tissue and mucous to make up their diet rather than blood. Monogeneans do not have an anus, so food is ingested and excreted through the pharynx. Species within the Monopisthocotylea may have eye spots, as seen in Figure 1, and they have a distinctive haptor composed of large anchors and smaller hooks, which may be marginal. The reproductive method may either be viviparous (giving live birth, as in the family Gyrodactylidae) or oviparous (egg-laying, as in the family Diplectanidae) and species within this suborder may inhabit freshwater, marine or brackish water fishes.

The other large suborder of the Class Monogenea is the Polyopsithocotylea (Figure 3), which is known for the distinctive haptor, composed of subdivided and paired muscular sucker-like attachment organs, known as clamps. Species within this suborder are primarily blood feeders, have an oral sucker or pair of buccal suckers, have two, primitive pairs of eyespots, are oviparious, and infect fresh, marine and brackish water fishes.

Figure 2. Species within the suborder Monopisthocotylea. Note the haptoral anchors, which have been focused on. The arrows point to the marginal hooks, which can only slightly be seen. Photo credit: FWC, Fish and Wildlife Health group. 
Figure 3. Species within the suborder Polyopsithocotylea. The arrows point to haptoral clamps, which help anchor the organism to the host. Photo credit: FWC, Fish and Wildlife Health group.

Impacts on Fish Health

In the wild, monogeneans generally exist at modest intensities, although non-native monogeneans have been known to cause epizootics when introduced to native fishes. Gyrodactylus salaris, is a non-native viviparous monogenean belonging to the Monopisthocotylea suborder, which has had widespread detrimental impacts on native stocks of Norwegian salmon, Salmo salar, after it was accidentally introduced with its native Baltic salmon host, a subspecies of Salmo salar. It feeds on the epithelial cells in the skin and gills of salmon parr, thereby causing harm by hindering their osmoregulatory function and delaying their maturation to smolt or even causing mortality.

In fish culture, monogeneans can become serious pests and sometimes cause widespread epizootics in overcrowded conditions and when the water quality is poor. They have a direct life cycle requiring no intermediate host, and quickly reproduce and grow.  Although most monogenean species are fairly host-specific, together they infect a wide variety of hosts, including eels, elasmobranchs, poeciliids, salmoniids, ictalurids, lutjaniids, centropomiids, sciaeniids, and many more. In culture, they can easily spread from fish to fish, eggs can attach to substrate in the tank or pond, or they may be transmitted through a shared water source to infect new hosts. The grazing activity of the monopisthocotyleans removes epithelial tissue, thereby causing the fish to flash, secrete excess mucous, or cause localized areas of erosion or ulceration in severe infestations. This results in impaired osmoregulatory function and may lead to secondary bacterial infections.  The blood sucking activity of the polyopsithocotyleans may lead to anemia in the host, but these monogeneans generally elicit a less severe host response.

Luckily, outbreaks of these parasites can be dealt with in aquaculture environments by improving the water quality, decreasing crowding conditions, and using pharmaceutical drugs, such as Praziquantel, or salt or freshwater dips to kill the parasites that are present. Most monogeneans have a very specific salinity tolerance, so an aquarist can perform saltwater dips on freshwater fish for a few minutes to make freshwater parasites fall off, or the vice versa for saltwater parasites. The good news is, humans are in no danger of acquiring these parasites, but if you have a home aquarium and add a wild fish to the habitat, you may be endangering its new tank mates, especially if they are of the same species. Sometimes, the monogeneans will appear as little white flakes on the surface of the scales, or you may see the fish flashing or rubbing against the glass, which are all signs that you may have an infested tank. So, keep an eye on your fishy pets and next time you take a duckface selfie, remember who you are imitating! Happy fishing :)