Duplicibothrium minutum

In June 2010 at Folly Beach, South Carolina, the local community was shocked and dismayed by the sight of millions of dead dwarf surf clams (Mulinia lateralis) that carpeted the beach with rotting shellfish. During the course of an investigation into what might have caused this die-off, researchers discovered some tapeworm larvae in the clams which had not been previously reported from that area.

image of Duplicibothrium minutum from figures in the paper

Because the larval stages of tapeworms have few of the morphological features that usually serve as diagnostic markers to identify different species, the researchers looked to their DNA for clues on their identity. They sequenced a section of the tapeworms' DNA and compared it with known DNA sequences of tapeworms (we have previously featured a study which used the same technique, know as DNA barcoding, to figure out the life cycle of a Great White shark tapeworm.) They were able to determine that the most common species of parasite in those clams was the tapeworm we are featuring today; Duplicibothrium minutum. Out of the 200 clams that the researchers dissected, 150 of them were infected with D. minutum, while four of clams were infected with another species of tapeworm - Rhodobothrium paucitesticulare (three of which were also infected with D. minutum).

photo of Rhodobothrium paucitesticulare 
from figures in the paper

The larvae of these two tapeworms occupied different part of the clam's body - whereas D. minutum were often found in pairs in the bivalve's digestive glands, R. paucitesticulare larvae tucked themselves away at the gap just beneath the clam's fleshy mantle. Both tapeworms are gut parasites of the Cownose ray, (Rhinoptera bonasus) which commonly feed on mollusks and other invertebrates that they suck up from sandflats and crush with their hard dental plates. Rhodobothrium paucitesticulare only infects the dwarf surf clam and one other species of clam (Donax variabilis), while D. minutum has a much wider host range and has also been found in two other species of clams (Donax variabilis and Angulus versicolor) as well as the Florida crown conch (Melongena corona).

Unlike some parasitic flukes that can alter the burrowing behaviour of clams and other bivalves, neither tapeworm caused much noticeable harm to the host clams. The presence of D. minutum caused some minor enlargement at the opening of the digestive glands, but there were no signs of inflammation, and the R. paucitesticulare larvae seem to be completely benign and did not affect the clam's health at all. So while those tapeworms seems to be very common in the clam population, they were not causing nearly enough harm to be considered responsible for the mass die-off.

Reference:
de Buron I, Roth PB, Bergquist DC, Knott DM. (2013) Mulinia lateralis (Mollusca: Bivalvia) die-off in South Carolina: discovery of a vector for two elasmobranch cestode species. Journal of Parasitology 99: 51-55

Ieredactylus rivulus

As you can probably tell from the name, asphalt lakes are not nice places to live. Also known as tar pits, they are natural deposits of bitumen that leak up to the surface, filling the water above with all kinds of nasty substances including volcanic ash, hydrocarbons, sulphur, and metal compounds. There are only five such natural asphalt lake sites in the world, one of which is the well known La Brea tar pits.

The largest asphalt lake in the world is Pitch Lake on the southwest coast of Trinidad and surprisingly, it is actually home to a variety of organisms. Not just bacteria and other hardy microbes, but animals such as aquatic insects, a species of frog (Pseudis reticulata), and some fish have also made it their home sweet home. Despite the inhospitable surroundings, there might be a perk to living in an asphalt lake. Such an harsh environment might also be intolerable for parasites, especially any external parasite which would be exposed to the asphalt-contaminated water.

Ieredactylus rivulus
image from here

In their natural habitat, guppies are commonly plagued by many parasites, especially ectoparasitic flatworms call monogeneans in the genus Gyrodactylus, and in heavily infected populations as many as three-quarters of the fish will be infected. The guppies living Pitch Lake are almost completely free of parasites - except the parasites that we are featuring today - Ieredactylus rivulus. While it is the only parasite to infect Pitch Lake guppies, it is not very abundant and they are found on fewer than five percent of the fish in any given population. Apart from Pitch Lake guppies, this parasite is only found on the giant rivulus Anablepsoides hartii (previously known as Rivulus hartii); another hardy inhabitant of Pitch Lake. Furthermore, the giant rivulus is also known for wandering onto dry land every now and then, so a parasite that lives on the skin of such a fish must be pretty robust.

In the paper we are featuring today, a group of scientists conducted a series of experiments to see how the asphalt lake environment affected the guppy's parasites. In one experiment, they tested whether the Pitch Lake guppies are innately resistant to infections by placing some Pitch Lake guppies in a tank filled with dechlorinated aquarium water. Within a week, seven out of the ten guppies in the aquarium water became infested with various bacterial and fungal infection, whereas all but one of the guppies kept in the original Pitch Lake water were free from infections.

In another experiment, they tested the effect of exposure to Pitch Lake water on monogenean parasites. They collected guppies that are naturally free of monogeneans parasites from a site at the Upper Naranjo, and experimentally infected them with Gyrodactylus by exposing them to parasite-laden guppies from the Lower Aripo, a site with high parasite prevalence. After those guppies had acquired some parasites from their infected cousins, the scientists transferred one group of the newly-infected guppies into a tank filled with water they collected from Pitch Lake that has been diluted to a quarter of its original concentration, and another group into tank of dechlorinated aquarium water. Within 48 hours, the guppies transferred into the diluted Pitch Lake water had lost their newly-acquired parasites, whereas those transferred into the aquarium water were stuck with their new parasites.

Both of those experiments showed that the Pitch Lake water was playing a key role in keeping the Pitch Lake guppies free from (most) infection, and that I. rivulus must have some special adaptations which allows it to survive on fish swimming in a pond filled with bitumen. So if I. rivulus can survive on asphalt lake guppies, what is to stop them from taking on guppies living in less noxious surroundings? Perhaps in the extreme environment of the Pitch Lake, I. rivulus does not face competition from other parasites and can have the host all to itself, whereas in other guppy populations they will be competing with rapidly breeding parasite like Gryodactylus and get shoved aside.

So while asphalt lakes might not be attractive places to live, such extreme environments can provide their inhabitants with a refuge from all but the most hardy parasites.

Reference:
Schelkle B, Mohammed RS, Coogan MP, McMullan M, Gillingham EL, van Oosterhout C, Cable J. (2012) Parasites pitched against nature: Pitch Lake water protects guppies (Poecilia reticulata) from microbial and gyrodactylid infections. Parasitology 139:1772-1779