SUMMARY

The habitat of Atractosteus tropicus, the Mexican gar, captured in the State of Tabasco, and 3 of its parasites are described and illustrated. Pictures of them and graphs of their lifecycles are included. Because of the shape of its head, and hard ganoid scales the Mexican gar, also called “tropical gar” is locally known as “pejelagarto” that translates as “crocodile fish”.

Key words: Atractosteus tropicus, gar, pejelagarto, Clinostomum complanatum, Cystoopsis atractostei, Proteocephalus singularis, Perezitrema bychowsky,  Centla  Swamps

INTRODUCTION

On October 22 of 2005, the Wilma Hurricane swept with its winds of up to 300 km/hour over the city of Cancun and stayed over it for 36 distressing hours. When finally it slowly retired, there was not left a single leaf on the trees that still stayed up in the city and their environs, showing their skeletons against the shady sky. The city, with its destroyed buildings, seemed a bombed city, the population was at the border of chaos, looters invaded the stores, and the neighbors were organizing themselves for their defense and survival. There was no electricity, and to illuminate the streets at night, bonfires were ignited with the moist remnants of the fallen vegetation. The black and pungent smoke covered the sky.

I received an asylum offer in the city of Villahermosa, Tabasco, and, with my family, I fled from the city. There is no other word to describe it. And we were amongst hundreds of those that fled.

Tabasco has also been whipped by hard hurricanes, before, and, mainly after, our visit. After a year the city of Villahermosa itself was flooded almost totally by overflowing waters of the rivers that surround it.

Those used to employing GoogleEarth can see Villahermosa City and pictures of its building, parks and floods!!.

But at the moment of our visit it was flooded by the sun, and the boisterousness of the busy life of the flourishing “oil city” that it is, and was really green because of its very abundant  vegetation.

The city is framed by the Grijalva River, and the Carrizal River, and is seated over a marshy plain that is always present because of the existence of innumerable lagoons, pools and little swamps, scattered throughout the city structure. For a microscopist interested in the aquatic microfauna, Villahermosa (and all Tabasco) is an Eden. 

And, around the city there is the jungle, in which man has opened extensive prairies to cultivate cattle. But which is always present, and is easy and extremely tempting to walk under its sun speckled shadow.

 Surrounded by a church-like silence it is possible to see the bromeliads hanging from tree branches, and, from time to time, some of the multiple species of tropical orchids that decorate it.

And while you ramble across the city and its environs, it is common to see along the shores of the rivers, what Mexicans call a “lagarto” (lizard): the crocodile, of which there are two species, Crocodylus moreletii (the swamp crocodile), and Crocodylus acutus (the river crocodile) like the one whose picture I took while crossing a bridge.  

But it is still more surprising to see this scene when you enter a restaurant.

NO, it is not really a small crocodile (although people also eat the meat of these), it is a fish. Not any fish of course. It is a survivor with a history of millions of years, a living fossil: The PEJELAGARTO (crocodile fish), the “tropical gar”

Atractosteus  tropicus

This is the frightening aspect of its head. The long and narrow shape, and its sharp teeth, was worth its nickname by comparison with the head of a crocodile

 The Atractosteus belongs to an old group of fish, the Lepisosteidae, with a history of more than 140 million years, whose fossils are already found even in the Permian. It is characterized by its special ganoid scales, (see http://www.amonline.net.au/fishes/what/scales/index.htm) so different from those of the common fish. The Family is now restricted to two genera, and is limited to the south  of the United States (Texas and Louisiana - 5 species), Mexico (1 species), Central America (1 species), and Cuba (1 species). When adults these fishes can reach a length of about 2 meters.

We can summarize its taxonomic position according to these categories:

 Kingdom: Animalia  
Phylum: Chordata
  Class: Actinopterygiids
  Order: Lepisosteiformes
  Family: Lepisosteidae

The family has two genera Atractosteus and Lepisosteus, with these species that I list with their geographic distribution

Atractosteus spatula          United States
Atractosteus tristoechus    Cuba ("Pantanos de Zapata", Zapata Swamp)
Atractosteus tropicus         México and Central América
Lepisosteus oculatus           United States

Lepisosteus osseus              United States
Lepisosteus platostomus      United States

Lepisosteus platyrhincus      United States

Normally gars vary between 0.6 and almost 2.0 m in total length, but Atractosteus spatula is the champion reaching more than 3.0 meters. In order to have a clear idea of its dimensions it is sufficient to enter “Atractosteus” in a browser and click “images”. Most of the pictures are examples of the enormous trophies captured in different fishing competitions in southern USA.

 All are powerful predators. In the United States the fishermen consider them a pest because they feed on the fish that they wish to fish, and they shoot them or, according to a journal article (dated June 2007), they even use bows and arrows (to classify this as a sport surely).

 In Mexico we eat them. I can attest that its meat, cooked with due care, is really tasty. It is the gastronomical emblem of Tabasco and is offered in all type of restaurant. It is not cheap, because until recently, due to indiscriminate fishing, it was considered a species threatened with extinction. I must give notice here that this fondness for gars is partially based on the alleged aphrodisiac properties of the meat.

