Digestive System

Diet & Dentition

Marine iguanas are herbivores, and the only known species of lizards that feed on macrophytic marine algae (specifically red, brown, and green algae), also known as seaweeds. Smaller individuals feed on algae in the intertidal zone that is exposed at low tide, while larger individuals will feed in the subtidal zone by diving. Terrestial plant matter is often scarce, which most likely lead to the gained adaptation of being able to dive underwater for food.

In order to feed on algae on rocks, the marine iguana will twist its head sideways and graze algae off the rocks with blunty cusped teeth. All of its teeth are similar in size, and are weakly attached with soft tissue. This is most likely for shock absorption during feeding.

Paprella and W. Caldwell

Theresa Guise, National Geographic

Paprella and W. Caldwell

Ingestion

As mentioned above, marine iguanas use their bluntly cusped teeth to scrape algae off of hard rock surfaces. Marine iguanas have a short snout compared to other iguanas, and a unique nasal system, known widely for its part in salt excretion. The short snout facilitates the scraping of algae from the rocks. As the head moves side to side, a greater toothed proportion of the skull can make contact with the algae.

In order to expel excess salt from their bloodstreams, marine iguanas expel concentrated salt through their nasal passageways. This admittedly cute adaption appears like a salty sneeze.

I. Machie et al.

Digestion & Absorption

*Note: There is not an extensive amount of available information on the specific workings of the marine iguana's digestive system. Though research has been conducted to study the diverse bacteria found in their gut microbiome, the actual processes that take place during digestion are not well known. The following digestion information is therefore also based on the marine iguana's two closest relatives, the green iguana (Iguana iguana) and the black spiny-tailed igauna (Ctenosaura similis).

The digestive tract of the marine iguana is adapted to process the tough cellulose found in algae. Unlike humans, marine iguanas (as well as other species of iguanas) do not have any digestive enzymes in their saliva. Instead, their mucous saliva simply helps food to pass through their digestive tract. After ingestion, the algae pass through a short and flexible esophagus, where more mucous is secreted and stretching and contracting help to move food. Next, food passes to a rather large stomach, which contains a large proportion of the gut content. The stomach, as well as the small intestine, does not possess high levels of volatile fatty acids (VFA), which indicates little breakdown of structural carbohydrates in this area. In contrast, concentrations of VFAs drastically increase in the colon. Gastric juices are produced in the stomach, which break down food in water-soluble molecules. In the small intestine, which is relatively short compared to other species, water and minerals are absorbed, which help to nourish the marine iguana's body.

The large intestine plays an important role in the absorption process. Marine iguanas have a large colon with adaptations that restrict outflow. The capacious colon is partitioned into 10 or more segments by transverse mucosal folds, as the colon in marine iguanas is specialized for fermentative digestion, the process by which microbes break down carbohydrates.

Gut Microbiome

Most herbivorous mammals depend on a symbiotic relationship with complex microbial populations residing in their digestive track. These microbes aid in the breakdown of high amounts of plant fiber. These microbial populations break down plant cell wall material and nonstarch polysaccharides that are typically indigestible to the host herbivore. The proportional capacities of the digestive tract of the marine iguana are slightly higher than its closely related iguana relatives, indicating that marine iguanas are unique reptilian herbivorous that have a large colonic gut compartment, which is specialized for fermentative digestion.

I. Machie et al.; confocal scanning laser microscopic images of the stomach and colonic samples of a marine iguana specimen. Images (C,D) show a dense and diverse bacteria population.

Elimination

The cloaca is the single exit point for excretory and gential products in the marine iguana's body. The is where all waste is expelled.

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