Wednesday, February 13, 2013

PARASITES, POOP, AND PALEONTOLOGY




Perhaps a delicate flower? Nope, just the scolex of the extant rhinebothriidean cestode Rhinebothrium. But you already knew that.

Life comes in a kaleidoscope of sizes, morphologies, and ecologies.  From the majesty of a soaring eagle to the beauty of an orchid, the natural world has inspired philosophers, artists, and scientists from antiquity.  A simple walk through a forest can be soothing and inspirational, even if you don’t know the name of any of the plants and animals you pass by.

But then there’s those damn parasites.  Like the crazy uncle everyone tries to ignore, parasites are an uncomfortable reminder that “Except for the grace of natural selection, there go I.”  Parasites cause immeasurable pain, discomfort, or death to billions of organisms every year.  But they are also, as Carl Zimmer has written, important drivers of evolution, control populations, and sometimes crucial to the functioning and survival of entire ecosystems. Nature does not have morality and parasites are not mean, bad, or evil.  They are just the result of the relentless selection pressures of evolution. Putting aside human prejudices, parasites are really wonderful creatures specialized in remarkable ways to live amazing lives. 

Parasite of the Day (http://dailyparasite.blogspot.com/) is a fascinating blog dedicated to highlighting the diversity of plants, animals, and unicellular organisms that live the parasitic lifestyle.  You should visit it regularly.  Take some time to peruse the back posts --- they will reveal a weird and wonderful world that few are aware of.  In a post from Jan 1, 2011 they estimate that by featuring one parasite per day, it would take their blog several millennia to document the incredible diversity of parasites (http://dailyparasite.blogspot.com/2011/01/after-one-year-just-tip-of-iceberg.html).   



Cymothoa just doing its tongue thing.

One of my all time personal favorite parasites is Cymothoa exigua, a crustacean that eats the tongue of a fish and then attaches to the fleshy stub to function as a new tongue, getting sustenance from the fish’s blood and mucus!  The parasitized fish seem to survive and do fine with their new tongue.

As can be seen at Parasite of the Day, parasites are often, of necessity, of small size and many of them have few, if any, hard parts. Because of these facts their chances for fossilization are very low.  Occasionally, however, we are afforded a glimpse into parasites of the ancient past.  They are often best seen in amber, the fossilized, oozing sap produced by a variety of trees.  Preservation in amber can be exquisite with even the finest of details easily seen.



Arrows point to four parasitic mites attached to the abdomen of an adult midge in amber.

There are many cases where external parasites, such as mites, are preserved in amber still attached to their hosts. 



The sac bearing a parasitic dryinid wasp larva on an insect in amber.

Living dryinid wasps are parasitic, with larvae that live in sacs on the outside of their insect hosts, and slowly consume their host from the inside. Adult dryinids have been found in amber along with specimens of other insects bearing the external sac of the wasp’s parasitic larva.

However, many parasites are internal but even their fossilized remains show up. Nematode worms are parasitic on a wide variety of arthropods, growing inside their host.  In some cases large numbers of juveniles are shed that mature later outside the body.


Over a hundred juvenile nematodes can be seen trapped in amber as they emerged from their female fruitfly host.

A single, large nematode, preserved in amber in the act of  bursting through the body wall of a midge.

In other nematode parasites a single worm grows to adulthood inside its host, until bursting through the body wall, a la the chest bursting xenomorphs on Acheron (LV-246) in the Zeta II Reticuli system.  More than one insect in amber has been preserved in its death throws with the parasitic nematode, sometimes longer than its host, partially sticking out of its host.

A just published paper by Paula Dentzien-Diaz and colleagues is an important contribution to palaeoparasitology and describes a spectacular (but very small) parasite of ancient sharks. This  is not, however, from amber but from a coprolite, i.e. fossilized feces.  As part of a paleoecological study, the authors collected some 500 fish coprolites, from ancient pond deposits in the Rio do Rasto Formation in the Parana Basin of Brazil.  These rocks are Permian in age, abut 270,000,000 years old.  As part of the study, a number of the coprolites were thin sectioned at various angles and examined for inclusions, such as bone fragments, that might provide insight into ancient predator-prey relationships. In one of these coprolites there was an unexpected scientific discovery. 



The Permian shark coprolite from Brazil in external view. Note spirals concentrated at left end.