 Natural (and spectacular) habitats of tropical gars are the Centla Swamps (los Pantanos de Centla) even if they are widely distributed in Tabasco. Centla is a large expanse of shallow waters, with plentiful aquatic vegetation and surrounded by tropical forests, one of whose notable characteristics is the high fish species richness, most of which are edible and of great quality.

The “Pantanos de Centla”, are located in the neighborhood of the city of Frontera, and their coordinates, for whoever who wishes to use Google Earth, are

Longitude: from 18º40’ to 18º02’ N

Latitude: from 92º16’ to 93º05’ W

Theoretically, commercial fishing is forbidden within all its area. But in defined areas, and through the innumerable marshes included around the Centla, the “pejelagarto” lives and is fished intensely in artisan form.

This is a picture from a dedicated photographer (ventolinmono) that has grasped the atmosphere of the area in his beautiful collection of pictures published in Flickr   http://www.flickr.com/photos/dystopiatv/sets/72157605278930063/

The culture of gars 

The only way to defend this species from man, the only enemy who is able to threaten their millions of years existence, has been to establish its culture, with aims to repopulate the swamps, or even to try commercial aquaculture. To do this it required not only a deep study of its own anatomy, physiology and ecology, but of its parasites also.

 Lacking my microscope, and interested by my first gastronomical contact with such an attractive “living fossil”, I approached the University Juárez Autónoma of Tabasco and its departments of Aquaculture and Parasitology. The admiration the people of Tabasco have for their crocodiles (the true ones, of course) is demonstrated by the entrance to its buildings where these have been beautifully sketched in their admission portal.

In the aquaculture area I could observe the large (and small) tanks in which the gars are grown step by step, from the alevine stage (the youngest fish, which is born from the egg) until the breeding age.

There is even a local market, for the local aquaculturists to buy and care for the little animals, with the understanding to return them to their natural habitat when they are uncomfortable in their house aquariums. The rest of the country is excluded from this hobby. With a good ecological criterion the exportation of living gars is banned out of Tabasco.

Breeding examples are impressive animals, being almost two meters in length and large in diameter. Unfortunately due to lighting conditions, I did not obtain any decent photo of the breeding tanks or the nursery.

But the hatchery is successful at the species reproduction level and is able to produce juveniles in high quantities (300,000 a year it is said) to be released at the Tabasco marshes to aid with the recuperation of the wild population of Atractosteus. Experimental farms, with relatively rustic, extensive aquaculture conditions, have been also installed to grow the gars to an appropriate harvest size. (2007)

I was more fortunate in Parasitology, where Dr. Leticia García Magaña not only kindly gave me copies of their papers on the parasites of atractosteus, but also put at my disposal some microscopes, and mounted preparations of several of them.

Dra  Leticia  García  Magaña

A first note for those who are not familiar with fish parasites, is that these can be classified in three great groups: the Nematoda, the Trematoda, and the Cestoda.

The reader interested not only in the images, but in the organisms which they represent, if they do not have enough knowledge of Zoology, would have to read at least some of the following articles on the Internet, where in addition they will learn the technical terminology applicable to the anatomy of each group:

http://en.wikipedia.org/wiki/Trematodes, follow up the references for aspidogastrea, digenea and monogenea

http://cas.bellarmine.edu/tietjen/images/platyhelminthes.htm

 http://en.wikipedia.org/wiki/Nematoda

http://www.1911encyclopedia.org/Nematoda

 http://www.itg.be/itg/DistanceLearning/LectureNotesVandenEndenE/imagehtml/ppages/CD_1071_086c.htm  A good sketch of digenean anatomy

http://home.austarnet.com.au/wormman/wltape.htm focused on human parasitology (Cestoda)

http://www.aber.ac.uk/parasitology/Edu/Cestodes/CestTxt.html another interesting site

The reader must be warned that as human beings host several parasites, and human parasitology has great sociological and clinical importance, most of the general parasitological information on the Internet refers especially to human parasites.

Although I worked with a Leitz Photomicroscope, my lack of familiarity with it and my hurried visit made that, finally, the better pictures I took with my Canon Powershot A300, by the simple artifice of applying the lens to the exit pupil of the eyepiece of a normal binocular microscope.

In the State of Tabasco (Salgado-Maldonado, et al, 2005, Salgado-Maldonado, 2006) there are detected on Atractosteus

Adult trematodes. Perezitrema bychowskyi

Trematode  Metacercariae: Clinostomum complanatum, Diplostomum sp., Posthodiplostomum mínimum;

Adult cestodes: Proteocephalus singularis

Adult Nematodes: Cistoopsis atractostei, Procamallanus (S.) rebecae

Nematodes larvae: Contracaecum sp. tipe 1, Contracaeucum sp. 2, Spiroxys sp.

