This blog was written by UCL Culture Museums Volunteer Margaret Peng.

A classic fact about the Grant Museum is that it has more zoology specimens on display than the Natural History Museum in South Kensington. So there is always a new specimen waiting to be discovered by even the most frequent visitor. One such specimen is this week’s Specimen of the Week with its tantalising label telling of swim bladders that can also function as lungs…

**The Garpike**

LDUCZ-V1446 Lepisosteus sp.

Garpikes, also known as gars, have an elongated and flattened body with sharp, fearsome teeth. They are excellent ambush predators and mainly prey on fish. Seven species of garpike exist today across North and Central America. But fossils have been found as far afield as Africa and South Asia, suggesting they used to be far more widespread.

A Breath of Fresh Air

Perhaps one of the reasons they have survived for so long is that their swim bladders can perform a similar function to lungs. The primary role of the organ is to maintain buoyancy for the fish to keep afloat. But it also allows garpikes to draw in oxygenated air at the surface so that they can even survive waters with the lowest levels of oxygen. The aerial breathing complements gill-facilitated breathing to create two methods of obtaining oxygen for respiration. This adaptation allows them to live in a wide range of environments and would have increased their resilience to changes in environmental oxygen levels in the last one hundred million years.

Swim Bladder

The swim bladder of a garpike is derived from the oesophagus, to form a sac (figure 1). The sac is further divided into smaller and smaller compartments, akin to the alveoli in lungs, to maximise the surface area available for gas exchange. The garpike family is regarded as a more basal family of fish and therefore, still retains the pneumatic duct that connects the swim bladder to the oesophagus.

This allows them to gulp air at the water’s surface which passes along the duct into the swim bladder. The oxygen in the air then diffuses into the bloodstream, across the lining of the swim bladder. This diffusion process is facilitated by the extensive network of blood vessels in the swim bladder’s walls. It maximises the number of entry points into the bloodstream and ensures as much oxygen as possible diffuses into the blood vessels.

The use of this aerial mode of breathing depends on the environmental conditions. For instance, eutrophication might make the water hypoxic; lacking in oxygen. So garpikes will switch to aerial breathing and come up to the surface for oxygen. Similarly, high temperatures might cause the fish to switch to aerial breathing. A higher temperature means less oxygen is dissolved in the water so less is available for respiration in the fish. Therefore, aerial breathing is an important tool that allows garpikes to endure a wide range of challenging environments.

However, aerial breathing is not unique to the garpike. For example, it is also found in lungfish which is of a differnet family to the garpike. It is thought that air breathing arose independently in multiple groups of fish. This highlights how important obtaining enough oxygen for respiration is for all fish and vertebrates, not just the garpike. So surprisingly, a garpike won’t breath as awkwardly as a fish out of water as one might think.

References:

Icardo J M, Colvee E, Lauriano E R, Capillo G, Guerrera M, Zaccone G. (2014) “The structure of the gas bladder of the spotted gar, Lepisosteus oculatus”. Journal of Morphology. 276: 90-101.

Omar-Ali A, Baumgartner W, Allen P J, Petrie-Hanson L. (2016) “Fine Structure of the Gas Bladder of Alligator Gar, Atractosteus spatula”. International Journal of Scientific Research in Environmental Science and Toxicology. 1(1): 8.

Winchell A. (1864) “Description of a Gar-Pike, Supposed to Be New: Lepidosteus (Cylindrosteus) oculatus”. Proceedings of the Academy of Natural Sciences of Philadelphia. 16:184-185.

https://www.in.gov/dnr/fishwild/files/Gar_animal_info_series_for_web_2012.pdf [last accessed 15th October 2019]

https://www.nationalgeographic.com/environment/freshwater/gar/ [last accessed 14^th October 2019]