Inside the Giants: The Anatomy of a 220-pound tarpon

Take a tour of the outside and insides of a 220 pound Atlantic tarpon (Megalops atlanticus).

Subject tarpon courtesy of Timmy Deacon

Photos: FWC except where otherwise noted

Warning: This article includes graphic images of dead animals

In the beginning it started out like all other tarpon larvae known as a leptocephalus, a small fish barely an inch long.

leptocephalus
Then she grew!

Captured tarpon and angler

Date of capture: June 13, 2007

Tarpon was caught at Egmont Key, north end in the hole.

Rough coordinates might be:

Latitude:    27 36.006
Longitude: 82 45.413

Total Length: 2,215 mm (88.6 inches)
Fork Length: 2,026 mm (81.0 inches)
Standard Length: 1,908 mm (76.32 inches)
Girth: 1,100 mm (44 inches)
Weight at necropsy: ~97kg.

It was weighed the night it was caught at 220 pounds.

Photo courtesy of Suncoast Tarpon Roundup

What were Fish and Wildlife Research Institute scientists able to do with her?

Histology samples | Brevetoxin samples | Gut contents | Parasite identification

Outreach samples | Otoliths | Biofuel

Histology samples:

Sex: Female.

Ovarian tissue samples were processed in three different solutions to evaluate maturity stages of egg development. These oocyte (egg cells) samples were taken from the posterior of the right ovary. One sample from the anterior end of the right ovary was also placed in 10% formalin.

Ovary

This image shows the right ovary of the 220-pound tarpon on the table next to the fish. It is one of two ovaries inside the tarpon. This ovary was larger than a man's arm and during the spawning season, a tarpon's ovaries will fill most of the inside body cavity of the fish. A female tarpon can release between 4 and 20 million eggs each year.

Oocyte

Oocyte cells of a female tarpon can be seen here at high magnification. Cells that are cream-colored contain yolk and are developing into mature cells to be released at spawning. The smaller clear cells in the image are new primary growth cells. These may have been a different batch of eggs to be released during another spawning event or perhaps the same one - this is an unknown for adult tarpon. The tiny cells are at the earliest stage of development and contain no yolk, yet.

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Were any other organs used to learn about tarpon? Yes.

Brevetoxin samples:

A huge first gill arch, a muscle chunk from the right shoulder or back of the fish, a liver sample and gut contents were used to establish baseline data on brevetoxin (the red tide toxin) concentrations found in healthy tarpon. If there is ever a red tide event in the future and a tarpon kill at the same time, we can sample the dead tarpon and compare brevetoxin levels from that dead fish to the levels of the toxin found in healthy tarpon like this one to see if was possible that the hypothetical tarpon's death was related to a red tide.

Tarpon gill arch

A single gill arch of a tarpon. Tarpon and other bony fishes have four gill arches on each side of their body. The gills are the major respiratory organ for tarpon and are very fragile. Special care should be taken when handling a tarpon that no damage is done to their gills. When gills are damaged you put the tarpon at risk for death upon release. Never handle a live fish through the gills or place a tow line or gaff through the gills.
Liver sample
A small sample of liver is being placed into a baggie to go back to the lab for calculating brevetoxin concentrations.

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Gut contents :

Gut Contents The gut contents of this tarpon as seen inside the cut-open stomach were also measured for brevetoxin levels. The dark color you see is the liver of the tarpon which is quite large and surrounds the outside of the stomach.
Crabs

Blue crab, Calinectes sapidus (the bait): 107 mm

Pass Crabs, Portunus gibbesii: 43 mm, 45 mm, and 66 mm and various claws and pieces of crab legs.

Three or four parasitic worms in gut ~92 mm long.

The penny is there to provide a comparison of the size of the tarpon's prey. The fish did not eat the penny.

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New parasite identification:

Tarpon hearts and gill arch
The hearts and another gill arch from other tarpon were also sent to Mississippi's Gulf Coast Research Laboratory to see if the blood of Tampa Bay tarpon contains a unique parasite not previously described. We may help identify a new species. The heart of an adult tarpon fits easily on a man's hand.

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Outreach samples:

Two, gallon-size bags of scales were given to schools for educational purposes on aging fish through the Fish and Wildlife Research Institute's (FWRI) Outreach Coordination office.

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Otolith samples:

Otolith removal

Removing the otoliths, or ear stones, from a tarpon is no easy task. The skulls are thick and bony and a hacksaw is required. Otoliths are used for hearing and orientation by the fish. Biologists use them to estimate the ages of fish.

Sagittal_Otolith_Right

The small, right sagittal (relating to the skull) otolith can be seen here inside the skull lying in its cavity (at tip of arrow) located behind the brain which has already been removed. Otoliths occur in pairs and the sagittae are the largest of the three sets found in bony fishes.

A whole saittal otolith (top) was removed and sectioned (sliced) to age the tarpon. A thin cross section from the center the otolith (bottom) was placed under a microscope to count the rings in order to age the fish. Tarpon can live up to 50 to 70 years or perhaps longer. .Whole Sagittal Otolith

Small sets of otolith

The two smaller sets of otoliths are located inside the inner ear. Here you can see one of the inner ears that has been removed and placed on top of the tarpon. You can see how small they are relative to the size of a finger tip (light blue).

Mesentaries.jpg

Most major organs inside the body cavity of a tarpon are encased in these strong mesentaries (sheaths of tissue) that help hold everything in place.

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Breathing air

It was mentioned that gills are the major respiratory organ for tarpon, but a tarpon can also breathe air. It uses a swim bladder (see image) that has a direct connection from the specialized bladder to its esophagus (throat) so a tarpon can come to the surface and gulp air to fill this swim bladder. Tarpon are the only fully marine species of fish able to breathe air.

Swim Bladder

Here the swim bladder has been dissected open length wise exposing three rows (see arrows) of spongy tissue lining the bladder. The bladder contains four highly vascularized rows of special lung-like tissue that is capable of using the oxygen from the air to supplement the tarpon's breathing. You can see slight peachy color of the left ovary of the tarpon underneath the mesentery, or membrane, that surrounds it at the lower part of the image.

The rest of this tarpon was cut into thirds and recycled by using it for biofuel.

If you drive diesel, you may be running on tarpon fumes.

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FWC Facts:
Tribal societies in Central America, West Africa, Australia and Papua, New Guinea consider sawfish symbols of strength, spirituality and prosperity.

Learn More at AskFWC