BIOL630 TERM PROJECT
DIGESTIVE SYSTEM
To break down and absorb the nutrients obtained from their diets, the octopus utilizes a digestive system with special adaptations, but a recognizable layout. Octopuses have a two-way digestive system that starts with the mouth. They have a beak, like the look of a bird’s beak, made of keratin that is used to break down the food. Along with the physical breaking down of the beak, the octopuses have salivary glands with special enzymes for chemically breaking down food. These enzymes are especially useful for removing tissue from hard-to-reach portions of a crab’s shell (“Super Suckers…”, 2009).
After ingestion, the food goes from the crop to the stomach (directly if there is no undigested food in the crop already), where more enzymes from the liver pass through ducts to break down food in the stomach. A study conducted in 2017 found that one hour after feeding, zymogens – acidic and alkaline enzymes - are released within the gastric juice and are activated by the food bolus (Gallardo et al, 2017). The type of enzyme found, whether it is acidic or alkaline, changes during the process of digestion. In another study, the researchers found enzyme activity was higher at the beginning of the process at a pH between 3 and 5, and towards the end of digestion, alkaline proteases peaked significantly (Bastos et al, 2020). The change from acidic to alkaline shows the dynamic nature of the digestive process in response to the enzymes releasing OH- ions in the first stage. Any undigested material is passed from the stomach to the intestine and is moved along to the siphon. Uneaten food exits through the anus.
Figure 1. The figure shows the anatomical path that the food bolus takes, from ingestion at the mouth and expulsion out of the anus. (Courtesy of Cosgrove and McDaniel)
The important nutrients found in the food are broken down into amino acids and glucose, both with different end goals. The glucose is converted into glycogen for energy storage, while some notable amino acids including phenylalanine, glutamine, and serine are used as “metabolic fuel” (Gallardo et al, 2017).
Depending on the type of octopus, researchers have noted key differences in the digestive process. For example, O. maya, found in higher temperature habitats between 22 and 30°C, have a fast digestive process that consists first of rapid ingestion of soluble nutrients used for the production of muscular proteins, then a second process of more complicated nutrients sent to for storage within the muscle of the octopus. O. mimus, which is found in lower temperature habitats between 14 and 22°C, undergoes this same two-part process but at a much slower rate. This energy-budgeting relates to the environments that they live in, since an octopus in a warmer habitat can afford to burn more energy quickly, unlike the octopus in the colder waters (Gallardo et al, 2017).
Figure 2. This diagram shows the flow of the food bolus through the octopus. At number (5), the distinction is made between extracellular digestion ending (the remaining waste exiting the octopus through the anus) and the intracellular digestion beginning (amino acids and glucose moving through the hemolymph to be used in tissues or muscles)
(Courtesy of Gallardo et al, 2017).