The species I could observe are written in blue. Aside from the pictures that I could obtain, and in order to complete the information and make more useful this article, I have added complementary images whose origin is detailed in each one.

Trematodes

Clinostomum complanatum (Rudolphi, 1819)  

(modified from  http://www.wildlife.state.nh.us/Fishing/Fishing_images/grubby_lifecycle.gif )

 

At Centla there are many aquatic birds that are possible hosts for Clinostomum. The "Blue Heron" is one of the known hosts

"Flying blue heron" Picture by Joe  Orban (see http://www.flickr.com/photos/vidular/3556033031/)

This is an image of a metacercaria, pulled out of its cyst, stained with Carmine and mounted in Canada Balsam. It is a four image mosaic. Although it occupies much more space, I present it in a vertical format so that its morphology is better appraised. The complete original image is a 6 Mpx one.

This is also a mosaic of which I think is a pre-adult, probably stained with Hematoxylin.

Adults have their uterus full of eggs, which is not the case here as you see in the third picture. Above it has been reduced, to show its general anatomy, but in the following images I show it through 4 images at a bigger magnification. Each original picture was 2 Mpx.

For Perezitrema , Posthodiplostomum, and Diplostomum I present the following images only with the intention to better complete the panorama of this fish's parasites.

Perezitrema bychowsky	Posthodiplostomum minimum	Diplostomum sp.

NEMATODES

Cystoopsis atractostei Moravec and Salgado Maldonado. 2003

It is a relatively large nematode, that lodges under the Atractosteus skin. Its description is at

http://www.ibiologia.unam.mx/pdf/directorio/s/salgado/cystoopsis.pdf

from where I reproduce the identifying drawings, and where you can investigate the meaning of the different drawings. For those not familiar with the group, this could be a good occasion to read a complete scientific description of a new species.

And this is its head.

Clearing the nematode with chloralphenol you can see the structure of the head.

 a –buccal bulb, e – œsophagus, v – vulva, va – vagina. The bar is 40µm long

reproduced from Moravec & Salgado-Maldonado

Life cycle is unknown, but we can hypothesize that it is similar to the cycle of Cystoopsis acipenseri, a better known species.  It must be relatively simple and similar to the one show below.

CESTODES Proteocephalus singularis La Rue, 1911 Cestodes (tapeworms) are basically characterized by the anatomy of their constituent proglottids (segments). This is the site of the sexual organs (whose morphology is essential for species differentiation). Nevertheless, the scolex (the first segment, or head, the holdfast organ) has generally also a distinctive structure, based on the suckers, grooves, hooks or spines that ornate some species. Both structures serve to fix the tapeworm to the wall of the intestine of their host. The genus Proteocephalus has a scolex with a very simple structure as it is seen in the following frontal figure. The preparation has been stained with Hematoxylin and mounted in Canada Balsam. It is similar enough to the one of another genus named Ophiotenia. Differentiation between both genera is based on the distribution of the testicles in the proglottid, in Ophiotenia they are distributed in two distinct lateral fields, whereas those of Proteocephalus form an ample, central, and unique field, as the following image (left) shows, the right image is a preparation of one mature proglottid. Salgado-Maldonado et al.,2003, states that “The life cycle of this cestode in Mexico is unknown, but we assume that a copepod serves as an intermediate host. Plerocercoids of proteocephalids have been found in the viscera of several freshwater fishes in Tabasco”. This is consistent with what is known about the life cycles of the cestoda, that can be so summarized: The adult of Cestoda (1) lives in the intestine of the definitive host fish (in this case the Atractosteus), where it sheds free proglottids which free its eggs that enter the water with the feces (2). The eggs are generally eaten by a copepod (still now unknown) (3) and develop into a first larva named procercoid. If the infected copepod is eaten by a small fish  (4) (also unknown but there are a lot that are food for the tropical gar) the parasite penetrates the gut and develops in the internal organs or the mesentery into a second larval stage (plerocercoid). Finally, the definitive host  (5) eats an infected small fish and the adult cestode develops in its intestine. So they think the cycle is similar to the one of the proteocephalid of the black bass shown above and reproduced from an original drawing in https://www.msu.edu/~gillilla/basstapeworm.html I want to offer thanks here for the corrections that Prof. Leticia García Magaña made to my draft, which improved the article.   NOTE:  if you like to know better the State of Tabasco, I recommend that you visit this incredible collection of pictures, most of them in HD:  http://www.skyscrapercity.com/showthread.php?t=518975 Comments to the author, Walter Dioni , are welcomed.   Microscopy UK Front Page Micscape Magazine Article Library © Microscopy UK or their contributors. Published in the June 2009 edition of Micscape. Please report any Web problems or offer general comments to the Micscape Editor. Micscape is the on-line monthly magazine of the Microscopy UK web site at Microscopy-UK © Onview.net Ltd, Microscopy-UK, and all contributors 1995 onwards. All rights reserved. Main site is at www.microscopy-uk.org.uk with full mirror at www.microscopy-uk.net .