Swimming into Deep Time

This week, the Biodiversity class took a break from random numbers tables and rarefaction curves, and visited the Tennessee Aquarium in Chattanooga. Our goal: to study representatives of the most ancient lineages of vertebrate animals. The rivers of the south-eastern U.S. are home to several such venerable creatures and the aquarium has examples of these local species as well as species from more far flung places.

The ancient pedigree of gar is revealed by their skin. They have diamond-shaped ganoid scales, a character they share with extinct relatives and with paddlefish (who have a few such scales around their tail). The scales interlock all over the body, making armor-plating. The scales are very tough and Native Americans used them as arrow heads. Early European settlers covered plough blades with gar skin, literally tearing up the earth with the hides of these fish.

Spotted gar, Lepisosteus oculatus. Note the long mouth: gar grab prey by side-swiping these jaws

Paddlefish were also well represented in the aquarium’s tanks. Like gar, paddlefish date back to the late Cretaceous, so they swam around the ankles of river-stomping dinosaurs. Their long “nose,” the rostrum, is used both as a sense organ and as a stabilizing device. Paddlefish feed by cruising with their large mouths wide open, creating backwards drag that is counteracted by the rostrum.

Paddlefish, Polyodon spathula

The paddlefish is native to the mid-western and south-eastern U.S., although its range is now much smaller than it used to be, partly due to overfishing and partly due to habitat degradation. Some states now have restocking programs for the species. The only other living paddlefish species is found in the Yangtze River in China. It may well be extinct now: no living specimens have been found since 2003.

Paddlefish are not the only ancient lineage to suffer from the ill effects of modernity. Sturgeon rival sharks as the oldest group of fish still alive, with relatives dating back in the fossil record to the Devonian, 400 million years ago. The IUCN now classifies sturgeon as the most endangered group of animals on the planet. Unfortunately for sturgeon, a hyper-abundant species of Old World ape regards sturgeon eggs as a delicacy and a status symbol. Our taste for caviar is hard for the slow-reproducing, long-lived sturgeon to sustain; their populations cannot persist in the face of our nets.

Beluga or European Sturgeon, Huso huso. The four barbs on the lower surface of the snout are used to find food in muddy river bottoms. This individual is about six feet long.

This Stellate Sturgeon shows the heteroceral tail that characterizes many of these old lineages. The vertebral column and associated muscles extend into the top lobe of the tail. This design provides a lot of power, but because the tail is asymmetrical, it twists the fish as it swims and this twisting force has to be counteracted by the pectoral fins.

The Tennessee Aquarium, along with TWRA and other partners, have been reintroducing thousands of Lake Sturgeon in east Tennessee. Hopefully, some of these animals will survive long enough to help buoy the population. This species is listed as endangered by the State of Tennessee, although compared to most other sturgeon species worldwide it is faring rather well, with small but seemingly secure populations.

All these species carry Deep Time in their blood. They are the incarnation of our family tree, ancestors swimming alongside us.

I’ll close with a quote from E. O. Wilson, who became a biologist along the banks of Alabama’s streams:

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