The coprolite is classified as a spiral heteropolar coprolite, meaning that it has closely spaced whorls concentrated at one end, a structure indicative of shark origin.  The coprolite is not large, 5cm x 2 cm (2 in  x .8 in).  Within the coprolite are nearly very 100 small, ovoid, smooth shelled eggs packed together in a segment 4mm x 1mm.   


Thin section of the coprolite slowing the mass of small eggs (left) and a close up (right).


Photo of the fossilized tapeworm egg with preserved embryo still inside (left) and interpretive drawing of specimen (right).
 
The details of the morphology of these fossil eggs is very similar to those of modern tapeworms.  One of the eggs in the Rio do Rasto coprolite is so preserved well that parts of a developing embryo can be seen inside.



A modern tapeworm.  The scolex is at the thin end and attaches to the hosts gut wall.  The segments grow larger as they move away from the scolex and break free when they are full of fertilized eggs.

Tapeworms live in the intestines of vertebrates and can reach lengths of  37 feet and live up to 25 years in humans.  The head end, or scolex, is small and the scolex of each species bears a unique arrangement of  hooks, cups, and other structures by which it attaches to the gut wall of the host. The body is composed of segments that are continually added at the head and become larger with age.  Each segment contains eggs and when the eggs are mature the segment breaks off and leaves the body in the feces, to eventually infect other hosts.  Simply delightful!

Tapeworms often have complicated lifecycles, with different stages infecting different kinds of hosts, only reaching their reproductive state when comfortably ensconsed in the guts of their final host.  So the eggs in the coprolite indicate that these Permian tapeworms used sharks as their final host.

This tapeworm discovery is a fine example of serendipity in paleontology.  Although Paula Dentzien-Diaz  and her colleagues were studying the coprolites, they were not looking for parasites. But careful observation turned up these small, delicate, yet well preserved fossil eggs. Only one of the coprolites that were sectioned contained the eggs and one of the thin sections fortuitously passed close to the eggs so they could be seen, and thus the oldest case of tapeworm infestation became known.  If things had been done only slightly different, these ancient parasites might well have remained unknown.




Tapeworms have long been a plague in humans, living for decades in the gut, absorbing nutrients from the food that passes through the intestine, and producing hundreds of thousands of eggs that are passed out in the stool. Those with a tapeworm infestation suffer a wide variety of ailments from the infestation, and since the tapeworm is stealing nutrients, loss of weight is one. Between 1900 and 1920 there were several diet aids introduced trumpeting that they contained tapeworm eggs. Take the aid, get infected, eat to your heart’s desire, AND STILL LOSE WEIGHT!  The truth of the ads is uncertain as no samples are available for analysis and one cannot rule out that the diet aid was just a bit of food, with no eggs at all.  Can you imaging false advertising claims for a weight-loss product?  Who could have imagined!

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SOURCES
Dentzien-Dias, P., Poinar, G.O., de Figueiredo, A. E. Q., Pacheco, A.C.L.,  Horn, B.L.D., and Schultz C.L.  2013. Tapeworm eggs in a 270 million-year-old shark coprolite. PlosONE 8(1): e5507. doi:10.1371/jpounral/pone.0055007 Free at

Poinar, G.O. 1992. Life in Amber. Stanford University Press, Stanford: 350 pages.

Poinar, G. and Poinar, R. 1994. The Quest for Life in Amber. Helix Books, Addison Wesley Publishing Co.: 219 pages.
 
Zimmer, C. 2000. Parasite Rex: Inside the Bizarre World of Nature’s Most Dangerous Creatures.  Free Press: 298 pages.
 
PHOTOS
 Rhinebothrium scolex:

Cymothoa

Parasitic mites, dryinid wasps, large nematode: Poinar 1992

Creative Commons CCBYNCND 3.0 license


Shark coprolites and tapeworm egg fossils:  Dentzien-Dias et al. 2013






1 comment:

  1. Paleontologists and geologists can glean some information from the rock structures that contain fossils. Sedimentary rocks usually stack themselves in layers called strata. First, the law of superposition states that the oldest strata lies at the bottom and that newer strata is near the top layers. Paleontologists also understood that fossils called index fossils were widely dispersed organisms that were known to be alive in certain periods of time.

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